Istructe Recommendations For The Permissible Stress Design Of Reinforced Concrete Building Structures.pdf

The Institution of Structural Engineers

February 1991

Recommendations for the permissible stress design of reinforced concrete building structures

Published by the Institution of Structural Engineers

Constitution D. K. Doran, BSc(Eng), DIC, FCGI, CEng, FIStructE, FICE (Chairman) (Consulting Engineer) W. E. A. Skinner, CEng, FIStructE, FICE, FHKlE (Vice-Chairman) (Mitchell McFarlane & Partners) A. N. Beal, BSc(Eng), CEng, MIStructE, MICE (Thomason Partnership) J . E. C. Farebrother, CEng, FIStructE, FICE, FCIArb (Consulting Engineer) A. C. Morton, BSc(Eng), CEng, MIStructE (Travers Morgan Group) A. F. Mottram, BSc(Eng), CEng, MIStructE (Lewisham District Surveyor's Office) A. R. Pemberton; BSc(Hons), CEng, MIStructE, MICE (Scott-White & Hookins) D. J . Rolton, BSc(Eng), CEng, MlStructE (Consulting Engineer) R. J . W. Milne, BSc (Secretary) (The Institution of Structural Engineers) Thanks are due to F. N. Pannell, BSc, MScTech, PhD, CEng, MIStructE, who prepared the column design charts given in Appendix B.

01991: The Institution of Structural Engineers This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any copying under the UK Copyright Designs and Patents Act 1988, no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without permission of the copyright owners. Multiple copying of the contents of the publication without permission of the copyright owners contravenes the aformentioned Act. 2

IStructE RC permissible stress recommendations

Recommendations for the permissible stress design of reinforced concrete building structures Amendment No. 2, August 2009 This amendment has been prepared in order to keep the document up to date, so that it may be used as an alternative to BS 8110 or Eurocode 2. This amendment covers changes made in Amendments 1-4 to BS 81101:1997, the introduction of BS EN 206-1, BS 8500-1 and BS 8500-2 and also changes in the Building Regulations. Because some of the latest changes in these documents affect items which were previously altered in Amendment Slip No. 1, for ease of use Amendment No. 2 is presented in consolidated form. It therefore supersedes Amendment No. 1. Background BS 8110-1: 1997 reduced the materials safety factor on steel reinforcement compared with previous editions. However the latest amendment to BS 8110 restores this partial safety factor to its previous value and the characteristic stress of high tensile reinforcement is also increased from 460N/mm² to 500N/mm². In addition, the BS 8110 recommendations for specifying cover and concrete mixes for durability have been replaced by recommendations in BS EN206-1 and BS 8500-1. Amendments (the following text supersedes Amendment No. 1) General Throughout the document, all references to ‘concrete grade’ should be changed to ‘concrete strength class’, in accordance with BS 8500. 2.1 Delete existing text and insert: ‘The cement, or combinations of cement with ggbfs or pfa should be in accordance with BS 8500-2 Table 1.’ 2.2 Aggregates Delete all and insert: ‘Aggregates should comply with BS 8500-2 Cl. 4.3.’ 2.3 Reinforcement Delete all and insert: ‘Reinforcement should be designated by its type and effective diameter and should comply with BS 4449, BS 4482 or BS 4483 as appropriate. Characteristic yield strengths are given in these standards as 250N/mm² for mild steel and 500N/mm² for high yield steel.’ 2.4.3 Specification Delete ‘BS 1014’, insert ‘BS EN 12878’; delete ‘BS 5075’, insert ‘BS EN 934’. Cl.2.5 Concrete Delete existing text and insert: ‘Guidance on specifying concrete is given in BS EN 206-1 and BS8500. Concrete compressive strength class is specified generally in terms of required 28 day cylinder strength and cube strength: e.g. strength class C32/40, means that the cylinder strength is 32N/mm² and the cube strength is 40N/mm². The different types of mixes are listed below.’ 2.5.1, 2.5.2 and 2.5.3: delete and insert: 2.5.1 Designated concrete A range of designated concrete mixes to cover most normal applications is specified in BS8500-2. BS 8500-1 gives guidance on their use. Designated mixes are: (i)

GEN0, GEN1, GEN2 and GEN3 for non-structural and unreinforced concrete;

(ii)

RC20/25, RC25/30, RC28/35, RC30/37, RC32/40, RC35/45, RC40/50 and RC40/50XF for reinforced concrete;

Recommendations for the permissible stress design of reinforced concrete building structures – Amendment No.2, August 2009 The Institution of Structural Engineers Page 1 of 6

(iii)

PAV1 and PAV2 for external hardstandings and concrete subjected to freezing when wet.

(iv)

FND2, FND2Z, FND3, FND3Z, FND4, FND4Z and FND4M for foundation concrete subject to attack by chemicals including sulphates.

2.5.2 Designed concrete As an alternative to standard designated concrete mixes, designed concrete may be used. A designed concrete mix is specified by its strength class (e.g. C25/30) plus any other specific design requirements such as minimum cement content, maximum water/cement ratio, etc. 2.5.3 Prescribed concrete For prescribed concrete, the specifier prescribes the composition and constituents of the concrete mix. 2.5.4 Standardised prescribed concrete BS 8500-2 specifies a range of standardised prescribed concrete mixes for use on small sites where the concrete will be site-batched, or obtained from a supplier without third-party accreditation. 2.5.5 Proprietary concrete Proprietary concrete is designed and produced by a supplier to meet specified performance requirements. The supplier must produce evidence to the third-party certifier or the specifier to show that the mix meets the specified requirements. Table 2 For high tensile steel, amend heading to ‘high yield steel to BS4449 and BS4483’ and increase pst to 275N/mm² and psc to 235N/mm². 3A.5 Loadings Delete ‘and CP3: Chapter V’. 3A.6.1 Table 1 delete ‘grade of concrete’, insert ‘concrete strength class’, in column 1 replace ‘15 ... 60’ with ‘C12/15, C16/20, C20/25, C25/30, C28/35, C32/40, C35/45, C40/50, C45/55, C50/60’. 3A.6.4 Shear Stress Delete sentence: ‘Where d exceeds 400mm, the term 4√(400/d) should be taken as unity.’. 3A.8 Delete existing text and insert: Increases of permissible stresses for wind forces or earth pressures The permissible stresses in concrete and in the reinforcement may exceed those given in subsections 3A.6 and 3A.7, respectively, by not more than 25% in the following situations: (i)

where the increased stress is caused by wind loading, or

(ii)

where the increased stress is caused by earth pressure calculated in accordance with BS8002 and water pressure calculated on the basis of a maximum credible water level. (NB if water and earth pressures are calculated in accordance with CP2, no increase in stress is permitted.)

3A.11 In beams, where fst = 275N/mm², the clear distance between bars in tension should not exceed 170mm. Tables 5, 6 and 6c Replace with revised tables: Table 5 – Anchorage bond and lap lengths as multiples of bar size for fully stressed bars reinforcement type

grade 250

grade 500

grade 500

plain

deformed type 2

fabric (see subsection 3A.6)

concrete strength class 25/30 tension anchorage and lap length

36

38

29

compression anchorage length

25

26

20

compression lap length

31

33

25

(see clause 3A.12.12)

Recommendations for the permissible stress design of reinforced concrete building structures – Amendment No.2, August 2009 The Institution of Structural Engineers Page 2 of 6

Table 6 Basic span/effective depth ratios steel tensile stress (N/mm²)

140

275

cantilever

9

6.9

simply supported

25

19.6

continuous

32

25.5

Table 6c Span/effective depth limits for slabs (pst = 275N/mm2) total dead and imposed load, kN/m2

slab

flat slab

1-way simply supported

1-way continuous

cantilever

2-way simply supported

2-way continuous

no drops

drops

5

30

41

12

34

47

38

42

10

26

35

11

30

42

33

37

20

23

31

9

26

37

29

32

Delete existing Note 1, insert: ‘Note 1: For pst = 140N/mm², ratios may be increased by 20%. Intermediate stresses may be interpolated.’ 3A.12.6 Dimensions of hooks Delete existing text and insert: ‘Where hooks are used, they should be of the U- or L- type shown in Fig. 4. In both types, for high-yield bars (type ‘H’): (i) the internal radius of the bend should be at least 2 × bar diameter (bars up to 16mm diameter), or 3.5 × bar diameter (bars 20mm or greater diameter)’; (ii) the length of straight bar beyond the end of the curve should be at least 4 × bar diameter.’ Cl. 3B.2.1 Add at end: ‘the Table 6a modification factors for br /b = 1 can be calculated from the formula: 0.55 + 1.18/(0.6 + M/bd²) ≤ 1.6. The Table 6b modification factors can be calculated from the formula 1 + (100Asc/bd)/(3 + 100Asc/bd) ≤ 1.5.’ 3B.10.1 Table 10, Fig. 9 Delete ‘d ≥ 400’, insert ‘d = 400’. 3C.6.1 Shear at column face Delete ‘3B.12’, insert ‘3B.11’.

Recommendations for the permissible stress design of reinforced concrete building structures – Amendment No.2, August 2009 The Institution of Structural Engineers Page 3 of 6

3C.6.3 Shear reinforcement Amend first and second paragraphs to read: If the effective shear stress exceeds the permissible value pv, shear reinforcement should be provided. This may take the form of links, bent-up bars or fabricated components. However the effective shear stress on the critical perimeter should not exceed 2pv The design of bent-up bars or other components should be justified by established theory and/or test data. Links in flat slabs should be designed in a similar manner to those in beams, with Av taken as the total area provided on one perimeter of links (see Clause 3B.10.2). Spacing of link legs along the perimeter should not exceed 1.5d. Where the effective shear stress on the critical perimeter exceeds 1.75pv, the permissible stress pst in equation (19) should be reduced: where the effective shear stress is 2.0pv, pst should be taken as 0.75 times the normal value; intermediate values for stresses between 1.75pv and 2.0pv may be interpolated.’ 3J.1.6 Air Entrainment Delete ‘When concrete lower than grade 50 is used ... aggregate’ and insert ‘When concrete with strength class lower than C40/50 is used, the minimum air content by volume of fresh concrete at the time of placing shall be: 5.5% for 10mm max. aggregate, 4.5% for 14mm max. aggregate, 3.5% for 20mm max. aggregate, 3.0% for 40mm max. aggregate’. Add paragraph at end: “When air-entrained concrete is specified with a strength class of C32/40 or over, or with a cement content greater than about 350kg/m3, problems may be encountered in achieving the required strength, compaction and surface finish. The inclusion of air in concrete reduces the compressive strength and the concrete producer may increase the cement content to achieve the specified compressive strength." 3J.1.7 Requirements for durability of concrete Table 22 replace existing table with the following: Table 22 Durability and concrete cover Conditions of exposure (BS 8500-1 exposure class)

Nominal cover to all reinforcement (mm) and designated concrete mixes cover

concrete

25

RC20/25

40

RC28/35

50

RC40/50

45

PAV1

Internal except poorly ventilated rooms

cover

concrete

cover concrete

35

RC32/40

30

RC40/50

40

PAV2

30

RC40/50XF

with high humidity (XC1) External concrete (general) (XC3/4, XF1) External concrete in coastal areas (XS1) External concrete subject to saturation and freezing (no de-icing salts) (XF3) Notes: 1.

Refer to BS 8500-1 for recommendations for other exposure conditions and guidance on specifying designed concrete.

2.

Where cover is controlled by suitable spacers and checked by a supervisor before concreting, stated nominal covers may be reduced by 5mm.

3.

In no case should the nominal cover to main bars be less than the diameter of such reinforcement.

4.

Concrete cast against blinding: nominal cover 50mm; concrete cast against earth faces: nominal cover 75mm.

Recommendations for the permissible stress design of reinforced concrete building structures – Amendment No.2, August 2009 The Institution of Structural Engineers Page 4 of 6

Table 23 40mm aggregate: delete ‘-30’, insert ‘-20’. 3J.1.8 Reactive aggregates - alkali-silica reaction At end: delete Concrete Society Report 30, add BRE IP 1/02, BS8500-1 and BS 8500-2. 3J.2 Resistance to chemical attack Delete ‘Table 24 indicates the requirements for concrete exposed to sulphate attack’. Insert new para.: ‘Table 24 gives recommendations in accordance with BS8500 for 20mm aggregate concrete at least 140mm thick exposed to sulphate attack in (i) natural ground with mobile water and pH > 5.5 or static water with pH > 3.5 or (ii) brownfield site with mobile water and pH > 6.5 or static water with pH > 5.5. For concrete over 450mm thick, the requirements of classes 2, 3 and 4 may be reduced by one class. A ‘brownfield’ site is one which might contain chemical residues from previous industrial use or imported waste. For more detailed recommendations covering other conditions and concrete mixes, refer to BS8500-1 and BS8500-2 and BRE Concrete in Aggressive Ground, Special Digest 1. Delete existing Table 24 (p. 81) and replace with the following: Table 24 Recommendations for concrete exposed to sulphate attack Design sulphate class

Concentration of sulphate1

Designated concrete

In groundwater

In soil or fill

SO4 (g/l)

By 2:1 water:soil/extract (SO4)

1

<0.4

<0.5

RC28/35

2

0.4 to 1.4

0.5 to 1.5

FND2

3

1.5 to 3

1.6 to 3.0

FND3

4

3.1 to 6

3.1 to 6

FND4

Notes: 1.

Classification by groundwater samples is preferred. For analysis methods, see BS1377 Part 3 and BRE Research Report 279, which also gives methods for magnesium. To convert SO3 results to SO4, multiply by 1.2.

2.

For sulphate class 4, designated concrete FND4m should be specified if Mg exceeds 1g/litre in groundwater or 1.2g/litre in water/soil extract.

3.

Designated concrete FND mixes are strength class C25/30.

4.

For more detailed guidance, including specification of designed concrete mixes, see BS 8500-1.

3K.1 General Add at end: ‘Table 25 is based on the recommendations in BS8110-2. Alternatively, the cover recommendations in BS8110-1 may be used.’ 3L.1.5 Delete 3L.2 Ties (iii) Add at end: ‘Horizontal ties should be connected directly and robustly to the vertical structure. For columns, this can generally be achieved by ensuring that a minimum of two bottom bars in each direction pass through the column. Where top bars are used as ties, they should be restrained by links.’

Recommendations for the permissible stress design of reinforced concrete building structures – Amendment No.2, August 2009 The Institution of Structural Engineers Page 5 of 6

(iv) Delete and insert: Vertical ties Each column and wall carrying vertical load should be continuously tied from the lowest to the highest level. The tie should be capable of resisting a tensile force equal to the maximum load received by the column or wall from any one storey, calculated in accordance with 3L.4. Where a column or wall is supported at its lowest level by an element other than a foundation, the structural layout should be carefully checked to ensure that there is no inherent weakness and that adequate means exist to transmit the dead, imposed and wind loads safely from the highest supported level to the foundations. Insert: ‘(v) Bars should be lapped, welded, or mechanically jointed together in accordance with 3A.12.11-16. A tie may be considered anchored to another tie at right angles if the bars of the former tie extend either 12 diameters or equivalent anchorage beyond all the bars of the other tie, or an effective anchorage length (based on the force in the bars) beyond the centre-line of the bars of the other tie. At re-entrant corners or at substantial changes in construction, care should be taken to ensure that the ties are adequately anchored or otherwise made effective.’ 3L.3 Acceptable limits of damage For buildings in Class 2B of Building Regulations Approved Document A which do not comply with the requirements of subsection 3L.2, the structure should be designed such that if any element of structure were to fail or be forcibly removed, due to misuse or accident, the structural failure consequent on such removal would be localized within an area not exceeding 70m² or 15% of the area of the storey, whichever is less. Furthermore, the failure would be localized within the storey in which the element occurs, the storey next above (if any) and the next storey below (if any). Where the removal of such an element would result in an extent of damage exceeding the above limit, then the element should be designed as a ‘key element’ in accordance with 3L.5. 3L.5 Key elements and Bridging elements (where required in buildings of Building Regulations Approved Document A Class 2B or Class 3) Where a structural member is deemed to be a ‘key element’, the member and its supports should be capable of resisting a load of 34kN/m² acting in any direction on the whole surface of that member plus any building components attached to it, with reactions from the latter limited to the maximum that might reasonably transmitted taking into account the strength of the attached components and their connections. If a vertical key element relies on a horizontal member for stability, the relevant horizontal member (or part of the horizontal member) should also be considered as a key element.

Bridging elements should be designed by considering, at each storey in turn, the loss of each vertical loadbearing element in turn (other than key elements). If catenary action is assumed, allowance should be made for the horizontal reactions necessary for equilibrium. In this analysis, the length of loadbearing wall considered as a single element should be either the length between adjacent lateral supports or between a lateral support and a free edge, subject to a maximum of 2.25 times the storey height. For the purposes of this analysis, a lateral support to a loadbearing wall is either (a) a stiffened section of the wall (maximum 1m length) capable of resisting a horizontal force of 1.5Ft kN/m, or (b) a partition (mass at least 100kg/m²) at right angles to the wall, connected with ties capable of resisting 0.5Ft kN/m, where Ft = 20+4no < 60 (no = number of storeys). 5.1.1, 5.1.2 Delete ‘BS 5328’, insert ‘BS 8500’ 5.2.2 Delete ‘BS4466’, insert ‘BS8666’. 6.1Methods of testing concrete After ‘BS1881’ insert ‘and in BS EN 12350, BS EN 12390 and BS EN 12504’; after ‘BS 812’ insert ‘BS EN 933, BS EN 1097, BS EN 1367 and BS EN 1744’. Delete last sentence. Appendix A replace references to ‘BS 882’ with ‘BS EN 12620’. TECHNICAL NOTES (p. 122) Delete note on Clause 3C.6.3.

Recommendations for the permissible stress design of reinforced concrete building structures – Amendment No.2, August 2009 The Institution of Structural Engineers Page 6 of 6

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Contents

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Foreword 1 General 1 . 1 scope 1.2 Definitions 1.3 Symbols

2 Materials 2. I 2.2 2.3 2.4

Cements, ground granulated blastfurnace slags and pulverized-fuel ashes Aggregates Reinforcement Admixtures 2.4. I General 2.4.2 Types 2.4.3 Specification 2.5 Concrete 2.5.1 Designed mix 2.5.2 Special prescribed mix 2.5.3 Ordinary prescribed mix 2.6 Nominal mixes

3 Design considerations 3A General 3A. 1 Basis of design 3A. 1.1 Method of design 3A.1.2 Momcntsof inertia 3A.2 General stability 3A.3 Complete structures and analysis of structural frames 3A.3.1 General 3A.3.2 Frames supporting vertical loads only 3A.3.3 Frames supporting vertical and lateral loads 3A.3.4 Stress reversal 3A.4 Overturningstability 3A.5 Loadings 3A.6 Permissible stresses i n concrete 3A.6.1 Compressive and bond stresses and elastic modulus 3A.6.2 Compressive stress 3A.6.3 Modular ratio method 3A.6.4 Shear stress 3A.6.5 Bond stress 3A.7 Permissible stresses in reinforcement 3A.7.1 General 3A.7.2 Tensile stress 3A.7.3 Compressive stress 3A.7.4 Tensile stress due to shear 3A.8 Increases in permissible stresses due solely to wind forces IStructE RC pcrmissiblc stress rccommcndations

11 13 13 13 14 17 17 17 17 17 17 18 18 18 18 19 19

19 20 20 20 20 20 20 21 21 21 21 21 21 22 22 22 22 22 23 24 24 24 24 24 24 24

3

. 3A.9 Calculation of resistance moments of beams and slabs 3A.9.1 Basisof method 3A.9.2 Formulae for rectangular beam and slab sections 3A.9.3 FormulaeforT-or L-beams 3A.10 Concrete cover 3A.11 Distance between bars 3A. 12 Bond and anchorage 3A.12.1 Barsin tension 3A.12.2 Bars in compression 3A.12.3 Avoidance of bond failure 3A.12.4 Bearing stresses on bends 3A.12.5 Hooks and other anchorages 3A.12.6 Dimensionsof hooks 3A.12.7 Effective anchorage lengths for U- and L-hooks: high-yield bars 3A.12.8 Effective anchorage lengths for U- and L-hooks: plain bars 3A.12.9 Links in beams and transverse ties in columns 3A.12.10 Shear reinforcement 3A. 12.11 General requirements for connecting reinforcement 3A.12.12 Tension laps 3A.12.13 Compression laps 3A.12.14 Anchorage bond and lap lengths 3A. 12.15 Effective perimeter 3A. 12.16 Groups or bundles of bar 3A.13 Joints, crack control and minimum reinforcement requirements

3B Beams and slabs 3B.1 General 3B.l.l Effectivespan 3B.1.2 Slenderbeams 3B.1.3 Minimum reinforcement 3B.1.4 Compression reinforcement in beams 3B.1.5 T-beams 3B.1.6 L-beams 3B. 1.7 Effect of wear 3B.2 Deflection and stiffness of members 3B.2.1 General 3B.2.2 Simplified rules for slabs 3B.3 Bendingmoments 3B.4 Bending moments and shears in beams and slabs spanning in one direction 3B.4.1 Calculation of bending moments 3B.4.2 Detailingof beams 3B.4.3 Detailingof slabs 3B.5 Slabs spanning in two directions at right-angles with uniformly distributed loads 3B.6 Trimmings for openings 3B.7 Distribution of concentrated loads on slabs 3B.8 Bearings for slabs on steel joists 4

25 25 26 28 29 29 29 29 30 30 30 31 31 31 32 32 32 32 32 33 33 33 34 34 35 35 35 35 35 35 36 36 36 36 36 38 38 39 39 40 40 40 42 44 45

IStructE RC permissible stress recommendations

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3B.9 Slabs: ribbed and hollow-block construction 3B.9.1 General 3B.9.2 Blocks and forms 3B.9.3 Topping 3B.9.4 Calculation of resistance moment 3B.9.5 Resistance toshear 3B.9.6 Thickness of topping 3B.9.7 Size and spacingof ribs 3B.9.8 Reinforcement in ribs 3B.9.9 Supports parallel to ribs 3B.10 Resistance toshear 3B. 10.1 General 3B.10.2 Shear reinforcement 3B. 11 Loads near supports of beams: shear enhancement 3B. 12 Deep beams 3B. 13 Torsional resistance of beams 3B.13.1 General 3B. 13.2 Calculation of torsional rigidity 3B.13.3 Torsional shear stress 3B.13.4 Limit toshear stress 3B. 13.5 Reinforcement for torsion 3B.13.6 Torsion reinforcement 3B.13.7 Arrangement of links in T-, L- or I-sections

3C Flat slab construction 3C.1 3C.2 3C.3 3C.4 3C.5 3C.6

General Methodsofdesign Division of panels Notation for flat slab construction Thickness of slab Shear stresses in flat slabs 3C.6.1 Shear at column face 3C.6.2 Slabshear 3C.6.3 Shear reinforcement 3C.6.4 Openings 3C.7 Openings in panels 3C.8 Concentrated loads 3C.9 Bending moments in edge panels 3C.9.1 Slab supported by marginal beam 3C.9.2 Edge moments 3C.10 Column heads 3C. 11 Design of flat slabs as continuous frames 3C. 11.1 General 3C. 11.2 Bending moments and shearing forces 3C.11.3 Stiffnessof members 3C. 11.4 Maximum bending moments in slabs 3C.11.5 Design momentsforflatslabs 3C.11.6 Design moments in columns 3C.11.7 Arrangement of reinforcement

IStructE RC permissible stress recommendations

15 45 45 46 46 46 46 46 46 47 47 47 49 49 50 50 50 50 51 52 52 52 53 53 53 53 54 54 55 55 55 55 57

57 58 58 58 58 58 58 60 60 60 60 60 60. 61 61 5

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3C. 12 Emperical design of flat slabs 3C. 12.1 General 3C. 12.2 Applicability of method 3C.13 Critical sections for bending moments in flat slabs 3C. 14 Bending moments in flat slab panels 3C. 15 Widths of reinforcing bands 3C. 16 Arrangement of reinforcement in flat slabs 3C. 17 Bending moments in columns

3D Stairs 3D. 1 Distribution of loading on stairs 3D.2 Effective span of stairs

3E Columns 3E. 1 Reinforcement in columns 3E. 1.1 Longitudinalreinforcement 3E.1.2 Transverse reinforcement 3E.2 Permissible loads on columns 3E.2.1 Axially loaded columns 3E.2.2 Columnssubject to both direct load and bending 3E.2.3 Elastic theory 3E.2.4 Overturning 3E.2.5 Reduction coefficients for columns 3E.2.6 Bending moments in columns

3F Reinforced concrete walls 3F.1 3F.2 3F.3 3F.4

General Permissible loads Walls subjected to concentrated loads Shear stresses

3G Bases and pile caps 3G.1 Bases for reinforced columns and walls 3G.l.l Bendingmomentsin bases 3G.1.2 Reinforcement in bases 3G.1.3 Shear 3G.1.4 Bond in reinforcement of bases 3G. 1.5 Pockets for precast members 3G.1.6 Massconcrete bases 3G.2 Pile caps

3H Reinforced lightweight aggregate concrete 3H.1 3H.2 3H.3 3H.4 3H.5 3H.6 3H.7 6

General Permissible stresses in reinforced lightweight aggregate concrete Deflection and stiffness Permissible loads on columns Reinforced concrete walls Modular ratio Cover

61 61 61 62 62 62 62 64

64 64 64 66 66 66 66 67 67 67 69 69 70 71 72 72 72 73 73 73 73 73 73 73 73 74 74 75 75 75 75 76 76 76 76 76

IStructE RC permissible stress recommendations

35 Durability and resistance to chemical attack 35.1 Durability 35.1.1 General 35.1.2 Drainage 35.1.3 Waterlcement ratio 33.1.4 Chloride content of mixes 35.1.5 Admixtures 35.1.6 Air entrainment 35.1.7 Requirementsfordurabilityofconcrete 35.1.8 Reactive aggregates-alkali-silica reaction 33.1.9 Finishingand curing 35.2 Resistance to chemical attack

3K Resistance to fire 3K.1 General 3K.2 Robustness 3K.3 Elementsexposcd to fire 3K.4 Aggregates 3K.5 Average cover to main bars 3K.6 Cover to secondary bars 3K.7 Contribution tocover of additional protection 3K.8 Floor thickness 3K.9 Beam width 3K. 10 Beams and ribs 3K. 11 Continuity 3K. 12 Use of tabular data (Table 25)

3L Stability and disproportionate collapse 3L. 1 Stability 3L. 1.1 General 3L.1.2 Planform 3L.1.3 Vehicle impact 3L.1.4 Accidental loads 3L. 1.5 Exceptions 3L.2 Ties 3L.3 Acceptable limitsofdamage 3L.4 Loads 3L.5 Key elements 3L.6 Stresses

4 Precast and composite construction 4.1 Gcneral 4.2 Detailing 4.2.1 Handling stresses 4.2.2 Conncctions 4.2.3 Anchorage at supports 4.3 Stability 4.4 Framed structures and continuous beams 4.5 Design of slabs IStructE RC pcrmissiblc strcss rccommcndations

77 77 77 77 77 77 77 77 78 79 79 79 80 80 80 80 80 82 82 82 82 82 82 85 85 85 85 85 85 86 86 86 86 87 88 88 88 89 89 89 89 89 89 89 90 90 7

4.5.1 Wide unitsorseriesofjointed narrow units 4.5.2 Conccntratcd loads on slabs without rcinforccd topping 4.5.3 Conccntratcd loadson slabswith reinforced topping 4.5.4 Slabs carrying conccntratcd loads 4.6 Bearings for precast mcmbcrs 4.6.1 General 4.6.2 Net bcaringwidth 4.6.3 Effective bcaringlcngth 4.6.4 Pcrmissiblc bcaring strcss 4.6.5 Allowances for spalling at supports 4.6.6 Allowances for construction inaccuracics 4.7 Bearings transmitting comprcssive forccs from abovc 4.8 Horizontal forces at bcarings 4.9 Rotation of bearingsof flcxural mcmbcrs 4.10 Concrete corbcls 4.1 I Continuousconcrete nibs 4.12 Conncctions between prccast units 4.13 Site information 4.14 Continuity of reinforcement 4.14. I LOOPS 4.14.2 Slccves 4.14.3 Thrcadingof rcinforccmcnt 4.14.4 Welding 4.15 Otherconncctions 4. 15. 1 Jointswith structural steel inscrts 4.15.2 Resin adhesives 4.15.3 Compressivc joints 4.16 Joints transmittingshear 4.17 Composite construction 4.17.1 Gcncral 4.17.2 Horizontal shearstresscs 4.17.3 Nominal links 4.17.4 Designed links 4.17.5 Vertical shear I

5 Workmanship 5.1 General 5.1.1 Concrete quality 5.1.2 Transportation 5.1.3 Placing 5.1.4 Curing 5.1.5 Concreting in cold weather 5. I .6 Concreting in hot weather and drying winds 5.2 Reinforcement 5.2.1 Specification 5.2.2 Cuttingand bending 5.2.3 Fixing 5.2.4 Surface condition 5.2.5 Welding 5.2.6 Mechanical splices

90 90 91 YI 91 91 91 YI 91 Y3 93 93 94 94 94 94 95

95 Y5 95 95 06 96 96 Y6

96 Y6 Y6 97 97

98 98 YY YY

100 LOO 100 100 100 100 101 101 10 I 101 10I 101 102 102 102

IStructE RC pcrrnissiblc strcss rccommcndations

5.3 Formwork 5.3.1 Design and construction 5.3.2 Cleaning and treatment of forms 5.3.3 Strikingof formwork 5.3.4 Camber 5.3.5 Tolerances

102 102 102 102 103 103

6 Testing and inspection 6.1 Methodsof testing concrete 6.2 Rate of strength testing 6.3 Inspection 6.4 Load testing of structures or parts of structure 6.4.1 6.4.2 6.4.3 6.4.4 Assessment of results 6.4.5 Test criteria

104 104 104 104 104 104 104 104 105 105

7 Protection, maintenance and repair

106 106 106 106 106

7.1 7.2 7.3 7.4

General Protection Maintenance Repair

Appendix A Nominal concrete mixes

107

Appendix B Column design charts

109

Technical notes

122

Index

123

IStructE RC permissible stress recommendations

9

I , ,

Foreword Limit-state design became enshrined in British Standards with the publication in 1Y72 of CP 110: The structural use of concrete. However since that time a significant numbcr of engineers have expressed a wish to maintain permissible stress methods. In the spring of 1987 the Institution held a referendum which posed the question ‘Should permissible stress codes such as CP 114 and BS 445, be updated and made available for design purposes, in addition to partial factor codes such as BS 8110 and BS SYSO?’ The corporate membership voted in favour of retention by 2366 votes to 650. Subscquently a Task Group was formed to ‘draft a Type T1 Design Code for permissible stress design of reinforced concrete structures’. We were fortunate in having available a base document popularly known as the ‘red book’, a revised version of CP I14 prepared and circulated by the Campaign for Practical Codes of Practice. We wcrc also able to gain advantage from the considerable volume of public comment concerning the red book. Partly as a result of these comments we have added sections on torsion and precast and composite construction. The Task Group is confident that designs made using these recommendations will either match with or be slightly more conservative than for those using BS 8110. I t is o u r hope that we have produced simple rules for those who feel more comfortable using permissible strcss methods. Our efforts have received support from the London District Surveyors Association and other bodies who find the method acceptable. May I thank all members of the Task Group for their professionalism and tireless endeavour; independent engineers who have been constructive in their technical comments; to all who responded during the period for public comment -their work has helped enormously in shaping our final document. Finally, may I pay tribute to our Secretary Robert Milne w h o has, with patience and good humour, kept us on the road to our final goal.

DAVID DORAN Chairman of Task Group

IStructE RC pcrmissiblc strcss rccommcndations

11

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1 General 1.1 Scope This manual gives practical recommendations for the design of reinforced concrete building structures using working loads and working stresses, and is a comprehensive revision of CP 114: 1969 The structural use of reinforced concrete in buildings extended to incorporate precast and composite construction. The recommendations do not cover prestressed concrete or reinforced concrete construction of a specialized character such as water-retaining structures. Nor do they cover the structural use of concrete made with high-alumina cement. The recommendations are based on the assumption that the design of reinforced concrete is entrusted to chartered structural or civil engineers, for whose guidance the documcnt has been prepared and that the execution of the reinforced concrete work is carried out under the direction of a qualified supervisor. Where other structural materials such as masonry, steel, timber, etc., are used in conjunction with reinforced concrete, reference should be made to design Codes covering their use and particularly in relation to safeguards against disproportionate collapse. The recornmendations given in this manual are intended to apply to all normal reinforced concrete building structures without limitation of size, although for special or complex structures reference to specialist literature or other design guidance may be appropriate.

1.2 Definitions Bedded bearing Bearing length

Bearing width Braced structure

Bundle of bars Characteristic strength

Column head

Core of helically reinforced column

Drop

A bearing with contact surfaces having an intermediate padding of cementitious material The length of support, supported member or intermediate padding material (whichever is least) measured along the line of support The overlap of support and supported member measured at right-angles to the line of support Structure where resistance to lateral forces is provided by cross-bracing, shear walls or other such elements A group of not more than four bars in contact The characteristic cube strength of concrete is defined in BS 5328 The characteristic strength of reinforcement is defined in BS 4449 An enlargement of the top of a column supporting a flat slab so designed and constructed as to act monolithically with both the column and the flat slab The portion of the concrete enclosed within the centre-line of the helical reinforcement The portion of a flat slab above and immediately surrounding the column head and of greater thickness than the remainder of the flat slab panel

IStructE RC pcrmissiblc stress rccornmcndations

Previous page is blank

13

. Dry bearing Effective depth of a beam or slab Effective diameter Effective span

Exterior panel in a flat slab Flat slab

Net bearing width

Simple bearing

Wall

.

A bearing with no intermediate padding material The distance between the centre of tension and the edge of the compression section The diameter of a circle having an area equal to that of a bar or bundle of bars The lesser of the two following distances: the distance between centres of bearings the clear distance between supports plus the effective depth of the beam or slab A panel having at least one edge that .is noncontinuous A reinforced concrete slab with or without drops, supported, generally without beams, by columns with or without flared column heads For a simple bearing, the bearing width after deduction of allowances for ineffective bearing and constructional inaccuracies A supported member bearing directly on a support, the effect of projecting steel or added concrete being discounted A vertical loadbearing member whose length exceeds four times its thickness

1.3 Svmbols Ab

I

14

equivalent area of helical reinforcement (volume of helix per unit length of column) gross cross-sectional area of concrete, excluding any finishing material gross cross-sectional area of concrete core area of longitudinal reinforcement cross-sectional area of bent-up bars cross-sectional area of steel in compression cross-sectional area of steel in tension cross-sectional area of link distance from nearest support to the section under consideration distance from face of support to nearest edge of principal load breadth of a rectangular beam or breadth of flange of a T- or L-beam effective moment transfer strip from a flat slab to an edge column breadth of the rib of a T- or L-beam torsional constant column width diameter generally effective depth to the tensile reinforcement in a beam depth to the compressive reinforcement in a beam depth of concrete in compression in a beam depth to neutral axis depth of slab forming the flange of a T- or L-beam modulus of elasticity IStructE RC pcrmissiblc strcss rccommcndations

I

eccentricity of a load on a column additional eccentricity of load on column due to buckling effects P,~A,, or the anchorage value of the reinforcement, whichever is less characteristic strength of concrete stress in steel layer farthest from the compression face stress in steel layer nearest to the compression face stress in steel reinforcement characteristic strength of reinforcement shear modulus overall depth larger dimension of a rectangular section smaller dimension of a rectangular section stiffness of beam stiffness of beam on one side of a column stiffness of beam on the opposite side of a column stiffness of lower column stiffness of upper column reduction coefficient for columns coefficient depending on the ratio h,,,,/h,,,, length of a column or beam between centres of support (in flat slabs) length of panel in the direction of span (in flat slabs) width of panel at right-angles to direction of span limiting dimension for assessing tie forces in a floor to resist the effects of accidental loading average of L I and L2 effective span of beam or slab, or effective height of column lever arm of the resistance moment length of shorter side of slab spanning in two directions length of longer side of slab spanning in two directions bending moment (suffixes as required) bending moment at end of beam framing into a column, assuming fixity at both ends of the beam the maximum difference between the moments at the ends of two beams framing into opposite sides of a column, each calculated on the assumption that the ends of the beams are fixed and that one of the beams is not loaded total panel moment in a flat slab moment of resistance of a section to bending moment transferred to a column from a flat slab maximum bending moments, for spans I, and ly, respectively, on strips of unit width in slabs spanning in two directions moment capacity of a column for uniaxial bending about x- and y-axes, respectively modular ratio sum of perimeters of the bars in the tensile reinforcement direct load on a column column section capacity for eccentric load column section capacity for axial load permissible stress in concrete in average bond IStructE RC pcrmissiblc strcss rccommcndations

1s

.. permissible stress in concrete in average bond permissible stress in concrete in direct compression permissible compressive stress in concrete in bending permissible compressive stress in the reinforcement permissible tensile stress in the reinforcement permissible stress in concrete in shear spacing or pitch of links torsional moment clearance for grouting in pocket base for preeast column total shear across a section shear stress at a section of a beam or slab torsional shear stress total load o n beam or slab dead load imposed load total load per unit area of slab or per unit length of beam length of side of shear perimeter in a flat slab smaller centre-to-centre dimension of a rectangular link larger centre-to-centre dimension of a rectangular link angle of internal friction between the faces of a joint

Ph PCC

Pch

Psc

Pat Pv S

T t V V VI

W W'l Wi W X

XI

Yl a x LQ

ay Px

P, Y

I

16

=

bending moment coefficients for the short and long spans, respectively, for slabs spanning in two directions and simply supported on four sides

=

bending moment coefficients for the short and long spans, respectively, for rectangular panels supported on four sides and with provision for torsion at corners factor for computing moment of resistance

LQ

=

IStructE RC permissiblc stress rccommcndations

2 Materials 2.1 Cements, round ranulated blastfurnace slags and

pulverizef-fuel a&es

The cement, or combinations of cement with ggbfs or pfa, to be used should comply with the following: (i) Portland cement BS 12: Specification for ordinary and rapid hardening Portland cement BS 1370: Specification for low heat Portland cement BS 4027: Specification for sulphate-resisting Portland cement (ii) Cements containing ggbfs or pfa These cements are factory-produced intimate mixtures mainly of Portland cement clinker and either ground granulated blastfurnace slag or pfa. The proportion of ggbfs or pfa is given in brackets. BS 146: Portland-blastfitmace cement (not more than 65%) BS 4246: Low heat Portland-blastfurnace cement (between 50% and 90%) BS 6588: Portland pulverized-fuel ash cement (between 15% and 35%) (iii) Other cements BS 4248: Supersulphated cement (iv) Combinations of cements and ggbfs or pfa Combinations of Portland cement, generally to BS 12, with ggbfs or pfa to the appropriate British Standard may be included as part of the mix by simultaneously combining them with the other concrete materials at the concretc mixer. BS 3892: Pulverized-fuel ash, Part 1: Specification for pulverized-fuel ash for use as a cementitious component in structural concrete BS 6699: Ground granulated blastfurnace slag (v) High-alirmina cement These recommendations do not cover the use of high-alumina cement concrete

2.2 Aggregates Aggregates should comply with BS 882 for coarse and fine aggregates from natural sources or with BS 877, BS 1047 or BS 3797 as appropriate for lightweight aggregates.

2.3 Reinforcement Reinforcement should be designated by its type and effective diameter. Characteristic or yield strengths are given in BS 4449 and 4483 as 250 N/mm’ for mild steel and 460 N/mm’ for high-tensile steel.

2.4 Admixtures 2.4.1 General Admixtures may be added to concrete mixes to improve the properties of the concrete. While improving certain properties, an admixture can significantly affect others, and it is therefore important to know all the effects of any admixture and to IStructE RC permissible stress recommendations

17

.---. -

-

-

_--. ___.,

. . _ I _ I -

use the material in strict accordance with the manufacturer’s instructions. Two or more admixtures may not be compatible, and they should be combined only if tests or other suitable investigations prove satisfactory. The behaviour of admixtures with composite or supersulphated cements should be carefully investigated before use. The suitability and effectiveness of any admixture should be verified by trial mixes with the cements, aggregates and other materials to be used in the works.

2.4.2 Types The main types of admixture are: (i) water reducing, often referred to as ‘plasticizers’ which may be used to increase workability or, by reducing the water content of the mix, to increase its strength (ii) accelerating, .which increase the rate of gain of early strength (iii) retarding, which slow the setting time and thus extend the workability phase (iv) air-entraining, which introduce very small air bubbles into the concrete, improving workability and resistance to frost action (v) superplasticizing, which have highly efficient water-reducing effects (vi) waterproofing, which reduce the permeability of concrete to water and dampness (vii) pigments, which change the colour of concrete.

2.4.3 Specification Admixtures should comply with the following British Standards, where applicable: BS 1014: Pigments for Portland cement and Portland cement products BS 5015: Concrete admixtures Part 1 : Accelerating admixtures, retarding admixtures and water reducing udmixtures

Part 2 : Air entraining admixtures.

2.5 Concrete Concrete should be designated by a grade number (with a prefix C) for which appropriate design stresses are given in these recommendations. All concrete designed and prescribed mixes should be specified in accordance with BS 5328 by reference to the grade of concrete and type of mix. Guidance on specifications and forms for specifying different types of concrete are given in BS 5328. One of three types of mix should be selected:

2.5.1 Designed mix Designed mixes are intended for general use and arc designated by a grade number with the suffix D. The recommendations of this document apply to the following designed mixes containing normal-weight aggregates: C20D, C25D, C30D, C35D, C40D, C45D, C50D, C55D and C60D. For lightweight aggregate concrete an additional grade, C15D, may be used. The supplier may choose from the various types of cement and aggregates listed in BS 5328 unless the specifier wishes to restrict the choice. The specifier should also state: (i) the nominal maximum size of aggregate (ii) the minimum cement content (iii) the rate of sampling. 18

IStructE RC pcrrnissiblc strcss rccornmcndations

In addition the specifier may statc requirements for any or all of the following: (iv) workability (v) maximum free watedcement ratio (vi) maximum cement content (vii) special cements that should or may be used (viii) special requirements for aggregates (ix) type(s) of admixture specified and the quantity required (x) type(s) of admixture prohibited (xi) the air content of fresh concrete (xi) the maximum and/or minimum temperature of fresh concrete (xiii) the maximum and/or minimum density of fresh concrete (xiv) details of any required trial mixcs (XV)requircrnents for assessment and/or compliance (xvi) any other requirements.

2.5.2 Special prescribed mix Special prescribed mixes are intended for use in place of designed mixes when the specificr wishcs to stipulate the proportions of the constituent materials; they are designated by a grade number with suffix SP. The recommendations of this document apply to the same range of grades, from C15 to C60 as for designed mixes described in clause 2.5.1. The specification for special prescribed mixes is also similar to the specification for designed mixes, and the specifier should, or may, state his requirements as listed in items (i) to (xvi) in clause 2.5.1 and, in addition, the required mix proportions.

2.5.3 Ordinary prescribed mix Ordinary prescribed mixes, also known as standard mixes, are intendcd for use where small quantities of concrete are required or where the specifier wishes to restrict the concrete to a limited range of commonly used materials and mix proportions; they are designated by a grade number with the suffix P. The recommendations of this document apply only to the following ordinary prescribed mixes containing normal-weight aggregates:

C20P, C25P and C30P. The supplier may choose from the limited range of cements and aggregates listed in BS 5328 unless the specifier wishes to restrict the choice. The specifier should also statc: (i) thc nominal maximum size of aggregate (ii) thc workability. Admixtures or other special requirements should not be specified for inclusion in ordinary prescribed mixes. If these are required, the mix should be specified as a special prcscribed mix.

2.6 Nominal mixes Where nominal mixes specified by volume proportions are used, requirements are given in Appendix A. Nominal mixes are intended for use where small quantities of concrete are required and where standards of quality control necessary to achievc designed mixes are impracticable. IStructE RC pcrmissiblc strcss rccommcndations

19

3 Design considerations 3A GENERAL 3A.1 Basis of design 3 A . l . l Method of design The method of design should accord with the laws of mechanics and the general principles relating to the design of reinforced concrete. I t may be assumed that: (i) at any cross-section plane sections remain plane, and (ii) all tensile stresses are taken by the reinforcement except that the concrete may be assumed to resist shear within the limits of stress specified. For concrete made with lightweight aggregates, special recommendations are given in section 3H.

3A.1.2 Moments of inertia For the purpose of calculating bending moments in continuous structures, the moment of inertia may be estimated by considering; (i) the entire concrete section, ignoring the reinforcement, or (ii) the entire concrete section, including the reinforcement on the basis of the modular ratio, or (iii) the compression area of the concrete section combincd with the reinforcement on the basis of the modular ratio. Whichever method is adopted for the beams, the same method should be used for the columns, and care should be taken that the method adopted is appropriate to the particular circumstances.

3A.2 General stability The structure should be designed to support loads arising from its normal function, and there should be a reasonable probability that disproportionate collapse would not follow misuse or accident. In addition, because of the nature of a particular occupancy or use of a structure (e.g. flour mill, chemical plant, etc.), it may be necessary in the design concept or a design reappraisal to consider the effect of a particular hazard so that, in the event of an accident, there is a reasonable probability of the structure remaining after the event, even in a damaged condition. The recommendations given in subsection 3L on tying the structure together, and on the planform of the building, aim at enabling the structure to accommodate a limited amount of accidental loading that may occur as a result of causes such as construction loading, differential settlement of the supports, thermal movements, explosions, accidental impact, etc. which are not defined as normal loading. These accidental loadings may produce local damage, but the recommendations have as their objective the limitation of the extent of such damage. Irrespective of the design wind load, all structures should be capable of resisting a horizontal force at any floor level equal to not less than 1.25% of the total dead load above that level at stresses not exceeding those given in subsection 3A.8.

I

20

IStructE RC permissible stress recommendations

.

I

~

I.

,

-

.,. ' , ... * . .

. I , _ , . . . .

... .. .

.

.-

.

... .

. ,. . .,

,

3A.3 Complete structures and analysis of structural frames 3A.3.1 General The analysis of a structure should be carried out to determine a set of internal forces and bending moments in equilibrium with the design loads applied to the structure. The method of analysis should be based on as accurate a representation of the behaviour of the structure as is reasonably in accordance with the recommendations of section 3A. Alternatively, the methods below may be adopted if appropriate.

3A.3.2 Frames supporting vertical loads only In a braced frame where the individual beams and columns and their connections are not intended to resist lateral loads, the moments loads and shear forces may be determined from an elastic analysis of a series of subframes. Mcmber stiffness should be determined in accordance with clause 3A. 1.2. Each subframe should be taken to consist of beams at one level, together with columns above and below. The extreme ends of columns should be assumed fixed unless actual constructional details make the assumption of pinned ends more reasonable. Alternatively in determining moments and forces in an individual beam, a simplified subframe consisting of that beam, the columns at each end and the beams on each side of those columns may be used. The ends of the columns and beams remote from the beam under consideration should be assumed to be fixed unless the assumption of a pinned end is more reasonable. The beams either side should be assumed to possess half their actual stiffness, if they are taken to be fixed at their outer ends. All critical combinations of minimum and maximum loading on individual spans should be considered to determine the maximum possible design moments, loads and shear forces in the subframe members (see subsection 38.4).

3A.3.3 Frames supporting vertical and lateral loads In an unbraced frame where resistance to lateral loads and overall stability are provided for by the frame itself, it will be necessary to consider the effects of lateral loads and sway. The design of individual beams and columns should be based on the moments, forces and shears obtained by considering vertical loading only as in clause 3A.3.2 or, if more severe, from the sum of the moments and forces obtained by separate analyses for vertical and lateral loads allowing increased permissible stresses in accordance with subsection 3A.8. For lateral loading, the moments, forces and shears may be determined from an elastic analysis of the complete frame assuming points of contraflexure at the centre of all beams and columns.

3A.3.4 Stress reversal In certain cases such as structures with significantly disproportionate spans or with short spans adjacent to cantilevers, critical loading arrangements may cause a tendency towards stress reversal in particular members (see subsection 3A.4).

3A.4 Overturning stability The stability of the structure as a whole or of any part of it should be investigated, and weight or anchorage should be provided so that the least restoring moment, including anchorage, should be not less than the sum of 1.4 times the maximum overturning moment due to dead and wind loads and 1.6 times the maximum IStructE RC pcrmissiblc stress rccommcridations

21

overturning moment due to imposed loads. To check stability at all times account should be taken of probable variations in dead load during construction, repair or other temporary conditions. In complying with the requirements of this clause it is necessary to check that the resulting pressures and shear forces to be transmitted by the foundations to the supporting soil would not produce failure.

3A.5 Loadings The loadings should be in accordance with BS 6399 and CP 3: Chapter V, as appropriate. For the purpose of calculating dead loading, the weights of materials should, unless otherwise agreed, be taken to be as in BS 648: Schedule of weights of building materials. For ordinary construction the density of reinforced concrete may be taken as 2400 kg/m3, but where the amount of steel exceeds 2%, some greater weight may be more appropriate. Where lightweight aggregates are used a smaller appropriate weight may be taken. Guidance on the distribution of concentrated loads on slabs and on the distribution of loads on stairs is given in subsections 3B.7 and 3D.1, respectively.

3A.6 Permissible stresses in concrete 3A.6.1 Compressive and bond stresses and elastic modulus The compressive and bond stresses in reinforced concrete should not exceed those shown in Table 1 as appropriate for each grade of concrete. Values of short-term elastic modulus are given in Table 1. Where an ap roximate value is required for analysis of forces and moments a value of 14 kN/mm (rn - 15) may be taken.

P -

3A.6.2 Compressive stress Good agreement with test results is obtained if a uniform compressive stress of % of the cube strength is assumed. For details of the stress block when bending is present see subsection 3A.9. Since concrete in a structure is not placed or cured in laboratory conditions the design concrete strength is assumed to be % of the strength of test cubes. Adopting a factor of safety of 1.8, the permissible compressive stress is therefore pcc = (% =

X

%feu) + 1.8

0.275fc,

3A.6.3 Modular ratio method As an altenative to the above, stresses may be assessed by the modular ratio theory. This assumes that steel and concrete are elastic within the range of permissible stresses. The modular ratio, m, may be taken as 15 for normal weight concrete and 30 for lightweight aggregate concrete. The permissible extreme fibre stress in bending, pch, should not exceed 1 . 3 3 ~ ~ ~ . 22

IStructE RC permissible stress rccommcndations

Table 1 Basic permissible concrete stresses and moduli of elasticity grade of concrete

I basic permissible concrete stress, N/mm2 compression

fcu

15 20 25 30 35 40 45 50 55

60

PCC

'

4. 1 5.5 6.9 8.2 9.6 11.0 12.4 13.7 15.1 16.5

average bond(3)* Ph

mean E-values (short term 4 kN/mm()z )

1.3 1.5 1.7 1.8 2.0 2.1 2.2 2.4 2.5 2.6

23 24 25 26 27 28 29 30 31 32

* The hasic stresses for hond relate to type 2 deformed hars in tension. Notes: I . Grade IS may he used only for lightweight aggregate concrete. 2 . For normal huilding structures grades 4.5 and ahove are rarely used. 3. For type I deformed hars the wlues are Xll% o f those quoted. ' For plain hars they are SS% of those quoted. In a heam where nominal links (see suhsection 38.10) have not heen provided, the hond stress should he taken as that for plain hars. irrespective o f the type o f hlir used. For fahric to BS 44x3 the permissihle hond stress i s 1.3 x the value o f average hond stress given in Tahle I . provided that: the fahric is welded in a shear resistant manner complying with 8s 44x3. and (I,) the n.umher of welded intersections within the anchorage length is at least equal to a value of 4 x area o f steel requiredlarea o f steel provided. (U)

When condition (h) is not satisfied. the anchorage hond should he taken as that appropriate to the individual wires or hars i n the sheet. 4. The quoted short-term elastic moduli are average values. For long-term loads, creep effects will produce higher deformations. so that the total can he 2 to 4 times the short-term value. Where this i s critical. specialist literature on the suhject should he consulted.

For modifications of permissible stresse.s due to wind forces see subsection 3A.8. The derivations of the basic permissible concrete stresses in Table 1 are given in the clauses 3A.6.2 to 3A.6.5.

3A.6.4 Shear stress Adopting a factor of safety of 1.8, the permissible shear stress pvshould not exceed

I

Equation 2 may be used for values of fCu between 20 and 40 N/mm'. For normal weight aggregate concrete,fCushould not be taken as greater than 40 N/mm'. Where d exceeds 400mm, the term .\1/(400/d) should be taken as unity. Subsection 3B.10 gives values of p,. for various concrete strengths, effective depths and reinforcement proportions (see subsection 3H.2 for lightweight aggregate concrete). IStructE KC pcrmissiblc stress recomrncndations

23

3A.6.5 Bond stress For type 2 deformed bars in tension, the permissible average bond stress, ph, should not exceed Pb

= 0.7 d%fc- f 1.8 = 0.33 dfcu

(3)

3A.7 Permissible stresses in reinforcement 3A.7.1 General The tensile and compressive stresses in steel reinforcement should not exceed those shown as appropriate for the.type of reinforcement and its characteristic strength, f,.

3A.7.2 Tensile stress The values given in Table 2 are based on a factor of safety of 1.8 giving permissible tensile stresses, psi, which should not exceed pSl= f,/1.8 = 0.55fy

(4) High levels of tensile stress may lead to undesirable cracking, and lower limits than those given in Table 2 may need to be adopted in circumstances of exposure to corrosive environments.

3A.7.3 Compressive stress Table 2 gives permissible compressive stresses, psc,which should not exceed P ~ C

=

0.85 psi

(5)

The factor of 0.85 takes account of the concrete displaced by the reinforcement together with the buckling effect, assuming the reinforcement to be a fixed-ended strut restrained by links spaced at 12 times the bar diameter.

3A.7.4 Tensile stress due to shear The values of permissible tensile stresses in shear are the same as for permissible tensile stresses in bending or tension. Table 2 Basic permissible steel stresses basic permissible steel stress, N/mm2 high-tensile steel to BS 4483 mild steel to BS 4449

I

type of stress tensile, ps1 compressive, psc

I

I

140

120

I

I

250 215

3A.8 Increases of permissible stresses due solely to wind forces The permissible stresses in concrete and in the reinforcement may exceed those given in subsections 3A.6 and 3A.7, respectively, by not more than 25% provided that: (i) such excess is solely due to stresses induced by wind loading (ii) in no case does the stress in the reinforcement exceed 300 N/mm2. 24

IStructE RC permissible stress rccommendations

3A.9 Calculation of resistance moments of beams and slabs 3A.9.1 Basis of method Thc basic requirement of this method is a suitable load factor (i.e. the ratio of the ultimate strength of the beam or slab to its working load). As the failure load is approached, the stressktrain relationship of the concrete becomes non-linear, and may be assumed to follow the short-term stresslstrain curve of Fig. 1. Tests have shown that, at failure, a uniform rectangular stress block may be taken extending from the compression face over 90% of the depth to the neutral axis, provided that the depth to the neutral axis does not exceed half the effective depth of the beam or slab.

Stress

I Short-term stresslstrain relation for concrete A load factor of 1.8 should be provided, assuming an adjusted value of of the characteristic strength predicted by cube tests to allow for differences in curing and placing conditions, giving the stress diagram in Fig. 2. The resistance moment may be calculated on the following assumptions:

(i) The stress in the tensile reinforcement does not exceed the permissible stress, ps,,appropriate to the type of reinforcement, given in subsection 3A.7. (ii) The compressive stress in the concrete is equal to the permissible concrete stress in compression, pcc,given in subsection 3A.6, assumed to be uniform over 90% of the depth to the neutral axis. The depth to the neutral axis IStructE RC permissible stress recommendations

25

should not be taken as greater than 0.5 X the effective depth, and the lever arm should not exceed 0.95 X the effective depth. (iii) The stress in the compressive reinforcement does not exceed the permissible stress, psc, appropriate to the type of reinforcement, given in subsection 3A.7, nor does it exceed: 375

(1

d’

-

-

) N/mm2

where d’ is the depth to the compressive reinforcement and d, is the depth to the neutral axis*

land-df

2

Stress

diagram

3A.9.2 Formulae for rectangular beam and slab sections For beams and solid slabs of rectangular cross-section without compressive reinforcement and for qualities of concrete and steel within the range permitted by these recommendations, these requirements may be deemed to be satisfied if the resistance moment, M, (corresponding to working loads) is assumed to be the lesser of the two values calculated from equations (6) and (7) as follows:

* At a failure of beam or slab, the maximum compressive strain in the concrete has been shown by tests to be such that a reinforcing bar at the surface of the beam would develop a stress of 700 Nlmm’ with the steel of the requisite yield or proof stress. With a load factor of 1.8, the limiting stress to he used in design is thus 385 Nlrnrn’ at the compressive surface. Since tests show that the strain is roughly linear across the section, the limiting cornpressive stress at the depth d‘ is therefore 385(1 - d’/d.). With an allowance for displaced concrete this becomes 375(1 - d’ld.). 26

IStructE RC permissiblc stress rccommcndations

Based on tensile reinforcement

Mr = AstPstla Based on the strength of concrete in compression M, = pcc x b x 0.45d x 0.775d (see Fig. 2) = 0.35pc,bd2

where A,, is the area of tensile reinforcement pst is the permissible tensile stress in the reinforcement pcc is the permissible compressive stress in the concrete b is the breadth of the section d is the effective depth to the tensile reinforcement 1, is the lever arm. For convenience, values of lever-arm and neutral-axis-depth factors are tabulated for various values of Mlpccbd2in Table 3 (see also Fig. 3). Table 3 Lever-arm and neutral-axis-depth factors

I,ld

I I

d,ld

I

Mlpccbd2

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.95

0.95

0.92

0.89

0.85

0.82

0.77

0.11

0.11

0.18

0.25

0.33

0.41

0.50

0.95

0.90

L

0

c

s

U-

! 0.85 0 L

w

0

2

0 80

0.77 '

0

2

1

'/bd2,

3

4

Nlmm2

3 Lever-arm factors IStructE RC permissible stress recommendations

27

-

Where it is necessary for M to exceed O.35pCcbd2,compression reinforcement should be provided so that M = 0.35pccbd2+ A,g,,(d - d ’ )

(8)

where A,, is the area of compression reinforcement pbCis the permissible compressive stress in the steel as given in assumption (iii) of clause 3A.9.1, and the area of tension reinforcement should be such that the stress in this steel does not cxceed thc permissible stress.

3A.9.3 Formulae for T- or L-beams For T- or L-beams with a breadth of flange b , a rib width b , and a depth of slab forming the flange d,, the resistance moment when compressive reinforcement is not provided may be assumed to be the lesser of the two values given by equations (9) and (10) as follows: Based on the tensile reinforcement

Based on the strength of the concretc in compression, M r = ypccbd’

(10)

where the factor y has the values given in Table 4.

values of y for d,ld hrlh

20.5

0.3

0.2

0.1

0

1 .o

0.35 0.35 0.35 0.35 0.35 0.35

0.35 0.30 0.28 0.27 0.26 0.25

0.35 0.26 0.23 0.21 0.20 0.18

0.35 0.22 0.17 0. I5 0.12 0.10

0.35 0.18 0.11 0.07 0.04 0

0.5 0.3 0.2 0.1 0

* For intermediate values of b,lb and d,ld the value of y can be calculated from the following formula y = O . 3 5 -br+ b

1 (I---)(:) br

(2-+)

2

Where it is necessary for the resistance moment to exceed ypccbd2,compressive reinforcement should be provided so that Mr = Ypccbd’

+ Asgsc(d - d ’ )

(12)

IStructE RC permissible stress recommendations

3A.10 Concrete cover In addition to durability and fire-protection requirements (see subsections 35 and 3K) the following structural requirements apply: (i) for reinforcement in a slab, not less than 15 m m nor less than the diameter of such reinforcement (ii) for longitudinal reinforcement in a beam, not less than 25 mm nor less than the diameter of such reinforcement (iii) for a longitudinal reinforcing bar in a column, not less than 40 mm nor less than the diameter of such a bar. In the case of columns with a minimum dimension of 200 mm or less whose bars do not exceed 12 mm diameter, 25 mm cover may be used. For bar bundles, the cover should be taken as not less than the diameter of a single bar of equivalent area or 50 mm, whichever is less. Requirements for cover are exclusive of plaster or other decorative finishes, and where surface treatment such as bush hammering cuts into the face of the concrete, the expected depth of treatment should be added to the specified cover.

3A.11 Distance between bars The horizontal distance between two parallel steel bars or bundles of bars should not be less than the maximum size of coarse aggregate plus 5 mm, or the bar size, whichever is greater. Where there are two or more rows of bars or bundles of bars, they should be arranged vertically above each other and the vertical spacing between rows should not be less than the bar size or two-thirds of the maximum size of coarse aggregate, whichever is greater. In detailing, the spacing of the reinforcement should be carefully considered in relation to the ease of compaction of the concrete, and a space of not less than 75 m m betwcen bars or groups of bars should normally be provided to enable a poker vibrator to be inserted at appropriate intervals, unless other means of compaction are specified and agreed. In beams the clear distance between bars near the tension face should not exceed 185 m m = 250 N/mm’) or 300 mm U,, < 155 N/mm’), wheref,, is the steel tensile stress; limits for intermediate stresses may be interpolated. Where the design momcnt has been reduced by redistribution as permitted in subsection 3B.3, the bar spacing should be reduced proportionally. The clear distance between the corner of a beam and the nearest longitudinal bar in tension should not exceed half the above values. Reinforcement should be placed within the side faces of deeper beams if appropriate. For slabs up to 200 m m thick, the pitch of the main bars should not exceed three times the effective depth, that of distribution bars should not exceed five times the effective depth and neither should exceed 750 mm. These limits also apply to thicker slabs where AJbd is less than 0.3%; where AJhd is greater than 1%, the recommendations for beams apply, and for intermediate reinforcement percentages the limits may be interpolated.

3A.12 Bond and anchorage 3A.12.1 Bars in tension A bar in tension should extend from any section for a distance to the end of the bar IStructE RC pcrmissiblc strcss rccommcndations

29

such that the average bond stress does not exceed the permissible average bond stress in Table 1. This condition will be satisfied if the length measured from such section is not less than: the bar diameter

the tensile stress in the bar X

4 x the permissible average bond stress The bar should extend at least 12 bar diameters (or the effective dcpth if this is greater) beyond the point at which it is no longer required to resist stress. For the purpose of this subsection the length of bar so determined may have deducted from it a length equivalent to the value of the hook as givcn in clause 3A.12.7, but no deduction should then be made for the length of bar contained in the hook.

3A.12.2 Bars in compression A bar in compression should extend from any section for a distance such that the average bond stress does not exceed the permissible bond stress given above for bars in tension by more than 25%. This condition will be satisfied if the length measured from each section is not less than:

the bar diameter x

the compressive stress in the bar 5 x the permissible average bond stress

The bar should extend at least 12 diameters beyond the point at which it is no longer required to resist stress.

3A.12.3 Avoidance of bond failure Each bar should have adequate anchorage on both sides of any cross-section to develop the calculated force. .This may be assessed by direct calculation, or it may be assumed to be satisfied if the local bond stress does not exceed the permissible average bond stress by more than 25%.

V

local bond stress =

do

where V is the shear force across the section o is the sum of the perimeters of the bars in the tension reinforcement. In members of variable depth the effect of change of depth should be taken into account in calculating the bond stress.

3A.12.4 Bearing stresses on bends The bearing stress calculated from the formula: calculated force in bar at the start of the bend internal radius of bend x the bar diameter should not exceed 3pcc. For any bar, provided that the bar is not assumed to be stressed more than four diameters beyond the end of the bend, bearing stresses need not be calculated. 30

IStructE RC permissible strcss rccommcndations

.,, c

.

..

.. ,.. .

.* '

..

~

,.

,.

.

_. _ . '

I

.

. '. . '

.

.

..-.. .

:.:

. ,. ,

'

_ *.'

I

.

3A.12.5 Hooks and other anchorages '

Hooks and othcr anchorages of reinforcement should be of such form, dimensions and arrangement as will ensure their adequacy without overstressing the concrete or other anchorage material.

3A.12.6 Dimensions of hooks Where hooks are used they should be of the U- or L-type shown in Fig. 4. In both types, for high-yield bars: (i) thc internal radius of the bend should be at least 3 x the diameter of the bar or 4 x the diametcr of the bar for bars of 25 mm diameter and greater. (ii) the length of straight bar beyond the end of the curve should be at least 4 X the diameter of the bar.

3A.12.7 Effective anchorage lengths for U- and L-hooks: high-yield bars The cffcctivc anchorage length of hooks and bends as shown in Fig. 4 may be taken as: (i) U-hooks: 24 X thc bar diameter, or the actual length of bar in the hook, including thc straight portion, whichcver is greater (ii) L-hooks: 12 x the bar diameter, or the actual length of bar in thc hook, including thc straight portion, whichcver is grcater. Thc bcaring strcss should be chcckcd when the straight portion excccds four the diamctcr.

X

I D

I

I

i"

--------A

t

-I

24D Equivalent straight length

(a) U-Hook

(b) L-Hook

4 Standard hooks f o r high yield bars

IStructE RC pcrmissiblc stress rccommcndations

31

3A.12.8 Effective anchorage lengths for U- and L-hooks: plain bars When hooks are formed in plain round mild-steel bars, the internal radius of the bend should be at least twice the diameter of the bar. The length of straight bar beyond the end of the curve should be at least 4 X the diameter of the bar and effective anchorage lengths may be taken as 160 for U-hooks and 8D for L-hooks. The bearing stress should be checked when the straight portion exceeds 4 x the diameter.

3A.12.9 Links in beams and transverse ties in columns Notwithstanding any of the provisions of these recommendations, in the case of links and transverse ties complete bond- length and anchorage may be deemed to have been provided when4he bar is bent through an angle of at least 90" round a bar of at least its own diameter and the link or tie is continued beyond the end of the curve for a length of at least eight diameters or, through an angle of 180" with the link or tie continued beyond the end of the curve for a length of at least four bar diameters.

3A.12.10 Shear reinforcement All bent-up bars acting as shear reinforcement should be fully anchored in both flanges of the beam, the anchorage length being measured from the end of the sloping portion of the bar nearest to the anchored end.

3A.12.11 General requirements for connecting reinforcement All connections transferring stress may be lapped, welded or joined with suitable mechanical devices. Laps should be placed, as far as possible, away from points of high stress and should be staggered. Welded joints should conform with the appropriate Code of Practice, but welding of bars is not recommended where loading is predominantly cyclical. Where the smaller bar at a lap is larger than 20 mm diametcr and the cover is less than 1% x the bar size, links should be provided throughout the lap length. The spacing of the links should not exceed 200 mm, and their diameter should be not less than '/4 of the smaller main bar diameter. Where lapped bars are of different diameters, the length of the lap may be based on the diameter of the smaller bar. The minimum lap for bar reinforcement should be 15 diameters or 300 mm, whichever is greater, and for fabric reinforcement not less than 250 mm.

3A.12.12 Tension laps For bars in tension, the length of lap should not be less than the bar diameter

the tensile stress in the bar X

4 x the permissible average bond stress The following recommendations also apply to tension laps: (i) Where a lap occurs in the top of a member as cast and the minimum cover is less than twice the size of the lapped reinforcement, the calculated lap length should be increased by 40%. (ii) Where a lap occurs at the corner of a section and the minimum cover to either face is less than twice the size of the lapped reinforcement, or where the clear distance between adjacent laps is less than 75 m m or 6 X the size 32

IStructE RC pcrmissiblc strcss rccommcndations

I

of the lapped reinforcement, whichever is greater, the calculated lap length should be increased by 40%. (iii) I n situations where both (i) and (ii) apply, the calculated lap length should be doubled. At tension laps, the sum of the reinforcement sizes in a particular layer should not exceed 40% of the breadth of the section at that level.

3A.12.13 Compression laps For bars in compression the length of lap should not be less than the compressive stress in the bar

the bar diamcter x

4 x the permissible average bond stress

3A.12.14 Anchorage bond and lap lengths Values for minimum anchorage bond and lap lengths for fully stressed bars as multiplcs of bar size are given in Table 5 for grade 30 concrete. For other concrete strengths, these values should be multiplied by the modification factors given in Table SA. Table 5 Anchorage bond and lap lengths as multiples of bar size for fully stressed bars

reinforcement type

grade 460 plain

deformed fabric (see subsection type 2 3A.6)

deformed type 1

concrete grade 30 tension anchorage and lap length (see clause 3A.12.12) compression anchorage length

II I

compression lap length

36

25 31

I

I I

4

4

30 38

II I

27 35

24

23

30

29

Note: The values are rounded up to the next whole numher. and the lengths derived from these values may differ slightly from those calculated directed for each har or wire size.

Table SA Modification factors for various concrete grades

concrete grade

20

25

30

35

40

45

50

factor

1.20

1.06

1.00

0.90

0.86

0.82

0.75

3A.12.15 Effective perimeter The effective perimeter of a single bar may be taken as n times its nominal diameter. IStructE RC pcrmissiblc strcss recommendations

33

3A.12.16 Groups or bundles of bars The effective diameter of a group or bundle of bars may be taken as the diameter of a bar of equal total area. For calculation of bearing strcss under clause 3A. 12.4, the effective diameter may be taken as the diamcter of a bar of equal area. At laps between bundles of bars no more than four bars should be in contact at any point.

3A.13 Joints, crack control and minimum reinforcement requirements Consideration should be given to the extent of cracking of the structure and finishes arising from the effects of thermal and shrinkage movements in the concrctc. Somc structures, such as basements, are relatively well protected against scasonal temperature effects, and in such cases, the principal cause of cracking is likcly to bc drying shrinkage, the bulk of which takes place in a relativcly short period after casting and can often be controlled by leaving sections of concrete for casting after the concrete on each side has matured for a period (shrinkage bays). Shrinkage effects reduce with time, and stable conditions eventually occur; however for 'summer casting' conditions, where the effects of drying shrinkage and thermal contraction may be additive, enhanced provisions should be considered. Thermal movements are of greater importance for cxposed concrete structures, unprotected within a building envelope, such as cxtcrnal retaining walls, and while movements predominantly produced by shrinkage can often be largcly accommodated by shrinkage bays and spccified casting sequences, significant thermal movements require the provision of permanent movement joints. The effects of all movements tend to concentrate at weak points such as abrupt changes of section, and the location of joints should be considered in relation to their probable effectiveness and their impact on the overall design of the building and finishes. The design and detail of permanent joints should be such as will permit movements to occur without detrimental effects on stability, watertightness, durability, fire resistance or other essential functions of the structure. The question of the provision and spacing of joints is left to the discretion of the designer in view of the large number of factors involved, but if joints are provided their design should be carefully considered in relation to their probable performance over the lifetime of the structure. Generally, if permanent joints can be avoided (which may be the case where movements arise predominantly from shrinkage) this may be regarded as desirable. Other non-structural elements such as brickwork, blockwork, mosaic finishes, etc. will require to be jointed at considerably more frequent intervals than is necessary in a concrete framed structure. Special consideration should also be given to structures where the vertical supporting elements are of loadbearing masonry, which has less flexibility and capability of accepting horizontal movement of the floor slabs because of shrinkage and thermal effects than would be the case in a fully framed structure. The provision of joints may also be necessary to accommodate movements arising from differential settlement or other anticipated causes producing vertical or horizontal effects on the structure. In considering the provision for movement of concrete structures caused by thermal effects typical coefficients of expansion may be assumed as follows: aggregate of flint and quartzite aggregate of granite and basalt aggregate of limestone 34

12 x 1O4 per "C 10 x 104 per "C 8 x 1O4 per "C. IStructE RC pcrmissiblc strcss rccomrncndations

For further information o n this subject, reference may be made to technical literature and in particular to Concrete Society Report no. 22 and ClRlA reports no. 91 and 107. Under the specific design provisions of this manual, various recommendations for a minimum amount of reinforcement to be provided are made. These minimum steel quantities are given as general guidance and are suitable for many common structures. Some elements of the structure may be regarded safely as mass concrete without reinforcement and others by reason of their jointing, construction and restraint may resist cracking without the use of higher quantities of controlling reinforcement. These factors should be takcn into account in the design, where relevant, in determining the amount of reinforcement to be provided. For further guidance reference may be made to specialist literature and BS 8007.

3B BEAMS AND SLABS 3B.1 General 3 B l . l Effective span The effective span, 1, of a beam or slab should be taken as the lesser of the following: (i) the distance between the centres of bearings, or (ii) the clear distance between supports plus the effective depth of the beam or slab, the effective depth being the distance between the centre of tension and the edge of the compression section.

3B.1.2 Slender beams The clear distance between restraints should not exceed: (i) simply supported or continuous beams: the lesser of 606 or 250b2/d (ii) cantilevers with lateral restraints only at support: the lesser of 256 or 100b2/d where b is taken as the breadth of the compression face, at midspan in case (i) and at the support in case (ii). d is the effective depth, which need not be taken as more than the minimum value required to resist the design loads without compression reinforcement.

3B. 1.3 Minimum reinforcement The minimum area of high-yield reinforcement provided in tension should be as follows (areas of mild steel are given in parentheses). The area is expressed as a percentage of the gross concrete area in the case of solid sections and as a percentage of the rib area (6,h) in the case of T- or L-beams: solid slabs, solid beams, flanged beams with webs in tension and bJb at least 0.4 flanged beams with webs in tension and b,/b less than 0.4 T-beams with flange in tension (continuous support) L-beams with flange in tension (continuous support)

0.13% (0.24%) 0.18% (0.32%) 0.26% (0.48%) 0.20% (0.36%)

3B.1.4 Compression reinforcement in beams The compression reinforcement should be effectively anchored in two directions at right-angles over the distance where it is required to act in compression, at points IStructE RC permissible stress recommendations

35

._. .

not further apart, centre to centre, than 12 x the diameter of the anchored bar or 300mrn, whichever is less. Links used for this purpose should pass round, or be hooked over, both the compression and tension reinforcement. The amount of steel in compression should preferably not exceed 4%, but if it does, only 4% should be allowed for in the calculation of the resistance moment of the beam. This percentage should be calculated as follows: (i) in rectangular beams, on the total cross-sectional area (ii) in T- and L-beams, on an area equal to the total depth X the width of the rib. Where compression reinforcement is needed, the area should not be less than 0.2%, except in the case of webs to T- and L-beams, where it should not be less than 0.4%.

3B.1.5 T-beams In T-beams, the breadth of the flange assumed as taking compression should not exceed the least of the following: (i) Vj of the effective span of the T-beam (ii) the distance between the centres of the ribs of the T-beams . (iii) the breadth of the rib plus 12 x the thickness of the slab.

3B.1.6 L-beams In L-beams, the breadth of the flange assumed as taking compression should not exceed the least of the following: (i) '/h of the effective span of the L-beams (ii) the breadth of the rib plus V'z of the clear distance between ribs (iii) the breadth of the rib plus 4 x the thickness of the slab. When a part of a slab is considered as the flange of a T- or L-beam, the reinforcement in the slab transverse to the beam should cross the full breadth of the flange. The quantity of such reinforcement should be related to the shear stress in the slab produced by its acting as the compression member of the T- or L-beam, but should not be less than 0.15% of the slab area and placed near thc top surface.

3B.1.7 Effect of wear If the surface of a concrete slab is not adequately protected by a suitable finish against the effect of wear, an appropriate addition should be made to the structural thickness required.

3B.2 Deflection and stiffness of members 3B.2.1 General Reinforced concrete members should possess adequate stiffness to prevent such deflection or deformation as might impair the strength or efficiency of the structure or produce unacceptably large cracks in finishes or in partitions. Deflections may be calculated and compared with the serviceability requirements appropriate to each particular case, which may include consideration of such beneficial factors as secondary support, high-performance materials, reduction in early deformations, chartacteristics of loading and redundancy. However, in all normal cases the deflection of beams and slabs should not be excessive if the ratio 36

IStructE RC permissible stress rccommcndations

of spanleffective depth does not exceed the recommended values. In cases where aggregates give unusually high shrinkage it may be necessary to reduce these values , (see BRE digest 35). Span/effcctivc depth ratios are given below; they are based on limiting the total deflection to span/250, and this should normally mean that the part of the deflection occurring after construction of finishes and partitions will be limited to span/350 or 20 mm, whichever is less. Basic ratios are given in Table 6 and these should be modified by factors obtained from Tables 6A and 6B. The term ‘steel tensile stress’ in Table 6 and the terms Mlhd’ and 100A,Jbd in Tables 6A and 6B apply to t h e centre of the span or, in the case of a cantilever, to the support. Fig. 5 shows graphically the modification factors for spanleffective depth ratios. For T-bcams and ribbed slabs, the appropriate factor from Table 6A can be obtaincd by interpolation between the values of h,/h = 1 and hr/h = 0. Table 6 Basic spadeffective depth ratios for T-beams and ribbed slabs

I

steel tensile stress, N/mm’ cantilever simply supportcd continuous

140

I

250 7 20 26

9

25 32

Notes: I Intermediate values of the ratios may he obtained hy interpolation. 2 For spans greater than IOm. these ratios should be reduced hy the factor (IO/span in metres): cantilevers exceeding 4 m should he justified hy calculiition, with due regard to rotation of the support.

L

0

L

8

0

1

2

5 Modification factors for spanleffective depth ratios IStructE RC permissible stress rccommcndations

3

i M/bd2 ,Nlrnrn2

37

,

.

M/bd2, N/mm2

.o

so.5

b,Jb = 1 bdb = 0

1

1.6 1.2

1.3

1.1

'

.

2.0

3.0

24.0

1.o 1.0

0.9 0.9

0.8 0.8

lOOA,Jbd

0.25

0.50

0.75

1.0

1.5

2.0

3.0

factor

1.07

1.14

1.20

1.25

1.33

1.40

1.50

total slab flat slab dead and 1-way 2-way 2-way imposed 2-way no simply continuous cantilever simply continuous drops drops load, supported kN/m2 supported

5 10 20

31 27 23

42 36 31

13 11

9

36, 31 27

50 44 38

42 36 31

45 39 33

3B.3 Bending moments Bending moments in beams and slabs should be calculated for the effective span and all loading thereon. ' The bending moments to be provided for at a cross-section of a continuous beam or slab should .&the maximum positive and negative moments at such cross-section, allowing, in both cases, if so desired, for the reduced moments arising from the width of the supports, for the following arrangements of imposed loadings: (i) alternate spans loaded and all other spans unloaded (ii) any two adjacent spans loaded and all other spans unloaded. 38

IStructE RC pcrmissiblc strcss rccommcndations

Nevertheless, except where the approximate values for bending moments given in Table 7 are used, the negative moments at the supports for any assumed arrangement of loading may each be increased or decreased by not more than 1570, provided that these modified negative moments are used for the calculation of the corresponding moments in the spans. Redistribution of moments should be limited to 10% in structures over 4 storeys high where the frame acting in sway provides the structural stability. The computation of bending moments in beams and slabs is dealt with in subsections 3B.4 and 3B.5 and of bending moments in flat slabs in section 3C.

3B.4 Bending moments and shears in beams and slabs spanning in one direction 3B.4.1 Calculation of bending moments The bending moments in beams and slabs spanning in one direction may be calculated on one of the following assumptions: (i) Beams may be designed as members of a continuous framework, with monolithic connection between the beams and columns, and the bending moments calculated taking into account the resistance of the columns to bending. Where beams are framed into external columns they should be designed to resist bending moments in combination with the columns in conformity with clause 38.2.6. (ii) Beams and slabs may be designed as continuous over supports and capable of free rotation about them. Nevertheless, where the supports to beams or slabs are monolithic with them and stiff in relation to them, it is preferable to design the beams or slabs with due regard to such stiffness. (iii) For the purpose of calculating moments in beams or slabs in a monolithic structure, it will usually be sufficiently accurate to assume that members connected to the ends of such beams and slabs are fixed at their remote ends. (iv) Unless more exact estimates are made, the bending moments in uniformly loaded beams and slabs continuous over three or more approximately equal spans may be assumed to have the values given in Table 7, provided that the following limits are met: 0

0

beams: imposed load not greater than dead load, shortest span not less than 85% of longest slabs: imposed load not greater than either 5kN/m' (excluding partitions) or 1% times dead load, area of each bay between lines of support at least 30 m2.

(v) Where moments are calculated from Table 7, design shear forces may be taken as follows: beams: outer supports: 0.45 W , support next to end: 0.6W, interior support: 0.55". slabs: outer supports: 0.4W, support next to end: 0.6W, interior support: 0.5W IStructE RC pcrmissiblc strcss rccommcndations

39

middle of end span beams slabs

support next to end

middle of intcrior span

intcrior supports

+0.063 WI

-0.08 WI -0.063WI

3B.4.2 Detailing of beams Beams designed in accordancc with Table 7 may be dctailcd in accordancc with the following rules: (i) all reinforcement for negative moments should extend a distance of 0. ISL (or the bar anchorage length if greater) from the support face, 60% should extend 0.251 from this line and at lcast 20% should be continuous throughout the span (ii) all rcinforcement for positive moments should extend to within 0.1SL of the effective support and 30% extend into it at intcrior supports; at external supports, all positive reinforcemcnt should extend to within 0.08L of the effective support and 50% should extend into the support (iii) where negative moments may occur at an end support but have not been calculated, anti-crack reinforcement should be provided. This reinforcement should be equal to half the midspan reinforcement for positive moments and should extend 0.15L (or the bar anchorage length, if greater) i n t o the span.

3B.4.3 Detailing of slabs Slabs designed in accordance with Table 7 may be detailed in accordancc with the following rules: (i) all reinforcement for negative moment should extend a distance of 0.1SL (or the bar anchorage length if greater) from the support face and 50% should extend 0.3L from this line (ii) all reinforcement for positive moments should extend to within 0.2L of the effective support and 40% should extend into it.

3B.5 Slabs spanning in two directions at right-angles with uniformly distributed loads Bending moments in two-way slabs may be calculated by elastic analysis, yield-line analysis, Hillerborg’s strip method or other appropriate methods. Alternatively, the methods and coefficients given in (i) and (ii) below may be adopted. Simply supported slabs. When simply supported slabs do not have adequate provision to resist torsion at the corners and to prevent the corners from lifting, the maximum moments per unit width may be assumed to have the values given by the following equations: M, = 0(,w1,2 My = O(,W1,2

(14)

(15) IStructE RC permissible stress recomrncndations

~~~

-

~

.*

-

. r

r

_-

.

,

..

_,.

where M , and M y are the bending moments at midspan on strips of unit width and spans I, and ly, respectively w is the total load per unit area I , is the length of the shorter side 1, is the length of the longer side a, and ayare the coefficients from Table 8. At least 50% of t h e tension reinforcement provided at midspan should extend to the supports. The remaining 50% should extend to within 0.1 of the span from the support. Table 8 Bending moment coefficients for two-way slabs simply supported on four sides

1.0

1.1

1.2

1.3

1.4

1.5

1.75

2.0

2.5

3.0

0.062 0.074 0.084 0.093 0.099 0.104 0.113 0.118 0.122 0.124 0.062 0.061 0.059 0.055 0.051 0.046 0.037 0.029 0.020 0.014 (ii) Slabs restrained on four sides ( a ) Where the corners of a slab are prevented from lifting and adequate provision for torsion in accordance with subclause 3BS(ii)(e) is made, the bcnding moments may be assumed to have the values given in subclause 3BS(ii)(c). (b) Slabs are considered as being divided in each direction into middle strips and edge strips as shown in Fig. 6, the middle strip having a width of three-quarters of the width of the slab and each edge strip having a width of one-eighth of the width of the slab, except that, for slabs for which the ratio of the sides 1,11, exceeds 4.0, the middle strip in the short direction should be taken to have a width of I, - I, and each edge strip a width of 1,12.

(a) For span Ix

(b) For span ly

6 Division of slab into middle and edge strips IStructE RC permissiblc stress rccommcndations

41

(c) The maximum bending moments per unit width in thc middlc strip of a slab are given by the following cquations:

M , = pXwIx7 M y = pywlx7 where M , and M y arc thc maximum bending moments on strips of unit width in the direction of spans I, and Iy, respcctivcly w is the total load per unit area I, is the length of the shortcr sidc Iy is the length of thc longer sidc p, and p, arc cocfficicnts givcn in Table 9. By adopting thc rclationship givcn in equation (17) for M y , it is possible

to use a single cocfficicnt for p, for all ratios of Iy//x for cach condition of edge support. ( d ) N o reinforcement parallel to thc adjacent cdgcs of thc slab need be

inserted in thc edge strips above that rcquircd to comply with subscctions 3A. 1 I and 3B. I and subclause 3B.S(ii)(c).

(e) Torsion reinforcement should be provided at thc corncrs of a slab cxccpt at corners contained by edges over both of which thc slab is continuous. At corners contained by edges over neither of which thc slab is continuous, top and bottom reinforccmcnt should bc provided for torsion at the corners of the slabs. Both top and bottom rcinforccment should consist of two laycrs of bars placed parallel to the sides of the slab and extending in these directions for a distance of onc-fifth of the shortcr span. The area of the bars in each of the four laycrs, pcr unit width of the slab, should be thrce-quarters of the area required for the maximum positive moment in the slab. At corners contained by edgcs over only onc of which the slab is continuous, the torsional rcinforcement may be reduced to one-half of that required by the preceding paragraph. Any reinforcemcnt provided for the purpose of complying with other clauses of thcse rccommendations may bc includcd as part of the reinforcement required to comply with this clausc. (f) Where a slab ends and there is monolithic connection between the slab and the supporting beam or wall, provision should be made for the negative moments that may occur in the slab at such support. Thc negative moment to be assumed in these cases depends on the degree of fixity afforded to the edge of the slab, but for general purposes it may be taken as two-thirds of the momcnt obtained using Table 9 for the midspan of the slab. (iii) Loads on supporting beams The loads on supporting bcams may be assumcd to be in accordance with Fig. 7. Where appropriate, allowance should be made for the cffects of clastic shear.

3B.6 Trimmings for openings When openings in floors or roofs are required such openings should be trimmed where necessary by special beams or reinforcement so that the designed strength of 42

IStructE RC pcrmissiblc strcss rccommcndations

IStructE RC permissible stress recommendations

43

-

-

the surrounding floor is not impaired by the opening. Due regard should be paid to the possibility of diagonal cracks developing at the corners of openings. 'Y

k

Load included in this shaded area to be carried

CI

-I

B

Load included in this shaded area to be carried by beam B

7 Diagram showing the load carried by supporting beams

3B.7 Distribution of concentrated loads on slabs Allowance should be made for bending moments due to Concentrated loads, using methods based on the elastic theory, such as those of Piegaud, Westergaard or other acceptable method. Alternatively, allowance should be based on an appropriate plastic analysis such as Hillerborg's strip method or yield-line theory. If a slab is simply supported on two opposite edges and carries one or more concentrated loads in a line in the direction of the span, it should be designed to resist the maximum bending moment caused by the loading system. Such bending moment may be assumed to be resisted by an effective width of slab (mcasured parallel to the supports) as follows: (i) For solid slabs, the effective width may be taken as the sum of the load width and 2.4a(l - U N ) , where a is the distance from the nearest support to the section under consideration and 1 is the span (ii) For other slabs, except where specially provided for, the effective width will depend on the ratio of the transverse and longitudinal flexural rigiditics of the slab. When these are approximately equal, the value for the effective width as given for solid slabs may be used, but as the ratio decreases a smaller value should be taken. The minimum value that need be taken, however, is 4a

metres, where a and I have the same meanthe load width plus - (1 -A) 1 I ings as in (i); so that, for a section at midspan, the effective width is equal to 1 m plus the load width. (iii) Where the concentrated load is near an unsupported edge of a slab, the effective width should not exceed the value in (i) or (ii) above as appropriate, nor half that value plus the distance of the centre of the load from the unsupported edge (see Fig. 8).

I

44

IStructE RC pcrmissiblc strcss rccornrncndations

-

3B.8 Bearings for slabs on steel ioists Concrete casing to steel frame mcmbcrs should be reinforced with steel binding wire not less than 2.5 m m in thickness, not further apart than 300 m m (or the equivalent in steel fabric), passing under but clear of the edges and soffit of the bottom flange of the beam.

,

Load

Unsupported edge

Effective width

Load width

-

A

V

V

I

A

b

3B.9 Slabs: ribbed and hollow-block construction

I

3B.9.1 General This type of construction consists of a series of reinforced concrete ribs cast in situ between blocks that remain part of the completed floor or on forms that may be removed after the concrete has set.

3B.9.2 Blocks and forms Blocks and forms may be of any suitable material that will retain its shape and dimensions and is strong enough to support the concrete when placed. Blocks that are required to remain as part of the slab and to contribute to its structural strength should be of concrete or burnt clay and should have a crushing strength of at least 14 Nlmm’ measured on the net section when axially loaded in a direction corresponding with that in which they will function in the floor slab. Burnt clay blocks should also comply with BS 3921 Bricks and blocks of fired brick earth, clay or shale.

IStructE RC pcrmissiblc strcss rccommendations

45

3B.9.3 Topping The tops of the ribs may be connected by a topping of concrete cast in situ over the blocks or forms. The concrete used for the topping should be of the same quality as that used for the ribs and cast integral with the ribs or properly bonded to them.

3B.9.4 Calculation of resistance moment I n determining the bending resistance of hollow-block construction, the blocks may be neglected. Alternatively, they may be assumed to act in structural combination with the ribs and topping (when used), provided that the blocks are properly jointed with a 1:3 cement:sand mortar or that a topping of at least 25 mm is used. Where the thickness of the top of hollow blocks composed of material other than concrete is regarded as contributing to the structural strength of the floor slab, the permissible working stress in the blocks should not exceed one-fifth of their crushing strength. For the purpose of calculation, the elastic modulus of the material forming the block may be assumed to be the same as that for concrete.

3B.9.5 Resistance to shear Where the blocks are considered as adding to the strength of the floor, the thickness of one wall of the block may be added to the breadth of the rib. Alternatively, the walls of both the adjacent blocks may be taken into account, using a shear stress appropriate to the material.

3B.9.6 Thickness of topping When topping is used the thickness, after allowance has been made for the effect of wear if necessary, should not be less than the thickness given in (i) to (iii) below for various conditions: (i) in floors with permanent blocks regarded as contributing to the strength of the construction, and with a clear distance between the ribs not exceeding 500 mm, 25 mm. (ii) in floors with permanent blocks not regarded as contributing to the strength of the construction, 30 mm or one-tenth the clear distance between the ribs, whichever is greater. (iii) in all other cases, 50 mm or one-tenth the clear distance betwecn the ribs, whichever is greater. It should be noted that the maximum size of aggregate may need to be restricted if the minimum thickness of topping is used. The practical problems of constructing thin toppings should be considered.

3B.9.7 Size and spacing of ribs The breadth of the ribs should be not less than 65 mm. The depth, excluding any topping, should be not more than 4 x the width. The spacing should be not more than 1.5 m centre to centre. For longer spans, consideration should be given to the provision of transverse ribs, particularly where no topping is used.

3B.9.8 Reinforcement in ribs (i) General In floors continuous over supports, it may sometimes be impracticable to provide sufficient reinforcement to develop the full support moment on the 46

IStructE RC pcrmissiblc strcss rccommcndations

,

basis of continuity. Such floors may be treated as simply supported and the reinforcement in the slab determined accordingly. I f so treated, it is desirable to provide reinforcement over the support to control cracking; it is recommended that such reinforcement should have a cross-sectional area of not less than one-quarter of that in the middle of the adjoining bays and should extend at least 15% of the clear spans into the adjoining bays. At least 50% of the total main tensile reinforcement should be carried through at the bottom on to the bearing, and effectively anchored. (ii) Reinforcement in topping Consideration should be given to providing a single layer of mesh in the topping having a cross-sectional area of not less than 0.12% of the topping in each direction. Care should be taken that adequate cover can be provided if a mesh is used, particularly where laps occur.

3B.9.9 Supports parallel to ribs Where a slab reinforced in one direction only is built into a wall, or rests on a beam, parallel to the ribs, a rib should be placed along the wall or beam, the minimum breadth of such rib being that of the bearing. Consideration should be given to the necessity for some reinforcement at right-angles to the rib. Where a slab butts against a wall parallel with the ribs, there should be a rib against the wall at least 50 m m wide.

3B.10 Resistance to shear 3B. 10.1 General (i) The shear stress, v , at any cross-section in a reinforced concrete beam or slab should be calculated From the following equation:

(ii)

v = Vlbd (18) where V is the total shearing force across the section b is the breadth of a rectangular beam or the rib breadth, b,, of a Tor L-beam d is the effective depth of the section. Values of the permissible shear stress, p v , are given in Table 10 and shown in Fig. 9 for fCu = 30 N/mm’. For other concrete strengths, these should be multi lied byg(fCu/30), with the value of fCu not taken as less than 20 N/mm or more than 40 N/mm’ (see Table 10A). Where at any cross-section the shear stress, v , as calculated from equation (18), is less than %pV for a beam or pv for a slab, no shear reinforcement is required. For beams of structural importance it is, however, recommended that nominal shear reinforcement be provided. Where at any cross-section the shear stress, v, as calculated from equation (18), is greater than %pv for a beam or pv for a slab but does not exceed @, + 0.25), nominal links should be provided for the whole length of a beam or, for a slab, over that length where v exceeds pv. The area of nominal links should be not less than 0.12% of the concrete plan area for high-tensile steel and 0.18% for mild steel. Where at any cross-section the shear stress, v , as calculated from equation (18), is greater than (pV+ 0.25), shear reinforcement should be provided but, even with such reinforcement, v should not exceed 0.5dfcu.

r:

(iii)

(iv)

(v)

IStructE RC pcrmissiblc strcss rccommcndations

47

cffcctivc dcpth o f mcmbcr. tl 100A,,lhd

S 125

150

175

200

225

250

300

2400

S0.15 0.25 0.50 0.75

0.30 0.36 0.45 0.52

0.29 0.34 0.43 0.49

0.28 0.33 0.41 0.47

0.27 0.32 0.46

0.26 0.31 0.39 0.44

0.25 0.30 0.38 0.43

0.24 0.29 0.36 0.41

0.22 0.26 0.34 0.38

1.00 1.50 2.00 33.00

0.57 0.65 0.71 0.82

0.54 0.62 0.68 0.78

0.52 0.60 0.66 0.75

0.50 0.58 0.63 0.73

0.49 0.56 0.62 0.71

0.48 0.55 0.60 0.69

0.46 0.52 0.57 0.66

0.42 0.48 0.53 0.61

0.40

Table 10A Adjustment factors for various concrete strengths

strength, fCu

I

factor

-

S20

25

0.87

0.04

30

35

240

.oo

1.05

1.10

1

0.8- -

0.7--

ul

E

0.6--

L

8

-

c ul

-n w

'j, .-ul

o,s--

E

-

c

0.4 --

0.3-

0.2-0

1

9 Permissible shear stress 48

IStructE RC pcrmissiblc strcss rccommcndations

3B.10.2 Shear reinforcement (i) A link in reinforced concrete should pass round or be otherwise adequately secured to the appropriate tension reinforcement, and all such links should be adequately anchored (see clause 3 A . 12.9). (ii) Where a beam or slab is reinforced with inclined bars, the shear resistance at any section may be calculated on the assumption that the inclined bars form the tension members of one or more single systems of lattice girders in which the concrete forms the compression members. The shear resistance at any section may then be taken as the sum of the vertical components of the tension and compression forces cut by the section. Care must be taken that such assumptions do not involve greater stresses in the horizontal bars than the permissible stresses. (iii) Where two or more types of shear reinforcement are used in conjunction, their total shearing resistance may be assumed to be the sum of their resistanccs. In a bcam, at least one-half of the shearing resistance provided by the reinforcement should be in the form of links. (iv) Where links alone are provided, the cross-sectional area, A,, of those links should be not less than: hs( I’ - p”) A, =

PSI

where s is the spacing of the links along the member p,, is thc permissible reinforcement stress given in clause 3A.7.4. (v) Where links arc provided the longitudinal spacing should not exceed 0.75d. In beams the lateral spacing of the vertical legs should not exceed the effective depth; in areas of solid slab the lateral spacing should not exceed 1 ’/r x the effective depth. (vi) In slabs less than 200 mm thick shear links should not bc used unless bending and fixing details are employed that ensure their structural effectiveness.

3B.11 Loads near supports of beams: shear enhancement For loads applied close to beam supports, the shear strength is enhanced. This may be allowed for by using appropriate theory or as follows: (i) Where a beam supports an approximately uniform load, the portion of loading within a distance d from the support may be ignored in calculating the shear stress, with permissible shear stress calculated in accordance with subsection 3B. 10. (ii) Where a beam supports a concentrated load and the distance a,. from the face of the support to the nearest edge of the concentrated load is less than 2d, an enhanced permissible concrete shear stress may be taken in this zone if all thc main reinforcement is continued to the support and provided with an anchorage length (or equivalent) not less than either the effective depth or 20 x the bar diameter. Shear reinforcement as described in subsection 3B.10 should be provided over the length a\., but in equations (18) and (19), the term p,. may be replaced by p , x 2 d l 4 provided that this does not exceed the maximum value of allowed. For cantilever beams and corbels where a, < 0.6d, horizontal links should normally be provided, and special attention should be paid to the anchorage of the main reinforcement. Fig. 10 shows possible methods of anchoring the main tension reinforcement in corbels. 11

IStructE RC pcrmissiblc stress rccommcndations

49

..

. .. . . .

.I

3B.12 Deep beams Where appropriate, deep beams may be designed on the basis of truss theory. Reference should be made to relevant specialist literature for detailed guidance.

3B.13 Torsional resistance of beams 3B.13.1 General In normal slab and beam or framed construction specific calculations are not usually necessary, torsional cracking being adequately controlled by shear reinforcement. However, when the design relies on the torsional resistance of a member, the recommendations given in clauses 3B.13.2 to 3B.13.7 should be taken into account.

3B. 13.2 Calculation of torsional rigidity If required in structural analysis or design, the torsional rigidity (C X C) may be calculated by assuming the shear modulus, C , equal to 0.42 X the modulus of elasticity, E, of the concrete and assuming the torsional constant C equal to half the St. Venant value calculated for the plain concrete section. Values for modulus of elasticity are given in subsection 3A.6.

Main steel welded to a transverse bar of equal diameter

Main reinforcement in the form of horizontal loops

'

rizontal shear steel SV)as stirrupsover

rs provided anchor orizontal stirrups

(C 1 Outside edge of bearing to be kept clear of bend in main reinforcement (minimum ckarance D 1 bar diameter)

Detailing rules (1) hy 4. 0 5 h (2) 04S100Astlbd < 1.3 (3) 0.6S 100(Ast+A~v)lbdS2.0 (4)Other details as per diagrams

I

10 Possible methods for anchoring main tension reinforcement in corbels 50

IStructE RC pcrmissiblc strcss rccommcndations

The St. Venant torsional stiffness of a rectangular section may be calculated from equation (20)

C = k~h~minhrnax

(20)

where kT is a coefficient depending on the ratio hm,,/hminwith the values given in Table 11 h,,, and hminare the larger and smaller dimensions, respectively, of a rectangular section. Table 11 Values of coefficient kT

l 1 I 0.14

kr

1.5

2

3

5

>5

0.20

0.23

0.26

0.29

0.33

The St. Venant torsional stiffness of a non-rectangular section may be obtained by ‘dividing the section into a series of rectangles and summing the torsional stiffnesses of these rectangles. The division of the section should be arranged so as to maximize the calculated stiffness. This will generally be achieved if the widest rectangle is made as deep as possible.

3B.13.3 Torsional shear stress (i) Rectangular sections The torsional shear stress v , , at any section should be calculated assuming a plastic stress distribution and may be calculated from equation (21)

2T 1’.

=

(

hlnli,, h,,;,, -

A) 3

where T is the torsional moment acting on the member. (ii) T-, L- or I-sections T-, L- or I-sections are divided into their component rectangles; these are in the following expression: chosen in such a way as to maximize h3nlinhnlzlx The torsional shear stress, v,, carried by each of these component rectangles may be calculated by treating them as rectangular sections subjected to a torsional moment of:

h3niinhm:ix

( t(h3minhm;ix) ) (iii) Hollow sections Box and other hollow sections in which wall thicknesses exceed one-quarter of the overall thickness of the member in the direction of measurement may be treated as solid rectangular sections. (iv) Other sections For other sections, specialist literature should be consulted. IStructE RC pcrmissiblc strcss rccommcndations

51

3B.13.4 Limit to shear stress In no case should the sum of the shear stresses resulting from shear force and torsion ( v + v , ) exceed 0.5dfc, nor, in the case of small sections where yl < 550, should the torsional shear stress vt exceed 0.5vfc, X yl/.%O, where y l is the larger centreto-centre dimension of a rectangular link. Values of 0.5vfc, are given in Table 12. Table 12 Values of permissible torsion and shear stress, N/mm2

I

concrete grade

0.042

I

20

I I

25 30 40 or above

d

fCu

0.5

d feu

0.19

2.24

0.21

2.50

0.23

2.14

0.26

3.12

3B.13.5 Reinforcement for torsion (i) Torsion reinforcement, where required, should be in addition to that required for shear and bending. (ii) Where the torsion shear stress, v,, is less than 0.042dfc, no torsion reinforcement is required. Values of 0.042dfc, arc given in Table 12. (iii) Where the torsion shear stress, v , , is greater than 0.042dfc,, torsion reinforcement should be provided. (iv) Recommendations for reinforcement for combinations of shear and torsion are given in Table 13. Table 13 Reinforcement for shear and torsion

v c 'hp,.

nominal shear reinforcement; no torsion reinforcement

designed torsion reinforcement, but not less than nominal shear reinforcement

designed shear reinforcement; no torsion reinforcement

designed shear and torsion reinforcement

3B.13.6 Torsion reinforcement (i) Torsion reinforcement should consist of rectangular closed links, together with longitudinal reinforcement, and should be such that: '

T

A" -3

s A, 2

and

0.hI YIPst

+ YI)

A,.(Xl S

52

IStructE RC pcrmissiblc stress rccommcndations

I I

where

(ii) (iii) (iv) (v)

(vi)

is the area of longitudinal reinforcement is the area of two legs of closed links at a section is the spacing of the links is the smaller centre-to-centre dimension of a rectangular link xI is the permissible reinforcement stress given in clause 3A.7.4 is the larger centre-to-centre dimension of a rectangular link yl I n a section with multiple links only the legs lying closest to the outside of the section should be used as torsion reinforcement. The longitudinal spacing of the links, s, should not exceed the least of x,, y,/2 or 200 mm. The links should be of the closed type complying with shape code 74 of BS 4466. Longitudinal torsion reinforcement should be distributed evenly round the inside perimeter of the links. Thc clear distance between these bars should not exceed 300 mm, and at least four bars, one in each corner of the links, should be used. Additional longitudinal reinforcement required at the level of the tension or compression reinforcement may be provided by using larger bars than those required for bending alone. The torsion reinforcement should extend a distance at least equal to the largest dimension of the section beyond where it theoretically ceases to be required. Consideration should be given to the congestion that may arise in providing the required amount of reinforcement for torsion. This may result in the need for larger member sizes than would result from other considerations. A, A, s

3B.13.7 Arrangement of links in T-, L- or I-sections In the component rectangles, the reinforcement cages should be detailed so that they interlock and tie the component rectangles of the section together. Where the torsional shear stress in a minor component rectangle does not exceed 0.042 dfcu, no torsion reinforcement need be provided in that rectangle.

3C FLAT SLAB CONSTRUCTION 3C.1 General The term flat slab means a reinforced concrete slab with or without drops, supported, generally without beams, by columns with or without flared column heads (see Fig. I I). A flat slab may be a solid slab or may have recesses formed on the soffit so that the soffit comprises a series of ribs in two directions. The recesses may be formed by removable or permanent filler blocks.

3C.2 Methods of design Flat slabs may be designed: (i) as continuous frames using the method described in subsection 3C.11 or by any other method satisfying the principles of statics and continuity; or (ii) by the empirical method described in subsection 3C.12 to 3C.17 which is applicable only to the more common forms of this construction described in subsection 3C. 12. In both methods subsections 3C.3 to 3C. 10 apply. IStructE RC pcrmissiblc strcss rccommcndations

53

3C.3 Division of panels Flat slab panels should be assumed to be divided into strips (see Fig. 12) as follows: (i) Column strip The width of the column strip should be taken as one-half of the width of the panel. Where the panel is oblong the widths in both directions should be based on the shorter panel dimension. Where dimensions in adjacent panels lead to strips in one panel of a width different from those in the other, the dimension of the panels in the region of the common support should be based on the panel giving the wider column strip. Where drops arc used, the width of the column strip may be taken as the width of the drop. (ii) Middle strip The width of the middle strip should be taken as the difference between that of the panel and that of the column strip.

3C.4 Notation for flat slab construction I n the following subsections and formulae relating to flat slabs, L , is the length of the panel in the direction of the span, L2 is the width of the panel at right-angles

Any concrete in this orea is to be neglected in the calculations

(a) Slab without drop and column without column head

l+-r

I

(b) Slab withoutdrop and column with column head

I Any concrete in this a r m is to be neglected in the colplotions

(c) Slab with drop and column with column head

11 Types of column head 54

IStructE RC pcrrnissiblc stress rccomrncndations

(measured in each case from the centres of the columns), L , is the average of L , and L2, D is the diameter of the column o r column head (see Fig. 11 and subsection 3C.10), and w is the total load per unit area on the panel.

3C.5 Thickness of slab The limiting span/depth ratios are given in subsection 3B.2, but in no case should the total thickness of the slab be less than 125 mm.

3C.6 Shear stresses in flat slabs 3C.6.1 Shear at column face The effective stress on the perimeter of the column face or column head (as appropriate) should not exceed O . S d f c , (see subsection 3B. 12).

3C.6.2 Slab shear The effective shear stress in the slab o r drop calculated on a perimeter 1 % x the effectjve depth of slab or drop from a column, column head o r drop should not exceed the permissible stresses, p v , in subsection 3B. 12.

I

I

12 Divisions of panel into strips IStructE RC permissible stress recommendations

55

- . .. r----- 1

I

I

I

13 Shear perimeter for internal columns

i

Perimeter a

14 Shear perimeter for edge column

Critical sections for shear are given in Figs. 13 and 14. In some situations it may also be necessary to check shear on a plane across a panel. Where slab reinforcement in two directions is not equal, the capacity of the faces in each direction may be calculated separately and added to give the total capacity of the perimeter. To calculate the effective shear stress the applied shear force should be increascd to allow for the effect of moment transfer and stress concentrations. This should be determined as follows: (i) Internal columns In braced structures with approximately equal spans the increase may be taken as 15%. In other cases the effect of moment transfer should be calculated and the increase taken as (15OM,/Vx)% if this exceeds 15%, where: M,

V x

is the moment transferred to the column the shear the length of the side of the perimeter considered parallel to the axis of bending. IStructE RC permissible stress rccommcndations

(ii) Edge columns In braced structures with approximately equal spans the increase may be taken as 40%. I n other situations the effect of moment transfer and stress concentration should be calculated and the increase taken as (25 + 15oM,/Vx)%0or 40%, whichever is greater, where: M,

is the moment transferred to the column running parallel to the edge.

(iii) Corner columns The increase may be taken as 25% in all cases.

3C.6.3 Shear reinforcement I f the effective shear stress exceeds the permissible value, shear reinforcement should be provided. This may takc the form of links, bent-up bars or fabricated components. The design of bent-up bars or other components should be justified by established theory andlor test data. Links should be designed in a similar manner to those in beams with A, taken as the total area provided on one perimeter (see clause 3B.10.2). Spacing of link legs along the perimeter should not exceed 1 S d . The links required at the critical perimeter defined in clause 3C.6.2 should be provided uniformly between this perimeter and the face of the support. Shear stresses should then be checked at successive perimeters at 0.75d intervals outside the critical perimeter and appropriate shear reinforcement provided if the effective shear stresses exceed the permissible value.

3C.6.4 Openings When openings are less than 6 X the effective depth of the slab from the edge of a column then that part of the perimeter that is enclosed by radial projections from the centroid of the column to the openings should be considered ineffective as shown in Fig. 15.

Length deducted from shear perimeter

-1 I 1 I I

15 Effect of opening on shear perimeter I

IStructE RC pcrmissiblc stress rccommcndations

57

f-.-

..

-

3C.7 Openings in panels Except for openings complying with (i), (ii) or (iii) below, openings should be completely framed on all sides with beams to carry the loads to the columns, and an opening should not encroach on a column head or drop. (i) Openings of a size such that the greatest dimension in a direction parallel to a centre-line of the panel does not exceed 0.4L may be formed in the area common to two intersecting middle strips, provided that the total positive and negative moments specified in clause 3C.11.5 or subsection 3C.14 are redistributed between the remaining principal design sections to meet the changed conditions. (ii) Openings of aggregate length or width not exceeding one-tenth of the width of the column strip may be made in the area common to two column strips, provided that.'the reduced sections are capable of carrying the appropriate moments specified in clause 3C.11.5 or subsection 3C.14. (iii) Openings of aggregate length or width not exceeding one-quarter of the width of the strip may be made in any area common to one column strip and one middle strip, provided that the reduced sections are capable of carrying the appropriate moments specified in clause 3C.ll.5 or subsection 3C.14.

3C.8 Concentrated loads Significant concentrated point and line loads should be carried on a framework of beams carried back to the columns unless a more exact analysis is adopted.

3C.9 Bending moments in edge panels 3C.9.1 Slab supported by marginal beam Where the slab is supported by a marginal beam with a depth greater than 1.5 the thickness of the slab, or by a wall then:

X

(i) the total load to be carried by the beam or wall should comprise those loads directly on the wall or beam plus a uniformly distributed load equal to onequarter of the total load on the slab; and (ii) the bending moments on the half-column strip adjacent to the beam or wall should be one-quarter of the bending moments specified in clause 3C.11.5 or subsection 3C.14.

3C.9.2 Edge moments Unless an edge strip or edge beam is designed for the necessary torsion the moment transmitted to edge columns should be limited to M,,,, = 0.1fcub,d2 where b, is defined in Fig. 16. Calculated edge moments that exceed M, max should be reduced to this value and positive moments adjusted accordingly. However, in no circumstances should the calculated edge moment exceed 2M,

3C.10 Column heads Where column heads are provided, the heads of interior columns and such portions of the heads of exterior columns as will lie within the building should satisfy the following requirements: 58

IStructE RC permissible stress recommendations

-.

. . -

'

, ,i

8

(i) the angle of greatest slope of the head should not exceed 45" from the vertical (ii) the diameter of the column head, D, should be taken as its diameter measured at a distance of 40 mm below the underside of the slab or the underside of the drop where provided, as shown in Fig. 11

a b

b

4

Edge of slab

b

ba=b+d

1

.

I

L

b j be= b + y +Column strip

16 Definition of breadth of effective moment transfer strip, b,. IStructE RC pcrmissiblc strcss rccommcndations

59

(iii) the diameter, D,should be not more than O.25Lm (iv) where the column and column head are not of circular cross-section the term diameter used in this subsection should be deemed to mean the diameter of the largest circle that can be drawn within the section.

3C.11 Design of flat slabs as continuous frames 3C.ll.l General Flat slabs may be designed as continuous frames on the assumptions given in clauses 3C.11.2 to 3C.11.7. Subsections 3B.3 and 3C.3 to 3C.10 are also applicable to this method of design.

3C.11.2 Bending moments and shearing forces The bending moments and shearing forces may be determined by an analysis of the structure as a continuous frame and the following assumptions may be made: (i) the structure may be considered to be divided longitudinally and transversely into frames consisting of a row of columns and strips of slab with a width equal to the distance between the centre-lines of the panels on each side of the row of columns (ii) each frame may be analysed in its entirety, or each strip of floor and roof may be analysed as a separate frame with the columns above and below assumed fixed at their extremities. The spans used in the analyses should be the distances between the centres of the supports except where the slab is supported by a wall, when thc span should be the distance to the face of the wall plus one-half the depth of the slab.

3C.11.3 Stiffness of members For the purpose of determining the relative stiffnesses of the members, the moment of inertia of any section of a slab or column may be assumed to be that of the gross cross-section of the concrete alone based on the full width of the slab. Variations of the moment of inertia along the axes of the slabs and columns should be taken into account. The joints between the columns and slabs may be assumed to have an infinite moment of inertia. Flat slab structures should not normally be designed as unbraced frames. Where sway resistance is used, a conservative value of effective slab width should be used.

3C.11.4 Maximum bending,moments in slabs The maximum bending moments near the midspan of a slab and at the centre-line of the supports should be calculated for the following arrangements of the imposed loads: (i) alternate spans loaded and all other spans unloaded (ii) any two adjacent spans loaded and all other spans unloaded.

3C.11.5 Design moments for flat slabs The slab should be designed for the bending moments so calculated at any section, except that provision need not be made for greater negative moments than those at a distance of D/3 from the column centre-line. In all cases the sum of the maximum positive bending moment and the average of the negative bending moments used in 60

IStructE RC pcrmissible strcss rccommcndations

the design of any one span of the slab should, for the whole panel width, not be less than:

wL2 8

(.,?)

2

where w is the total load per unit area on the panel and D is the diameter of the column heads supporting the slab concerned (see subsection 3C.10). Where the diameters of the column heads supporting the slab are not equal, D should be assumed to be the average of the two diameters. The bending moments for which provision is made should be divided between the column and the middle strips in the proportion given in Table 14. Table 14 Distribution of bending moments in panels of flat slabs designed as continuous frames

apportionment between column and middle strip expressed as percentages of the total negative or positive moment* column strip

middle strip

75 55

25 45

negative moments positive moments

3C.11.6 Design moments in columns The maximum bending moments in the columns may be assumed to occur when the imposed load is applied to alternate panels. The columns should be designed to resist that combination of bending moment and direct load consistent therewith which produces the greatest stresses in a column.

3C. 11.7 Arrangement of reinforcement Curtailment of reinforcement should be determined from the analysis. I n other respects reinforcement should be arranged in accordance with subsection 3C. 16.

3C.12 Empirical design of flat slabs 3C.12.1 General This empirical method is described in subsections 3C. 13 to 3C. 17. Subsections 3C.3 to 3C. 10 are also applicable.

3C.12.2 Applicability of method The bending moments given in subsection 3C.14 apply only when conditions (i) and (ii) below are satisfied. IStructE RC pcrmissiblc strcss rccommcndations

61

(i) Limitations regarding numbers and shape of a series of panels. The slabs should comprise a series of rectangular panels of approximately constant thickness, arranged in at least three rows in two directions at right-angles, and the ratio of the length of a panel to its width should not exceed 4 : 3. The lengths andor widths of any two adjacent panels in a series should not differ by more than 15% of the greater length of width. End spans may be shorter, but not longer, than interior spans. Where adjacent spans differ, the length should always be taken as that of the longer span in calculating the bending moments. (ii) Limitations regarding drops. Drops should be rectangular on plan, and have a length in each direction not less than one-third of the panel length in that direction. For exterior panels the width of drop at right-angles to the noncontinuous edge and measured from the centre-line of the columns should be equal to one-half the width of drop for interior panels.

3C.13 Critical sections for bending moments in flat slabs For interior panels, fully continuous, the critical sections for the bending moments given in subsection 3C. 14 are as follows: (i) positive moment along the centre-lines of the panel (ii) negative moment along the edges of the panel on lines joining the centres of the columns and around the perimeter of the column heads.

3C.14 Bending moments in flat slab panels The bending moments for which provision is made should be divided between the column and middle strips as shown in Table 15, where w L2

20

8

3C.15 Widths of reinforcing bands In slabs reinforced in two directions only, the reinforcement should be so disposed that each strip is reinforced over its full width.

3C.16 Arrangement of reinforcement in flat slabs In the following L is L , or

L2

as appropriate:

(i) In each strip or band all the reinforcement for positive moments should extend in the lower part of the slab to within 0 . 2 L , and at least 40% should extend to within 0.125L of the line joining the centres of the columns. (ii) The reinforcement for negative moments in the top of the slab should extend into adjacent panels for an average distance, measured from the line joining the centres of the columns, of not less than 0.25L, and no bar should extend less than 0.2L from this line. (iii) When D is less than 0.15L, two-thirds of the reinforcement required to resist the negative moment in the column strip should be placed in a width equal to half that of the column strip and central with the column. ~

62

IStructE RC pcrmissible stress recomrncndations

(iv) The above rules may be applied where the ratio of column thickness to slab thickness exceeds the value given in Table 16 for the relevant ratio of imposed to dead load. The values given have been calculated with 1.4 x the dead load and 1.6 x the imposed load applied to alternate spans, with bottom reinforcement stressed to 0.87fy.They are based on an assumed column height 10 X its thickness; for other heights the ratios may be multiplied by ' v ( L / l O h ) , where L and h are the length and thickness of the column. If the midspan reinforcement is increased by 20%, or a light (15% of support value) top reinforcement is provided throughout the span, the limits in brackets apply. Limits for intermediate ratios of imposed to dead load may be interpolated. Where no value is quoted or where imposed load exceeds twice the dead load, reinforcement for negative moments in the span should be determined from analysis.

Table 15 Distribution of bending moments in panels of flat slabs designed by the empirical method

apportionment of moments between the column and middle strips expressed as percentages of M,,

I

column strip

middle strip

50 20

15 15

lntcrior panels with drops

negative moments positive moments without drops

45 24

negative moments positive moments column supports

wall supports

column supports

wall supports

45 25 50

6 35 72

10 19 15

6 26 22

40

6 36 65

10

22 16

6 28 20

exterior panels with drops

exterior negative moments positive moments interior negative moments withoirt drops

exterior negative moments positive moments interior negative moments

28 48

Notes I Where the column strip is taken as equal to the width of the drop. and the middle strip is therehy increased in width to a value greater than half the width of the panel. the moments to he resisted by the middle strip should he increased in proportion to its increased width. The moments to be resisted by the column strip may then he decreased hy an amount such that there is no reduction in either the total positive or the total negative moments resisted hy the column strip and middle strip together. 2 Where end spans are shorter than interior spans. the moments given in this table may he suitably modified.

IStructE RC pcrmissible stress rccommcndations

63

column above and below I drops

column below only no drops I drops

1.5 (1.2) 1.8 (1.4) 2.1 (1.6)

1.8 (1.4) 2.1 (1.7) 2.5 (1.9)

no drops

w,= %W, w,= w, w,= 2w,

1.9 (1.4) 2.3 (1.7) 2.8 (2.0)

2.2 (1.7) 2.7 (2.0) 3.3 (2.4)

3C.17 Bending moments in columns External columns should be designed for the total negative moment at the edge specificed in subsection. 3C.14. Internal columns should be designed for the difference in total negative moment between adjacent spans calculated with dead load on one span and both dead and imposed load on the other. (i) These moments should be apportioned between the upper and lower columns in proportion to their stiffness. I n internal columns, the direct load acting with the moment may be reduced to allow for the panel on one side being free of imposed load. (ii) I n the case of external columns carrying portions of the floors and walls as a cantilevered load, the specified column moments may be reduced by the moment arising from the dead load on the cantilevered portion.

3D STAIRS

3D.1 Distribution of loading on stairs In the case of stairs with open wells, where spans partly crossing at right-angles occur, the load on areas common to any two such spans may be taken as one-half in each direction as shown in Fig. 17. Where flights or landings are built into walls a distance of not less than 100 mm and are designed to span in the direction of the flight, a 150 mm strip may be deducted from the loaded area and the effective breadth of the section increased by 75 mm (see Fig. 18).

3D.2 Effective span of stairs The effective span of stairs without stringer beams should be taken as the following horizontal distances: (i) where supported at top and bottom risers by beams or walls spanning parallel with the risers, the distance centre to centre of supports. (ii) where spanning on to the edge of a landing slab that spans parallel with the risers (see Fig. 19) a distance equal to the ‘going’ of the stairs plus, at each end, either half the width of the landing or 1 m, whichever is smaller. (iii) where the landing slab spans in the same direction as the stairs, they should be considered as acting together to form a single slab and the span determined as in (i) of this subsection, the going being measured horizontally.

I

64

IStructE RC permissible stress rccommcndations

I I

I I

UP

I SI E

W

Loading

I

I

Down

I

I I

I.

-4

L w 1 2

I

-L

The load on areas common to two systems to be taken a s one half in each direction

Loading

I7 Loading on stairs with open wells

18 Loading on stairs built into walls IStructE RC pcrrnissiblc strcss rccornmcndations

65

Going (G)

X I

-L

-

Y

I

I

L--+--

Max 1M

Max 1M Effective span

19 Effective span for stairs sirpportcd at cwch cwcl by lanrlings spanning purallrl with the risers

3E COLUMNS

3E.1 Reinforcement in columns 3E. 1.1 Longitudinal reinforcement A reinforced concrete column should havc longitudinal steel reinforcement, and the cross-sectional area of such reinforcement should not be less than 0.8% nor more than 8% of the gross cross-sectional area of the column required to transmit all the loading in accordance with these recommendations. I t should be noted that the use of 8% of steel may involve serious practical difficulties in the placing and compacting of concrctc, and a lower percentage would be recommended. Where bars from the column below have to be lapped with those in the column, the percentage of steel should usually not excecd 4%. A reinforced concrete column having helical rcinforccmcnt should have at least six bars of longitudinal reinforcement within this helical reinforcement. The longitudinal bars should be in contact with the helical reinforcement and equidistant around its inner circumference. For laps in longitudinal bars see subsection 3A. 12. The bars should be not less than 12 mm in diameter.

3E. 1.2 Transverse reinforcement (i) General. A reinforced concrete column should have transverse reinforcement so disposed as to provide restraint against the buckling of each of the 66

IStructE RC pcrmissiblc strcss rccommcndations \

longitudinal reinforcing bars. Every corner bar and each alternate bar in a column near the face should be properly linked by having at least one link with a change of direction at that bar. The ends of such transverse reinforcement should be properly anchored. No longitudinal bar within a compression zone should be further than 150 m m from a restrained bar. (ii) Pitch. The pitch of transverse reinforcement should be not more than the least of the three following distances: (a) the least lateral dimension of the column (h) 12 X the diameter of the smallest longitudinal reinforcement in the column ( c ) 300 m m .

(iii) Helical reinforcemenr. Helical reinforcement should be of regular formation, with the turns of the helix spaced evenly, and its ends should be anchored properly. Where an increased load on the column on account of the helical reinforcement is allowed for under subclause 3E.2.l(ii), the pitch of the helical turns should be not more than 75 m m or more than one-sixth of the core diameter of the column, nor less than 25 m m nor less than 3 X the diameter of the steel bar forming the hclix. In other cases the requirements of (ii) above should be met. (iv) Diameter. The diameter of the transverse reinforcement should be not less than one-quarter the diamctcr of the main rods, and in no case less than 5 mm.

3E.2 Permissible loads on columns 3E.2.1 Axially loaded columns The capacity of an axially loaded column is kf,, where k is a factor to cover the effects of buckling (see Table 17) and f,,is the section capacity calculated as in (i) or (ii) below: ( i ) Columns with lateral ties. For a column reinforced with longitudinal bars and lateral ties: where p,, is the permissible compressive stress for the concrete A, is the gross cross-sectional area of concrete p,, is the permissible compressive stress in the reinforcement A,, is the cross-sectional area of the longitudinal steel. (ii) Columns with helical reinforcement. The permissible axial load f, on a column reinforced with helical reinforcement is that given by the greater of equations (24) or (25): where A k is the gross-sectional area of concrete in the core A h is the equivalent area of helical reinforcement (volume of helix per unit length of the column) f,,calculated from equation (25) should not exceed 0.5 fc,Ac.

3E.2.2 Columns subject to both direct load and bending The permissible direct load is kf, and the permissible bending moment k M , IStructE RC pcrmissiblc strcss rccommcndations

67

where k is a factor to cover the effects of buckling (see Table 17) P, is the capacity for direct load coexisting M, is the capacity for bending moment

}

The column should be designed to have a load factor generally of 1.8; however in the strength calculations the cube strength should be taken as 75% of the actual cube strength. This allows for the difference in likely strength between concrete in the structure and that in wet-cured laboratory test cubes. I t should be assumed that the maximum concrete strain in compression does not exceed 0.35% at failure; that the compressive stress distribution on the concrcte at failure is rectangular, parabolic or such other shape as is shown by tests to be reasonable; and that the maximum stress in the concrete at failure does not exceed two-thirds of the cube strength of the concrete. I t is also necessary to see that the stresses at working loads are not such as to cause excessive cracking. These requirements will be satisfied for symmetrical columns of rectangular section with longitudinal reinforcemcnt in the two faces parallel to the axis of bending if the following procedure is adopted: (i) assume a depth to the neutral axis d, (ii) assume concrete stress is uniform (= pcc)over a depth of 0.9dn (iii) calculate stress in reinforcement as

(T) + 375 (T) + 375 (T) d' 385 (T)

*

d-d,

fbI

= - 385

or

f,r = or

d, - d

+ pbc(compression)

d,

+ plc (compression)

d'

- d,

+ p5, (tension)

-

(iv) P, = 0.9pC,bd,

+

Vbl

+ fd) 2

A,

(y) + *{ (Th - 0.9d

( v ) M, = 0.9p,,bdn

ps, (tension)

h

A

f,,

h

d ) +f,?

( T - d.,>,,7)

where d, is the depth to neutral axis from the compression face d is depth to the reinforcement farthest from the compression face d' is depth to the reinforcement nearest the cornpression face A, is the area of longitudinal reinforcement f,l is the stress in steel layer farthest from the comprcssion face f,* is the stress in steel layer nearest the compression face f, is the permissible direct load M, is the permissible moment. Note that in equations (26) and (27) 0.9pc,bd, first term should not be taken as less than zero. 68

#

pobh and in equation (27) the

IStructE RC pcrmissiblc strcss rccommcndations

.

. . . .L

_. . ._ ._ .. - 1. a

.

.

.

. . .. ..

.

.

.

.

.

. . ..

...

4,

.‘

..

-. :

’

Columns subjected to biaxial bending may be designed on the basis of calculations or design charts based on the above theory or the following equation:

(2,” (S,” +

M , and M y are the applied moments and M,, and M y , the corresponding moment capacities for uniaxial bending. O( may be taken as I where PIP, 0.2 and 2 where PIP, 2 0.8, with intermediate values obtained by interpolation. PO is the capacity of the column section in pure compression ( P O = pccAc+ pbcAbc). See clause 3E.2.5 for values of k . For convenience, equation (28) is shown graphically in Fig. 20.

20

3E Alternatively, column design may be based on elastic theory with a modular ratio of 15 and the permissible stresses given in subsections 3A.6, 3A.7 and 3A.8. The direct load on an eccentrically loaded column should not exceed that permissible for an axially loaded column.

33.2.4 Overturning Where appropriate, the factor of safety against overturning should be checked in accordance with subsection 3A.4. IStructE RC permissible stress recommendations

69

.

.

..,

.

.

,.

. - ..

... . .

__

3E.2.5 Reduction coefficients for columns (i) The permissible direct load and bending moments calculated for a column should be reduced by the appropriate factor from Table 17 to allow for buckling effects based on the ratio of effective column length to least lateral dimension (for rectangular columns) or radius of gyration (for other crosssections). Where the permissible load on a helically reinforced column is based on the core area, the least lateral dimension should be taken as the diameter of the core. Where column proportions comply with the limits for slender beams (see clause 3B.1.2), the value of k applied to moment resistance may be taken as that corresponding to the slenderness ratio in the plane of bending. Thus k for major axis bending may be based on Llh,,,,, and k for minor axis bending based on a L/hm,i,l, where h,,;,, and hmi, are the larger and smaller column dimensions, respectively.

Table 17 Reduction coefficient k for columns Effective length + least lateral dimension

20

0 17 34 52 69 86 104 121 138 172 207

0 5 10 15 20 25 30 35 40 50 60

1.oo 0.95 0.89 0.81 0.69 0.56 0.45 0.34 0.26 0.13 0

50 1 .oo

0.95

0.95 0.85 0.71 0.56 0.43 0.32 0.23 0.18 0.09 0

Effective column length

Type of column

0.75 L

( a ) Properly restrained at both ends in

position and direction (b) Properly restrained at both ends in position and imperfectly restrained in direction at one or both ends (c) Properly restrained at one end in position and direction and imperfectly restrained in both position and direction at the other end 70

Cocfficient concrete grade

Effective length + least radius of gyration

A value intermediate between 0.75L and L depending on the efficiency of the dircctional restraint A value intermediate between L and 2L depending on the efficiency of the imperfect restraint

IStructE RC pcrmissible strcss rccommcndations

Where, due to bending, the maximum moments on the column occur at the ends of the column and the effective length is in category ( a ) or (b) of Table 18 the permissible load may be determined without reference to the reduction coefficient for sections within one-quarter of the column length L from the centre-line of the beams. Where a column is in category ( c ) , the eccentricity, e , of the applied load at the restrained end should be increased by additional eccentricity L PccA ce where - 5 15 = - 0.25h h P,, L where =0 e;ldd = 0 h where h is the depth of the section in the plane of bending Intermediate values may be interpolated. (ii) Determination of effective length. Effective length should be determined from Table 18 where L is the length of the column from floor to floor, or between adequately restrained supports. The effective column length values given in this Table refer to typical cases only and embody the general principles that should be cmploycd in assessing thc appropriate valuc for any particular column.

-

33.2.6 Bending moments in columns Bending moments in internal columns supporting an approximately symmetrical arrangement of beams and loading need not be provided for except in the case of flat slab construction (see section 3C). Bending moments in external columns and in internal columns supporting an arrangement of beams and loading not approximately symmetrical should be calculated and provided for. The expressions given in Table 19 may be used for estimating the moments. Table 19 Moments in columns

Moments for frames of one bay

Moments for frames of two or more bays

External (and similarly loaded) columns

Moment at foot of upper column

Moment at head of lower column Internal columns

Moment at foot of upper column

Moment at head of lower column

IStructE RC pcrmissiblc strcss rccommcndations

71

is thc bending momcnt at thc cnd of thc bcam framing into thc column, assuming fixity at both cnds of thc bcam M,, is thc maximum diffcrcncc bctwccn thc niomcnts at thc cnds of thc two beams framing into opposite sidcs of thc column, cach calculatcd on thc assumption that thc cnds of the bcams iirc fixcd a n d assuming onc of thc bcams unloadcd Kh is thc stiffncss of thc bcam K b l is the stiffness of thc bcam on one sidc of thc column Kb2 is thc stiffness of thc bcani on thc othcr sick of thc column is the stiffncss of thc lowcr column KI is thc stiffncss of thc uppcr column. K, For the purposcs of this Tablc, thc stiffncss of ii mcmbcr may bc obtaincd by dividing the momcnt of inertia of a cross-scction by tlic lcngth of thc mcmbcr, providcd that the mcmbcr is of constant cross-section throughout its Icngth. The equations for thc momcnt a t thc hcad of tlic lowcr column may bc uscd for columns in a topmost storey by taking K,, as zcro. Whcrc the bcnding momcnt is calculatcd in tlic intcrnal columns it is pcrniissiblc to take into account thc rcduction in dircct loxl rcsulting from thc bcam on onc sick of the column being fully loadcd and thc bcam o n the othcr sidc hcing londcd with dead load only. The cffects of shcar arc not gcncrally significant in column dcsign. Howcvcr i n some circurnstanccs thcy may warrant considcration. whcrc M,

3F REINFORCED CONCRETE WALLS 3F.1 General Sections with a Icngth/thickncss ratio of grcatcr than 4:l should bc treatcd as reinforccd concretc walls (as opposed to columns) providcd t h a t thcy havc thc minimum reinforcement required to comply with this clausc. Wherc reinforced concrete walls arc intcndcd to carry vcrtical loads, thcy should be designed gcnerally in accordancc with the rccommcndations given for columns. The cross-sectional area of thc vcrtical rcinforcemcnt may howcvcr be rcduccd to not less than 0.4% and the lateral rcinforcemcnt parallel to thc wall facc to not less than 0.25% (high-yield steel), 0.3% (mild stccl). Wherc the vertical bars arc not assumcd to assist in rcsisting compression. tlic provisions of clausc 3E.1.2 with regard to transverse rcinforccmcnt to prevent buckling need not be takcn to apply, and the minimum vcrtical rcinforccmcnt may be reduced to 0.25% (high-yicld stccl), 0.3% (mild stccl). The minimum pcrcentages of reinforccmcnt specified in thc subscction may not always be sufficient to providc adequate rcsistancc to thc effects of temperature and shrinkage. These factors should be takcn into account if thcy iirc liable to bc significant. In general, the wall thickness should not be less than 100 mni.

3F.2 Permissible loads The permissible load on any storey height should be calculated in thc general manner specified for columns (omitting, however, the contribution of the vcrtical reinforcement in compression if transverse reinforcement satisfying clausc 3E. I .2 is not provided). 72

IStruetE RC permissible strcss rccommcndations

The effective height of the wall may be determined as for columns in accordance with subclause 3E.2.5(ii). Where, as may occasionally happen, the wall is stiffened by closely spaced crosswalls such that the length of wall between adjacent crosswalls is less than the effective height, the slenderness ratio may be asumcd to be the ratio of this length to the wall thickness.

3F.3 Walls subjected to concentrated loads Additional stresses of a purely local nature, as at girder bearings, column bases, lintels or other Concentrated loads are to be calculated, and the maximum stress resulting from these and other loads should not exceed the permissible compressive stress by more than 50%.

3F.4 Shear stresses Shear stresses need not be checked where the applied shear force is less than onequarter of the vertical compression on the wall. Otherwise, the recommendations of subsection 3B. 10 apply.

3G BASES AND PILE CAPS 3G.1 Bases for reinforced columns and walls 3Gl.l Bending moments in bases The bending moments at any section of a base for a reinforced concrete column or wall should bc taken to be the moment of the forces over the entire area on one side of the section. The critical section for bending in the base should be taken at the face of the column or wall.

I

I

3G.1.2 Reinforcement in bases

If the breadth of thc section exceeds 1S(c + 3 4 , where c is the column width, twothirds of the reinforcement provided to resist the bending moment should be concentrated in the central half width of the base beneath the column. Otherwise it may be spaced uniformly.

I

,

3G.1.3 Shear The critical sections for shear are: (i) a line across the width of the base, taken 1 l/r X the effective depth from the column face (ii) a punching pcrimeter 1% X the effective depth from the column face, with stresses assessed in the same manner as in flat slabs (sec subsection 3C.6). Where the shear stress does not exceed p v , no shear reinforcement is necessary.

, I

3G.1.4 Bond in reinforcement of bases Bond stresses should be checked in accordance with clause 3A.12.3. Anchorage bond should also be checked. Due care should be taken in the size of reinforcement chosen so that the anchorage bond length can be provided, and this may involve extending the bars through 90" up the face of the base. IStructE RC pcrmissiblc stress rccommcndations

73

3G1.5 Pockets for precast members Pockets are often employed in bases for prccast construction, a wedge-shaped pocket being formed in the base plan area such that a column can be inserted and grouted in. The following points should be considered (see Fig. 21):

X

t

m Plan

Section X - X

Section V - V

21 Base with pocket for precast column

(i) The walls surrounding the pockets should be sufficiently thick and adequately reinforced to avoid bcing damaged by wedges driven in between the precast column and the wall to secure the column. (ii) Dimension t should be a minimum of 75 mm to allow efficient grouting with a concrete composed of cement, sand and small aggregate. (iii) The wedge should be bevelled at such an angle as to carry in vertical compression the load from the column. If this cannot be achieved then the concrete in the base below the column must be checked for punching shear. (iv) The whole base should be reinforced as though it was monolithic to accept the critical bending moments and shears described in clauses 3G.l.l and 3G. 1.3.

3G.1.6 Mass concrete bases For an unreinforced concrete pad or strip footing the projection of the foundations beyond the wall or column face should not normally exceed the depth of the concrete in the foundations. 74

IStructE RC pcrmissiblc strcss rccommcndations

3G.2 Pile caps These should be designed in a similar manner to pad bases except: (i) I f pile spacing exceeds 3 pile diameters, tension reinforcement should be concentrated in bands 3 pile diameters wide across the pile heads. (ii) Where pile spacing does not exceed 3 x the pile diameter, the permissible shcar stress may be enhanced in accordance with subsection 3B.11. Where the spacing exceeds 3 pile diameters. this enhancement should be limited to strips 3 pile diameters wide which cross the pile heads. a, should be measured from the column face to one-fifth of the pile diameter inside the pile face. The dcsign of pile cap reinforcement may be carried out using truss analogy assuming that all tensile forces are carried by reinforcement determined in accordance with subsections 3A.7 and 3A. 12. The compression in the concrete should not exceed the permissible stresses in subsection 3A.6.

3H REINFORCED LIGHTWEIGHT AGGREGATE CONCRETE 3H.1 General The recommendations of all the previous clauses in Section 3 apply to lightweight aggregate concrete except for the modifications required in subsections 3H.2 to 3H.7 inclusive. In considering lightweight aggregate concrete the properties for any particular type of aggregate can be established far more accurately than for most naturally occurring materials, and the engineer should therefore obtain specific data direct from the aggregate producer in preference to using tabulated values taken from British Standard Codes of Practice or British Standard Specifications.

3H.2 Permissible stresses in reinforced lightweight aggregate

concrete The permissible stresses where lightweight aggregates are used are: (i) compressive stresses: as for normal-weight aggregate concrete (ii) shear stresses: for concrete grades of 25 or more, shear stresses should be 80% of those in subsection 3B.10; for concrete grade 15, shear stresses should be limited to those in Table 20; stresses for grades between 15 and 25 may be interpolated Table 20 Limitation of shear stress in lightweight aggregate concrete p, - grade 25 for effective depth of member, d

P V

100 A,,lbd 60. I 5 0.25 0.50 0.75 1 .oo 1S O 2.00 33.00

grade 15

0.09 0.10 0.11 0.12 0.13 0.14 0.15 0.17

6125

150

175

200

225

250

300

3400

0.22 0.26 0.33 0.38 0.42 0.49 0.53 0.61

0.21 0.25 0.32 0.37 0.40 0.46 0.51 0.58

0.21 0.25 0.31 0.35 0.39 0.45 0.49 0.57

0.20 0.24 0.30 0.34 0.38 0.43 0.47 0.54

0.19 0.23 0.29 0.33 0.37 0.41 0.46 0.53

0.19 0.22 0.28 0.33 0.36 0.41 0.45 0.51

0.18 0.21 0.27 0.31 0.34 0.39 0.43 ,0.49

0.17 0.20 0.25 0.29 0.32 0.36 0.40 0.45

IStructE RC pcrmissiblc stress rccommcndations

75

- __.

-

(iii) bond and laps: bond stresses should be limited to 80% of those given in subsection 3A.6; for foamed slag or similar aggregates stresses may have to be restricted further where reinforcement is in a horizontal position during casting, and the design should be based on test data (iv) bearing stress inside bends: stress should be limited to two-thirds of the value given in clause 3A. 12.4.

3H.3 Deflection and stiffness Where the imposed load on slabs does not exceed 4kN/mZ, the spardeffective depth limits in clause 38.2.2, Table 6C may be applied. Where the imposed load exceeds 4kN/m2, the limits should be taken as 93% of the relevant values in Table 6C. Beams should be based on clause 3B.2.1, with basic spanleffective depth ratios reduced to 85% of those given in Table 6.

3H.4 Permissible loads on columns Design should be in accordance with subsection 3E.2 except that the reduction factors in Table 17 should be replaced by those in Table 21, which have been prepared for concrete of a density of 1800 kg/m3. Where the density is less than this, the reduction coefficient should be based on a slenderness ratio equal to the actual ratio multiplied by (1800/dcnsity).

3H.5 Reinforced concrete walls Design should be in accordance with section 3F except that reduction factors should be as in Table 21. Table 21 Reduction coefficient for columns of lightweight aggregate concrete

Effective length t least radius of gyration

Effective length t least lateral dimension

20

30

0 17 34 52 69 86 104 121 138 172

0 5 10 15 20 25 30 35 40 50

1.oo 0.94 0.87 0.71 0.55 0.41 0.27 0.19 0.12 0

1.oo

1 .oo

1 .oo

0.92 0.83 0.66 0.49 0.35 0.23 0.16 0.11 0

0.91 0.82 0.63 0.46 0.32 0.21 0. I5 0.09 0

0.90 0.81 0.59 0.42 0.28 0. I9 0. I3 0.08 0

40

50

~

3H.6 Modular ratio Where the design calculations for the strength of a member are bascd on the elastic theory, the modular ratio for lightweight aggregate concrete should be assumed to be 15. However, calculation of the stiffness of members should be based on a modular ratio of 30.

3H.7 Cover Cover should be 10 mm greater than that specified in subsection 35.1. 76

IStructE RC pcrrnissiblc strcss rccornrncndations

3J DURABILITY AND RESISTANCE TO CHEMICAL ATTACK 3J.1 Durability 35. I. 1 General In a rcinforced concrete design one of the most important aims is the production of a durable structure. To produce it requires the integration of all aspects of design, materials and construction. Much of the damage to reinforced concrete structures arises from water penctration to the reinforcement causing rusting and subsequent cracking and spalling of the concrete cover, but the effects of other potentially deleterious substances need also to be considered. To prevent such damage, it is necessary that the cover specified is adcquatc for the conditions applying, that the concrete is of a suitable quality and that it is correctly placed to give a dense impermeable whole.

35.1.2 Drainage Care should bc taken that surfaces exposed to water are laid to adequate falls or other appropriate measures taken to avoid ponding.

35.1.3 Waterlcement ratio For low permeability, it is necessary that a concrete mix should have an adequate cement content and a sufficiently low waterlcement ratio and be fully compacted.

35.1.4 Chloride content of mixes Chlorides in concrete increase adversely affect the sulphate content of the constituents of cement (including ggbfs or pfa

the risk of corrosion of embedded metals and may resistance of the concrete. The total chloride-ion each mix, expressed as a percentage by weight of if used) in the mix, must not exceed the following:

concrete made with cement complying with BS 4027 or BS 4248 concrete containing embedded metal and made with cement complying with BS 12, BS 146, BS 1370, BS 4246 or combinations with ggbfs or pfa

0.2% 0.4%

Calcium chloride and chloride-based admixtures should never be added to reinforced concrete, prestressed concrete and concrete containing embedded metal.

35.1.5 Admixtures Where admixtures are used their effect on the durability of the concrete and the risk of corrosion of the reinforcement should be considered. The chloride-ion content of admixtures must not exceed 2% by mass of the admixture or 0.03% by mass of the cement.

35.1.6 Air entrainment Air-entraining agents are of value where severe frost conditions are likely to occur. The agent and dosage used should be such that the air content can be readily maintained within the limits specified at the time of placing. When concrete lower than grade 50 is used, the average air content by volume of the fresh concrete at the time of placing should be: IStructE RC pcrmissiblc strcss rccommcndations

17

7% 6% 5% 4%

for for for for

10 mm 14 mm 20 mm 40 mm

nominal nominal nominal nominal

maximum sized aggregate maximum sized aggregate maximum sized aggregate maximum sized aggregate

All concrete lower than grade 50 should contain appropriate amounts of entrained air where surfaces are subject to the effects of de-icing salts.

35.1.7 Requirements for durability of concrete Table 22 gives recommended maximum free waterkement ratios and minimum cement contents for various conditions of exposure and nominal cover for reinforced concrete using 20 mm nominal sized aggregate. Subsection 35.2 gives rccommendations for concrete mixes used below ground. Table 22 Durabilitv and concrete cover

qominal cover to all reinforcement for durability, mm

Conditions of exposure Mild

(Internal concrete)

25

20

20*

20%

-

35

30

25

-

-

40

30

-

-

50

40

0.65 275

0.60 300

0.55 325

0.50 350

Moderate

(Sheltered from severe rain and against freezing while saturated with water) Severe

(Exposed to driving rain, alternate wetting and drying, occasional freezing or severe condensation) Very severe

(Exposed to seawater spray, de-icing salts, corrosive fumes and severe freezing while wet) Maximum free waterkement ratio Minimum cement content, kg/m3 *

These covers may be reduced to 15 mm provided that the nominal maximum size of aggregate does not exceed 15 mm.

Notes: I For work against earth faces. the cover should not he less than 40 mm for all reinforcement in concrete cast against forms or protected hy blinding. For concrete cast directly against earth filces. cover should not he less than 75 mm. 2 Where concrete is subject to severe freezing when wet. air entrainment should be used. 3 Where the face of the concrete is protected hy a suitahle coating, it may he reasonahle t o vary these recommendations. 4 I n no case should the cover to the main hars be less than the diameter of such reinforcement.

Table 23 gives adjustments to minimum cement contents where other sized aggregates are used. However, any adjustment made is subject to the condition that the minimum cement content should not be less than 240 kg/m3. 78

IStructE RC pcrrnissiblc strcss rccornrncndations

Table 23 Adjustment to cement content for different sized aggregate Nominal maximum aggregate size

mm 10 14 20 40

Adjustment to minimum cement content in Tables 22 and 24 kg/m3

+40 20 0 -30

+

The above requirements have been found to be adequate for many common structures and circumstances. However where particular conditions might be relevant or an unusual design life is required, consideration may be given to such factors as concrete mix design, admixtures, cement replacements and enhancers, special surface treatment of steel, stainless steel and surface treatment or protection of the concrete surface. The influence of wear on the concrete surface should also be considered where relevant.

35.1.8 Reactive aggregates - alkali-silica reaction Some aggregates containing particular varieties of silica may be susceptible to attack by alkalis (Na20 and K20)originating from cement or other sources, producing an expansive reaction that can cause cracking and disruption of concrete. There are at present no British Standard tests for the reactivity of aggregates with alkalis. Damage to concrete from this reaction will normally occur only when all the following are present together: (i) a sufficiently strong alkaline pore solution (ii) a proportion of reactive silica in the aggregate lying within the sensitive range (iii) sufficient moisture in the concrete. Where there is a local history of alkali-silica reaction, an aggregate free from an alkali-reactive constituent should be used where possible. Where this cannot be achieved appropriate procedures need to be taken to avoid the conditions necessary to produce the reaction. Guidance is given in: Concrete Society report no. 30 BRE digest no. 330

35.1.9 Finishing and curing Good finishing practices are essential for durable concrete. Overworking the surface and the addition of water as an aid to finishing should be avoided: the resulting laitance will have impaired strength and durability, and will be particularly vulnerable to freezing and thawing in wet conditions and to the action of de-icing salts. The permeability of concrete will be reduced and its durability, enhanced the greater the extent of hydration of the cement, particularly in the surface zone of the concrete; thus i t is essential to use proper and adequate curing techniques (see Section 5).

3J.2 Resistance to chemical attack Concrete structures are susceptible to chemical attack either in the ground or from environmental pollution. Chemical reagents such as vegetable oils, fats and sugar IStructE RC pcrmissiblc stress rccommcndations

79

solutions, which are found in many industrial processes, also slowly attack Portland cement concrete. The most common chemicals in the ground or groundwater are sulphates, which may be of sodium, potassium, calcium or magnesium, and these can cause severe damage if appropriate precautions arc not taken. Scvcre pollution can be encountered when building on sites previously used for industrial purposes, or filled sites. Careful consideration must be given during the design proccss to assessing potential risks and specifying that a concrete is used capable of providing the required durability. Thinner sections are more vulnerable to damage from cheniical attack than thicker sections. Where high concentrations are present, i t may be necessary to provide a protectivc coating to the concrete, e.g. floors or foundations. Where permanently acidic conditions of pH value of 5.5 or less exist, the use of Portland cement in thc concrete is not recommended. Concretes containing supersulphated cements or mixes including ccmentitious materials containing slag or pfa can have some acid-resisting properties, but each situation requires its own individual solution. The choice of aggrcgatc may also be affcctcd by the type of chemical attack expected. Table 24 indicates thc rcquircments for concrete cxposcd to sulphate attack. In all cases it is essential that the concrctc is fully compaetcd and that adequate cover is provided (see Section 5). Specialist advice should be sought whcrc ncccssary.

3K RESISTANCE TO FIRE

3K.1 General

I

The following text gives simple tabular methods for checking that a structure has adequate resistance to fire for the required pcriods. Other methods of assessment may be used, such as the direct application of the rcsults of fire-resistancc tests, or calculations based on the reduced strength of steel and concrete at elevated temperatures.

3K.2 Robustness In the consideration of fire resistance, such factors as robustness, continuity and availability of alternative paths of support are important.

3K.3 Elements exposed to fire l

In considering the elements of structure, the surfaces normally considered to be exposed to fire are: walls: slabs: beams: columns:

one side soffit sides and soffit all sides fully exposed, or one or more sides depending on degree of protection afforded by adjacent walls.

In special circumstances it may be necessary to consider more adverse exposure conditions for walls.

3K.4 Aggregates Concretes made with lightweight aggregates have better fire resistance than normalweight aggregate concretes, and rarely exhibit spalling. Calcareous (limestone) 80

IStructE RC pcrmissiblc strcss rccommcndations

y

C

-

F m

C

c

-

-e

t

C a a c L

2

=

b-,

U

L

3

6

w

P

E

EL

E

c

e, x

e, v.

m

'c:

.c c

-a

r, L

0

d

e E

0 J

C

e

Y

3

3

N L

e,

I

IStructE RC permissible stress recommendations

5

81

aggregates are superior to siliceous (flints, quartzites, granites), but data on their relative performance are not available except for columns, where reductions in cover are possible.

3K.5 Average cover to main bars The cover to the main reinforcement should te taken as the minimum average cover measured from the surface exposed to fire, considering the areas of individual bars in each layer and their respective covers.

3K.6 Cover to secondary bars Where cover to all reinforcement including links exceeds 40 m m (normal-weight concrete) or 50 mm (lightweight concrete), measures should be taken to avoid spalling. A mesh of light supplementary reinforcement may be used, placed with 20 mm cover to the concrete face; however, this should not be used where durability problems could result and unless particular precautions are taken to maintain the reinforcement in its correct position during concreting. Consideration should be given to the use of additional protection.

3K.7 Contribution to cover of additional protection The following materials may be used in applications up to 25 mm total thickness. The tabulated concrete covers may be reduced by the following proportions of the thickness of additional protection: mortar and gypsum plaster lightweight plaster and sprayed . - lightweight - insulation I

vermiculite slabs

{0.6 x 1.0 x 2.0 x 1.0 x 1.5 x

thickness thickness up to 2h thickness above 2 h thickness up to 2 h thickness above 2 h

For example, 15 mm of gypsum plaster reduces the concrete cover requirement by 0.6 x 15 mm = 9 mm. 15 m m of sprayed lightwcight insulation reduces the cover requirement by 1 x 15 mm = 15 mm for ratings up to 2 h and 2.0 x 15 = 30 mm for ratings above 2 h.

3K.8 Floor thickness For a solid floor, the thickness may be deemed to be the actual thickness plus any screed or incombustible finishes. For hollow slabs with filler blocks, the effective thickness should be taken as the actual thickness X the proportion of solid material per unit width, plus the thickness of incombustible finishes. For ribbed slabs, the thickness should be taken as thc topping thickness plus the thickness of incombustible finish on the top.

3K.9 Beam widths The width of the beam rib should be taken as the width at the level of the lowest reinforcement. For I-sections the web thickness should not be less than half the appropriate tabulated beam width.

3K.10 Beam and ribs Ribs should not be placed at centres exceeding 1.5 m unless the tabular requirements for beams are satisfied. 82

IStructE RC pcrmissiblc stress rccommcndations

. _

. .

Table 25 Fire resistance of reinforced concrete Minimum dimensions mm, excluding any finish, for a fire resistance of Nature of construction and materials

l h

--

%t 2h

3h

4h

400 35 320 35

450 35 360 35

Fully exposed (a) Normal-weight

concrete (b) Lightweight concrete

width cover width cover

150 200 250 20 25 30 150 160 200 20 20 25

width cover width cover

125 20 125 20

160 200 200 300 25 25 30 25 130 160 185 250 20 25 30 25

350 35 275 30

thickness cover thickness cover

100 20 100 10

120 25 100 20

140 25 115 20

240 25 190 25

width cover width cover

80 20 80 15

120 30 100 20

150 200 240 70 40 60 I30 160 200 35 55 45

width cover width cover

80 20 60 15

80 20 80 20

120 35 90 25

150 200 240 50 70 60 110 150 200 45 35 55

thickness cover thickness cover

75 15 70

95 20 90

15

15

110 25 I05 20

I25 35 115 25

I50 45 135 35

170 55 1-50 45

thickness cover thickness cover

75

95 20 90 15

I10 20 105 20

125 25 I15 20

150 35 135 25

I70 45 150 35

300 35 240 35

50% exposed (a) Normal-weight

concrete (b) Lightweight concrete One face exposed ( a ) Normal-weight concre te (b) Lightweight concretc Reinforced concrete (simply supported) ( a ) Normal-weight concrete (6) Lightweight concrete Rein forced concrete (continuous) (a) Normal-weight concrete (b) Lightweight concrete Reinforced concrete (simply supported) ( a ) Normal-weight concrete (b) Lightweight concrete Reinforced concrete (continuous) ( a ) Normal-weight concrete (b) Lightweight concrete

IStructE RC pcrmissiblc strcss rccommcndations

15

70 15

-

-

-

160 200 25 25 130 I60 25 25

- -

-

280 80 250 65

83

, .

Table 25 Fire resistance of reinforced concretc continued)

Minimum dimensions m m , excluding any finish, for a fire resistance of

-

Nature of construction and materials Reinforced concrete (simply supported) ( a ) Normal-weight concrete

'/2

h

l h

70 75

90

thickness width cover thickness width cover

70 60 15

90 25 85 75 25

thickness width cover thickness width cover

70 75 15 70 70 15

90 80 20 85 75 20

150

150

1% steel Normal-weight aggregate thickness cover (concrete density 2400 kg/m3)

100 25

I20 25

More than 1% steel Normal-weight aggregate thickness (concrete density cover 2400 kg/m3)

75 15

(6) Lightweight concrete

Reinforced concrete (continuous) (a) Normal-weight concrete (b) Lightweight concrete

Less that 0.4% steel Normal-weight aggregate thickness

Lightweight aggregate (concrete density 1200 kg/m3) (Note: intermediate densities may be interpolated.)

thickness cover

15

-

75 15

I

2h

3h

4h

105 110 35 95 85 30

1 I5

125 45 100 100 35

135 150 55 1 I5 125 45

150 I75 65 130 150 55

I05 90 25 95 80 25

1 I5 110 35 100 90 30

135 125 45 115 100 35

150 150 55 130 125 45

140 25

160 200 25 25

240 25

-

-

I00 20

100 20

150 25

180 25

130 25

160 25

190 25

'/2

-

-

100 20

II

175 200

-

100 10

~

115 20

-

IStructE RC pcrmissiblc strcss rccommcndations

I

3K.11 Continuity Where provision is made for fixity in the resistance to normal loads by the presence of reinforcement properly detailed and adequately anchored into adjacent members, the tabulated data relevant to continuous structures may be used.

3K.12 Use of tabular data (Table 25) (i) Where a column is built into walls, the walls should have at least the same fire resistance as the column and extend to the full column height. They should be imperforate, except for external walls where openings should not occur within a minimum width of 600 m m each side of the column. (ii) The tabulated cover data given relate to members of the minimum width tabulated. Where greater widths are employed, the cover to main reinforcement may be reduced in accordance with Table 25A, but in no case should the cover provided be less than that given for plain soffit slabs, or walls (as appropriate), having the same fire resistance. Table 25A Decrease in cover for given increase in width Minimum increase in width mm

Decrease in cover Normal-weight Lightweight concrete aggregate concrete mm m,"

25 50 100 2 150

15

20

3L STABILITY AND DISPROPORTIONATE COLLAPSE 3L.1 Stability 3L. 1.1 General The ovcrall stability of the building including the stability during the period of construction should be considered in the design, together with the compatability of the design and details of the parts and components. This is particularly important where different engineers are involved in the preparation of the design and details.

3L. 1.2 Planforrn I n addition to designing the building to support loadings arising from normal use, there should be a reasonable probability that it will not collapse disproportionately IStructE RC pcrmissiblc strcss rccommcndations

85

or progressively under the effects of misuse or accident. N o structure can be expected to withstand the effects of high explosive, set deliberately, or to be fully resistant to the excessive loads and forces that could arise from an extreme overload condition, but it should not be damaged to an extent that is disproportionate to the original cause.'The layout of the structure on plan, returns at the ends of walls, interaction between intersecting walls, interaction between walls and other parts of the structure and the general strength and detailing of the structural components and their interconnection should be fully considered to provide a robust and stable design.

3L.1.3 Vehicle impact Where there is a possibility of a vehicle running into a building, the vulnerability of the lower parts of the structure to damage should be considered and permanent protection provided wherever possible by the use of bollards, raised flower beds, high kerbs or other construction that will shield the key elements of the structure. Reference may be made to BS 6180.

3L. 1.4 Accidental loads For the type of misuse or accident likely to be met in practice, minimum recommendations arc given in subsection 3L.2, based on current practice and experience. Enhanced design provisions and reduced limits of acceptable damage may be deemed necessary in the case of buildings where the consequences of failure could be particularly severe, e.g. certain public buildings.

3L.1.5 Exceptions Generally, it is not considered necessary to make special design provisions (other than the normal attention 10 the matters outlined in clauses 3L.1.2 and 3L.1.3) for buildings up to four storeys in height including the basement storey (if any).

3L.2 Ties Floors and roofs of all buildings should be capable of resisting the following tic forces, assuming no other forces arc acting:

(i) fnternal ties. These should be located at each floor and roof level in two directions approximately at right-angles. They should be effectively continuous throughout their length and should be anchored to the peripheral tics at each end. The total tie force may be spread over the width of the floor or grouped at uniform intervals in the top or bottom of the floor, or in the beams or walls (within 0.5m of the top or bottom of the floor slab). Spacing of ties should in no case be greater than ISL;,, where L a , is the greater distance (in metres) in the direction of the tie between the centres of columns, frames or walls supporting any two adjacent floor spans. In each direction, ties should be capable of resisting a tensile force (in k N per metre width) equal to the greater of:

+

F, = the lesser of (20 4N) or 60, where N is the number of storeys in the structure, or ( 6 ) ((dead + imposed floor loading)/7.5) X (L,/5) X ( F J , where L, is as defined above. (a)

86

IStructE RC pcrmissiblc strcss rccommcndations

For crosswall construction, where walls occur in one direction only, the value of L , should be taken as either the actual length of the wall, or the length that may be considered lost in the event of an accident, whichever is the lesser. The length that may be considered lost should be taken as the length between adjacent lateral supports or between a lateral support and a free edge. (ii) Peripheral ties. At each floor and roof level, an effectively uninterrupted peripheral tie should be provided, within a strip not more than 1.2m wide (or within the supporting wall or beam) to take a force equal to F1 as defined in (i) above. (iii) Horizontal column and wall ties. Every external column, and if the peripheral tie is not located within the wall, every metre length of external wall carrying vertical load, should be anchored or tied into the structure at each floor and roof level with a tie capable of developing a force (in kN) equal to the greater Of

(a) 2 x F,, or ( L J 2 . 5 ) x ( F 1 )if less (where L, is the floor-to-ceiling height in metres), or ( h ) 4'/2% of the total vertical load carried by the column or wall at that level. Where the peripheral tie is located within the wall, only such horizontal tying as is required to anchor the internal ties to the peripheral ties needs to be provided. Corner columns should be tied into the structure in two directions approximately at right-angles, with ties each capable of developing the tic force given above. (iv) Vertical ties. For buildings of 5 or more storeys, in addition to the ties described above, vertical ties should be provided effectively continuous through beam-to-column or slab-to-column (or wall) connections and should extend through the full height of the column (or wall). Vertical ties should be capable of resisting a tensile force equal to the dead load plus imposed load carried from any one storey. Vertical reinforcement should in no case be less than the minimum steel requirements for columns and walls given in subsection 3E. I and section 3F. Such ties are intended to allow for catenary, bridging or other action to be developed in the event of local damage to the supports, and ties should be provided in all cases unless alternative paths of support are available, or the configuration of intersecting walls is such as to provide effective safeguards against local collapse, or local failure can be shown to be limited as provided in subsection 3L.1.

3L.3 Acceptable limits of damage For buildings or parts of buildings of all occupancy classes having five or more storeys (including the basement storeys, if any) not complying with the requirements of subsection 3L.2, and for public buildings of any height that include a structure having a clear span exceeding 9 m between supports, the structure should be designed such that if any element of structure (other than one designed in accordance with subsection 3L.5) were to fail or be forcibly removed, due to misuse or accident, the structural failure consequent on such removal would be localized within an area not exceeding 70 m2 or 15% of the area of the storey, whichever is less. Further, IStruetE RC permissible stress recommendations

87

.

..

.:

..

.

. .* .. .

.. I-

the failure would be localized within the storey in which the element occurs, the storey next above (if any) and the next storey below (if any). For the application of this clause ‘public buildings’ may be defined as theatres, halls or other place of public resort, schools, churches, chapels or other places of public worship. I t need not apply to restaurants, shops, stores, warehouses or private houses to which members of the public are admitted.

3L.4 Loads In considering the ability of the structure to withstand removal of a supporting member as described in subsection 3L.3, the dead load of the structure above the level of collapse should be considered, together with one-third of the tabulated imposed loading. However, loads due to plant, machinery or other equipment should not be reduced. For buildings such as warehouses, storage buildings, factories and workshops where the imposed loading is likely fo be of a permanent nature, the full imposed loading should be considered. Wind loads should be taken as one-third of the normal design loading.

3L.5 Key elements In the case of any structural support member that is deemed to be an essential key element with no alternative path or safeguard against uncontrolled collapse of thc structure resultin from its removal, or which supports an area of floor exceeding g, the lesser of 70 m- or 15% of the area of any storey, such member and its supports should be capable of resisting a load of 34kN/m2 acting on the whole surface of that member in any direction. Where a key element is ticd to other construction, or where it may experience loading because of interaction with elements connected to it, an appropriate additional load related to the geometry and strength of the attached construction should be taken into account in addition to the loadcd area of the member under consideration.

3L.6 Stresses For all strength calculations arising from the emergency loading conditions described in section 3L, the permissible stresses may be taken as 1.75 X the values given in subsection 3A.7 for steel and subsection 3A.6 for concrete.

88

IStructE RC pcrmissiblc strcss rccommcndations

.

- .,

.-- .. .

.

.

. . .. . . . .

.

..

.

4 Precast and composite construction 4.1 General This Section covers the design and detailing of structures of partly or wholly precast construction. The general recommendations included elsewhere in these recommendations apply equally to precast and in situ construction, but where there are particular requirements related to precast work they are given in the clauses below.

4.2 Detailing 4.2.1 Handling stresses Precast units should be designed to withstand all stresses arising from handling, storagc, transportation and erection.

4.2.2 Connections Connections are of vital importance in precast construction and should be carefully checked to ensure adequate strength, practicability, compatability with other associated construction and durability.

4.2.3 Anchorage at supports All reinforcement used to provide structural integrity in bearings or in corbels and nibs should bc very carefully overlapped and anchored.

4.3 Stability Thc rcconimendations regarding stability and disproportionate collapse given in scction 3L apply also to precast and composite construction. In particular, the following points should be noted concerning ties: (i) All ties should be arranged so as to minimize eccentricity. (ii) Ties should be effectively continuous and may be provided wholly or partly within in situ toppings, in situ concrete or precast members. (iii) A tie may be considered effectively continuous if one of the following conditions is satisfied: ( a ) A bar in a precast member is lapped with a bar in in situ concrete

bounded on two opposite sides by rough faces of the same precast mcmber (see Fig. 22) (h) A bar in a precast member is lapped with a bar in in situ topping or other concrcte anchored to the precast member by enclosing links (see Fig. 23). The tensile capacity of the links should not be less than that of the tie (c) Bars projecting from the ends of precast members are lapped or interconnected in accordance with clauses 3A. 12.11, 3A.12.12, 3A. 12.13 and 3A. 12.14. (d) Bars are lapped within it1 situ topping or connection concrete and projecting links are provided from the precast concrete members to anchor the in situ concrete (see Fig. 24). IStructE RC pcrmissiblc strcss rccommcndations

89

.

.

.

.

.,

-

...

.-

\-Tie

22 Continuity of ties: bars in precast member tapped with bar in in situ concrete

23 Continuity of ties: anchorage by enclosing links

Tie

Tie

24 Continuity of ties: bars lapped within in situ concrete

4.4 Framed structures and continuous beams The analysis of precast, framed or continous beam structures designed to achieve full continuity and interaction between members should be carried out as for in situ construction.

4.5 Design of slabs 4.5.1 Wide units or series of jointed narrow units Slabs consisting of wide precast units or a series of narrow precast units with effective jointing between them capable of shear transfer may be treated as in situ slabs.

4.5.2 Concentrated loads on slabs without reinforced topping For concentrated loads (including partitions in the direction of the span), the width of slab that may be assumed to contribute to support should not exceed the width of three precast units and joints plus the width of the loaded area, nor extend more than a quarter of the span on each side of the loaded area. I n some forms of construction, e.g. long-span wide units, these limits may be inappropriate, and more detailed considerations may be made. 90

IStructE RC pcrmissiblc stress recomnicndations

..,.

..

.

.

. . ....

..

.

I

.

.

.

.

.

.

.

. . .:.. I

.

4.5.3 Concentrated loads on slabs with reinforced topping For concentrated loads on slabs with reinforced topping, the provisions of clause 4.5.2 apply in all respects except that, instead of three, the width of four precast units and joints may be allowed to contribute.

4.5.4 Slabs carrying concentrated loads Where test results justify a greater width of slab for dispersion of concentrated loads than those set out above, the effective width assumed on each side of the loaded area should not exceed one-quarter of the span.

4.6 Bearings'for precast members

.

4.6.1 General In the design of bearings it is important to make allowance for proper overlap of reinforcement in the supported member and the support, as well as for any loss of bearing through movement or rotation. A net bearing width is first calculated, and this is increased to cover these other effects. Fig. 25 is a schematic arrangement of the allowance to be made for bearing.

4.6.2 Net bearing width The net bearing width should be 60 mm or the value calculated from: net bearing width =

support reaction

effective bearing length x permissible bearing stress whichever is greater. The net bearing widtl must be increased to allow for spalling, constructional inaccuracies, effects of rotation, etc.

4.6.3 Effective bearing length This may be taken as the least of: (i) actual bearing length (ii) one-half of actual bearing length plus 100 mm, or (iii) 600 mm.

4.6.4 Permissible bearing stress The permissible bearing stress (assumed uniform) should be based on the weaker of the bearing surfaces and may be taken as follows: (i) for dry bearing on concrete: 1.0 pcC (ii) for bedded bearing on concrete: 1.5 pcc (iii) for contact face of a steel bearing plate cast into member or support and not exceeding 40% of the concrete dimension in either direction: 2.0 pcc Consideration should be given to the actual surface area of contact achieved with wide units because of tolerances in the units and/or bearing structure so that the recommended bearing stresses are not exceeded. Where in doubt, the units should be bedded, or dry packed after erection. Bearings using flexible padding may be designed using stresses intermediate between those for dry and bedded bearings. IStructE RC pcrrnissiblc strcss rccornmcndations

91

Elevation

-----

\

-L

A'\,

I

Net bearing width

Allowance for **Allowance for inaccuracies inaccuracies

* I

Ineffective bearing

Nominal bearing width

Maximum bearing width

m

*

I I I

Bearing length

I

Plan

I

a

b

Bearing width

25 Schematic arrangement of allowance for bearing

92

IStructE RC permissiblc stress recommendations

4.8 Horizontal forces at bearings Horizontal movements at bearings because of creep, shrinkage, temperature effects or other such causes should be provided for by suitable sliding bearings, or else reinforcement should be provided for the forces developed so that the strength of the bearing is not adversely affected.

4.9 Rotation of bearings of flexural members Where large rotations are likely to occur, particularly at end supports, suitable bearings designed to accommodate rotation should be used, or the bearing should be designed to accommodate the high stresses produced by such rotation.

4.10 Concrete corbels Concrete corbels should be designed in accordance with subsection 3B. 1 1 . Reinforcement should be provided for horizontal forces, which should be taken as not less than half the vertical load on the corbel unless calculations justify otherwise. At the front face of the corbel, the reinforcement should be anchored either by: (i) welding to a transverse bar of equal strength in which case the bearing area should stop short of the face of the support by a distance equal to the cover of the transverse bar, or (ii) bending back the bars to form a loop, in which case the bearing area should not project beyond the straight portion of the bars at the start of the bend. Shear reinforcement should be provided in the form of horizontal links distributed in the upper two-thirds of the effective depth of the root of the corbel. Such links should be adequately anchored and be of not less than one-half the area of the main reinforcement.

4.11 Continuous concrete nibs Generally, where a continuous nib is less than 300 m m deep, it should be designed as a short cantilever slab where: (i) The line of action of the load is assumed to occur at the outer edge of the loaded area (e.g. at the front edge of the nib without a chamfer, at the upper edge of a chamfer, or at the outer edge of a bearing pad), and (ii) The maximum bending moment is calculated at the nearest vertical leg of the links in the supporting member. The area of tension reinforcement in the nib should not be less than 0.8% for round bars and 0.45% for deformed bars. Such reinforcement should project from the supporting member, across the top of the nib to a point as near the front face of the nib as considerations of adequate cover will allow. I t should be anchored either by welding to a transverse bar of equal strength or by bending through 180" to form loops in the horizontal or vertical plane. For vertical loops the bars should not generally be greater than 12 mm. The bond and shear resistance of the nib should be checked as provided in subsections 3A.12 and 38.10. 94

IStructE RC pcrrnissiblc stress rccomrncndations

-. .. .:.._

. , _ .

.

.

4.6.5 Allowances for spalling at supports Distances to be assumed to be ineffective at bearings are given in Tables 26 and 27. Table 26 Allowances for effect of spalling at supports

Distance assumed ineffective Material at support

mm

Steel Concrete grade 30 or over, plain or reinforced Brickwark or masonry Concrete below grade 30, plain or reinforced Reinforced concrete less than 300 m m deep at outer edge Reinforced concrete less than 300 m m deep at outer edge, but with vertical loop reinforcement larger than 12 m m diameter

0 15 25 25 Not less than nominal cover to steel on outer face of support Cover plus internal radius of bend

Table 27 Allowances for effect of spalling at supported member

-

Reinforcement at bearing of supported member Straight bars, horizontal loops or vertical loops not exceeding 12 mm diamcter close to end of mcmber Straight bars exposed at end of member Vertical loop reinforcement of bar size cxceedine 12 mm

Distance assumed ineffective mm 10 or end covcr, whichever is greater 0 End cover plus inner radius of bend of bars

Notes: I Where unusual spalling characteristics are known to apply when particular constituent nliiterials are heing used. adjustment should he made to the distances recommended. '7 Chamfers occurring within areas suhject to spalling may he discounted. If steel packs are used for erection. they should he kept clear of areas suhject to spalling.

4.6.6 Allowance for construction inaccuracies For supported members up to 15 m span and with average standards of accuracy, allowances may be taken to be the greater of: (i) 15 m m or 3 mm per metre span for stecl or precast concrete supports (ii) 20 m m or 4 m m per metre span for masonry supports (iii) 25 m m or 5 m m per metre span for in situ concrete supports.

4.7 Bearings transmitting compressive forces from above Bearings required to transmit load from above from a column or wall should generally be bedded. However, for buildings of masonry construction up to four storeys where thc compressive forces are low, dry bearings may be used. IStructE RC pcrmissiblc strcss rccommcndations

93

4.12 Connections between precast units Connections between precast units should be designed by methods appropriate to reinforced concrete, prestressed concrete or structural steel. Where such methods arc not applicable the efficiency of the connections should be checked by appropriate tests. The following points should be considered in the design stage: (i) Where projccting bars or sections are required, they should be kept to a minimum and made as simple as possible. The lengths of such projections should not be more than necessary for security (ii) Fragile fins and nibs should be avoided (iii) Fixing devices should be located in concrete sections of adequate strength (iv) The practicability of both casting and assembly should be considered (v) Most connections require the introduction of suitable jointing material. Sufficient space should be allowed for the proper filling of the joint (vi) It may be desirable for levelling devices (e.g. nuts and wedges having no loadbearing function in the completed structure) to be slackened, released or removed. Where this is necessary, the details should be such that inspection can be carried out (vi;) Connections should be designed to maintain the standard of protection against weather, fire and corrosion required for the remainer of the structure (viii) The possibility of torsional strcsscs in supporting members because of eccentrically applied loads should be considered.

4.13 Site information Information on the following matters should be provided, where appropriate: sequence of forming the joint critical dimensions, allowing for tolerances critical details, e.g. accurate location rcquired for a particular reinforcing bar method of correcting possible lack of fit at the joint details of temporary propping and time when it may be removed description of general stability of the structure with details of any necessary temporary bracing (vii) extent to which the uncompleted structure may proceed above the completed and matured section (viii) details of any special materials (ix) fully specified weld sizes. (i) (ii) (iii) (iv) (v) (vi)

4.14 Continuity of reinforcement 4.14.1 Loops Where continuity of reinforcement is achicved by overlapping loops of reinforcement, the bearing stresses inside the loops should be checked (see subsection 3A. 12).

I

4.14.2 Sleeves Sleeves should be provided with cover not less than that specified for normal reinforcement. The detailing should be such that the ends of the two connccted bars can be accurately aligned in the sleeve. IStructE RC pcrmissiblc strcss rccommcndations

9s

4.14.3 Threading of reinforcement Connection by threading is permissible using couplers, steel plates or threaded anchors. Where there is a risk of a threaded connection working loose, e.g. during vibration of concrete, a locking device should be used.

4.14.4 Welding Welded connections should conform with the appropriate Code or Standard.

4.15 Other connections 4.15.1 Joints with structural steel inserts Where steel plates, rolled sections or bolted details are used in precast concrete connections they should be designed in accordance with the appropriate Standards. Permissible bearing stresses up to 2.0 pECmay be used unless other values can be justified by testing. Consideration should be given to the possibility of vertical splitting as a result of shrinkage effects and high localized bearing stresses, e.g. under narrow steel plates.

4.15.2 Resin adhesives Resin adhesives may be used only in compression joints when suitably protected against fire. They should not be used to resist tension or shear stresses.

4.15.3 Compression joints For simple compression joints such as most commonly occur at horizontal joints between loadbearing walls or columns, the joint should be designed to resist all the calculated forces and moments derived from the design coiisiderations. Generally, the area of concrete that may be considered in calculating the strength of the column or wall joint should be the greater of: (i) 75% of the area of contact between wall or column at the joint (ii) the area of the in siru concrete ignoring the area of any intruding floor or beam units. This area should not be taken as more than 90% of the gross column or wall area. Only those parts of the floor units that are solid over the bearing and properly bedded on concrete or mortar of adequate quality should be considered.

4.16 Joints transmitting shear These joints may occur when a wall acts as wind bracing or a floor acts as a horizontal girder and like situations. They may be assumed to be effective if the joint is grouted with a suitable concrete or mortar mix and the appropriate following condition is satisfied: (i) Joints fransmitting shear in plane. These should be restrained to prevent their moving apart. No reinforcement need be provided in or across the joint and the sides of the units may have a normal finish, provided that the shear stress does not exceed 0.15 N/mm2. Very smooth moulded finishes should be roughened. 96

IStructE RC pcrrnissiblc strcss rccornrncndations

(ii) Joints under compression in all design conditions. No reinforcement need be provided when the sides or ends of the units have a rough as-cast finish and when the calculated shear stress does not exceed 0.3N/mm2. In checking the presence of permanent compression across the joint, the provisions of subsection 3A.4 should be observed. (iii) Shear stresses less than 0.85N/mm2on the minimum roof area of a castellated joint. In such cases, separation of the units normal to the joints should be prevented either by steel ties across the ends of the joints or by the presence of a permanent compressive force across the joint under all loading conditions (see subsection 3A.4). (iv) Reinforcement provided to resist the entire shear force. I n this case, the shear force V should not excced: V

= 0.6 Fb tan

a,

where Fh = p,, A,, or the anchorage values of the reinforcement, whichever is less A,, = the minimum area of rcinforcement a, = the angle of internal friction between the faces of the joint. This may be determined by tests, or the value of tan afmay be taken as: for smooth interface (untreated concrete) tan a,= 0.7 roughened or castellatcd joint without continuous in situ strips across the ends of joints tan a,= 1.4 roughened or castellated joint with continuous in situ strips across the ends of joints tan af= 1.7

4.17 Composite construction 4.17.1 General Where in situ concrete is used in conjunction with precast units to provide a composite structure, provision for horizontal shear transfer should be made at the interface. The design should take into account construction methods, including the effects of propping on the stresses and dcflection in the structure. The relative stiffness of the composite materials should be based on the concrete gross or transformed section, making due allowance for differences of more than 10N/mm' concrete strength in the precast and in situ components. Where therc is an appreciable difference between the age and quality of concrete in the precast and in situ components, differential shrinkage effects should be considered, and in particular, the occurrence of tensile stresses, which may be significant. The minimum re'commended thickness of structural topping to precast units is 40 m m with a local minimum of 25 mm. The topping should be well vibrated on to a surfacc that has been dampened but is without standing water. IStructE RC permissible stress rccommcndations

97

Gradc of in situ concrctc 40 and 25 30 over

I

I

Prccast unit

Surface type

Without links

As-cast or extruded Brushed, screcded or rough-tamped Washed to remove laitcnce or treated with retarder and cleaned

0.25 0.38

0.34 0.41

0.41 0.47

0.44

0.47

0.50

As-cast or extruded Brushed, scrceded or rough-tamped Washed to remove laitance or treated with retarder and cleaned

0.75 1.13

1.13 I .25

I .25 I .38

1.31

I .38

I .56

With nominal links projecting into in situ concrete

Notes: 1 The description %-cast’ covers those cases where the concrete is placed and vibrated leaving a rough finish. The surface is rougher than would he required for finishes t o he applied directly without a further finishing screed hut not as rough a s would he ohtained if tamping. brushing or other artificial roughening had taken place. 2 The description ‘as-extruded’ covers those c:iscs in which an open-textured surface is produced direct from an extruding machine. 3 The description ‘Brushed, screeded or rough-tamped‘ covers those cases where some form o f deliberate surface roughening has taken place, but not t o the extent o f exposing the aggregate. 4 For nominal links see subclause 3B.IO.l(iv).

4.17.3 Nominal links Where provided, nominal links should have a cross-section of at least 0.15% of the contact area. The spacing of links in T-beam ribs with composite flanges should not generally exceed 4 x the thickness of the in situ concrete nor 600 mm, and all links should be properly anchored on both sides of the interface. 98

IStructE RC permissible strcss rccommcndations

4.17.4 Designed links Where the calculated horizontal shear stress exceeds the value given in Table 28 all the horizontal shear force should be carried on reinforcement anchored on either side of the interface. The amount of steel required (in mm2 per metre) should be calculated from: At, =

1000 b V I ,

AI

where vt, is the calculated shear stress.

4.17.5 Vertical shear The design of composite members resisting vertical shear should be carried out in accordance with subsection 3B. 10.

IStructE RC pcrmissiblc strcss rccommcndations

99

.

,.

'

5 Workmanship 5.1 Concrete 5.1.1 Concrete quality Concrete should be specified, produced and tested in accordance with the requirements of BS 5328.

5.1.2 Transportation Concrete should be handled from the place of mixing to the place of final deposit as rapidly as practicable by methods that will prevent the segregation or loss of ingredients. I t should be deposited as nearly as practicable in its final position to avoid rehandling. Ready-mixed concrete should be transported and delivered in accordance with the requirements of BS 5328.

5.1.3 Placing (i) Generul. The concrete should be placed before setting has commenced and should not be subsequently disturbed unless specifically permitted by the engineer. (ii) Construction joints. Concreting should be carried out continuously up to construction joints, the position and arrangement of which should be approved by the engineer. When work has to be resumed on a surface that has hardcned it should be scabbled, swept clcan and wetted, and it may be covercd with a layer of mortar composed of cement and sand in the same ratio as the cement and sand in the concrete mixture. This mixture should be freshly mixed and placed immediately before the placing of the concretc. Construction joints should be at right-angles to the general direction of the member and provided with joggles where appropriate. (iii) Compacting. Concrete should he thoroughly compacted by vibration or other means, worked around the reinforcement and embedded fixtures, taking care not to displace them, and i n t o the corners of the formwork to form a solid void-free mass having the required surface finish. Over vibration causing segregation should be avoided.

5.1.4 Curing After placing, appropriate curing procedures should start as soon as the concrete has lost its free surface water. The method adopted should prevent loss of moisture from the concrete surface and maintain a satisfactory temperaturc of the concrete during the curing process. Faces of concrete should be kept moist by approved means for 4 days after placing (Portland cement concrete) or six days after placing (concrete containing pfa or ggbfs). In cold dry weather these periods should be extended to 6 and 10 days, respectively. I n freezing conditions, special arrangements should be made to protect the concrete from frost. 100

IStructE RC pcrmissiblc strcss rccommcndations

.. ..

-

-

.-

.

5.1.5 Concreting in cold weather When depositing concrete at or ncar frcczing temperatures precautions should be taken to sec that the concrete has a tempcraturc of at least 5"C, or preferably 1O"C, until it has thoroughly hardened, and that formwork, reinforcement, etc. are free from snow, icc or frost. The procedures adopted should be approved in advance by thc engineer. Whcn ncccssary, concrctc materials should bc heated before mixing and carefully protectcd aftcr placing. I f the water is hcatcd abovc 60°C it should be mixed with the aggrcgatc bcforc coming into contact with the cement. Altcrnativcly, the ccmcnt contcnt of thc mix may bc increased or a rapidhardening ccmcnt uscd. I f accelerators arc uscd, thcy should be chloride free. Concrctc damagcd by frost should be removed.

5.1.6 Concreting in hot weather and drying winds Hot wcathcr or drying winds can cause rapid loss of moisturc and/or rapid stiffening of the concrete, making propcr compaction difficult. High temperatures and loss of moisturc aftcr compaction can cause plastic and thcrmal cracking and a reduction in strength and durability. The use of retardcrs to slow the rate of hardening, a watcr-rcducing agent to increase workability, or a cement with a low rate of hydration may assist in slowing down thc rate at which the concrctc hardcns, but will not ncccssarily prevent plastic and thermal cracking. Thc concrete should not havc ii temperature in excess of 30°C. The use of water to cool the aggrcgatc helps reduce the concrete temperature. Carcful attcntion should be paid to protective materials to prevcnt moisture loss during curing. The procedure adopted should be approved in advance by the cngincer.

5.2 Reinforcement I

5.2.1 Specification Reinforcement should comply with current British Standards.

I

I I

5.2.2 Cutting and bending Rcinforccment should be cut and bent in accordancc with BS 4466. Rcbending of reinforcement should be done only with the engineer's approval.

5.2.3 Fixing All reinforcement should be placed and maintained in the position shown on the drawing. Unlcss otherwise specified, the actual concrete cover should be not less than the specified cover minus 5 mm. Spaeers, chairs or other supports should be used to maintain the reinforcement in its correct position. Spacers should be of such materials or designs as will be durable, not lead to corrosion of the reinforcement or cause spalling of the concrete cover. Spacer blocks made from cement, sand and small aggregatc should match the mix proportions of the surrounding concrete as far as is practicable. IStructE RC pcrmissiblc strcss rccommcndations

101

5.2.4 Surface condition All reinforcement should be free from loose mill scale, loose rust, oil and grease, snow and ice or other harmful matter immediately before placing the concrete.

5.2.5 Welding Welding on site should be avoided if possible, but where suitable safeguards and techniques are employed, and provided that the types of steel have the required welding properties, it may be undertaken. All welding should be carried out in accordance with the relevant British Standards and the recommendations of the reinforcement manufacturers, and only with the engineer’s prior approval.

5.2.6 Mechanical splices Butt jointing of reinforcement with mechanical splices or couplers of approved design is permissible, but the cover to any sleeve should not be less than that specified for normal reinforcement. Mechanical couplers should be used in accordance with the manufacturer’s recommendations, and for bars in tension they should satisfy the following criteria: (i) When a test is made of representative gauge length assembly comprising reinforcement of the size, grade and profile to be used and a coupler of the precise type to be used, the permanent elongation after loading to 0.6 fy should not exceed 0.1 mm. (ii) The design ultimate strength of the coupled bar should exceed the specified characteristic strength by the percentage specified in BS 4449.

5.3 Formwork

5.3.1 Design and construction The design and construction of formwork should be in accordance with BS 5975. Reference may also be made to the IStructE/Concrete Society rcport on Formwork.

5.3.2 Cleaning and treatment of forms All rubbish should be removed from the interior of the forms before concrete is placed. Faces of the forms in contact with the concrete should be clean and treated with a suitable release agent. Release agents should be applied so as to provide a thin coating to the forms without contaminating the reinforcement.

5.3.3 Striking of formwork Forms should not be struck until the concrete reaches a cube strength of at least twice the stress to which the concrete may be subjected at the time of striking. Alternatively, Table 29 gives recommended minimum periods before striking which may be used. Formwork should be removed without such shock or vibration as wouid damage the partially hardened concrete. Where formwork is struck before the specified curing period is complete (see clause 5.1.4) appropriate alternative arrangements should be made to complete curing. 102

IStructE RC pcrmissiblc strcss rccommcndations

5.3.4 Camber Where it is desirable to give formwork an upward camber to counteract deflection or an apparent sag when the formwork is removed, the method adopted should be approved by the engineer.

5.3.5 Tolerances Tolerances should be in accordance with BS 5606, unless otherwise specified. Table 29 Minimum period before striking formwork or removing props (concrete made with ordinary or sulphate-resisting Portland cement)

Minimum period before striking Surface temperature of concrete

16°C and above

7°C

12 h

18 h

4 days

6 days

-days

Soffit formwork to beams and Props to slabs

10 days

15 days

-days

Props to beams

14 days

21 days

-days

Type of formwork Vertical formwork to columns, walls and large beams Soffit formwork to slabs

t°C (any temperature )etween 0°C and 25°C)

300 -h t + 10 100

f+lO

250

t +

360

t+

Further guidance is provided in ClRlA Report 67.

1

IStructE RC pcrmissiblc strcss rccommendations

10

10

..

-

.. , .

..

.

.

.

.,

6 Testing and inspection 6.1 Methods of testing - concrete

Thc methods of testing concrete are set out in BS 1881, and testing of aggregates is covered by BS 812. Both of these publications contain full details of sampling and testing procedures, and refcrence should bc made to them for such details.

6.2 Rate of strength testing Suggested rates of strength testing are: for highly stressed elements (e.g. columns) I test per 12 m3 for ordinary elements (e.g. beams and slabs) I test pcr 20 in3 for mass concrete I test per 50 m3. One of these or, if appropriate, a different rate should be adopted for designed mixes.

6.3 Inspection Immediately after stripping the formwork the concrete should be completely inspected. Any defects should be made good as soon as practicable.

6.4 Load testing of structures or parts of structures 6.4.1 Load testing of completed structures or components may be required by the specification or may be demanded if there is reasonable doubt a s to the adequacy of the strength of the structure or component. Such testing should not generally be carried o u t until the concrete may reasonably be expected to have obtained its design strength, and the objective should be to obtain sufficient information to enable an assessment of the adequacy of the structure to be made without affecting its longterm strength or durability. 6.4.2 During testing, struts or other supports should be provided sufficient to take the whole load in the event of collapse. Such supports should be designed so as not to inhibit the deflection of the structure or place undue constraints on the recording or reading of the test results. 6.4.3 The test load should in the first application be equal to and in latcr applications equal to 1% X the spccified imposed load used for the design, making due allowance for any finishes etc. that may not be in position at the time of test. However the total test load plus the dead load in place should not be less than 1 % X the total combined dead and imposed load used in the design. Test loads should be applied and removed incrementally, while observing all proper safety precautions. The test loading should be applied at least twice, with a minimum of 1 h between tests, and allowing 5 min after a load increment is applied before recording deformation measurements. A third application of load should be made that is left in position for 24 h. 104

IStructE RC pcrmissiblc stress rccommcndations

6.4.4 Assessment of results In determining deformation measurements, due allowance should be made for changes in environmental conditions that have occurred during the test.

6.4.5 Test criteria In assessing test data, the following criteria should be considered: (i) The deflection and cracking o n first loading should be in accordance with the design requirements (ii) Where significant deflections (exceeding span/1000) occur on second loading, the percentage recovery on removal of load should be at least 85% and at least equal to that for the first loading cycle (iii) The percentage recovery after third (24 h) loading should be at least 75% of the total deflection produced by the test (iv) The structure should be examined for defects, which should be noted, photographed where appropriate, and evaluated in the assessment of results (v) If during the test or on removal of the load, the structure shows signs of weakness, undue deflection or faulty construction, it should be reconstructed or strengthened as necessary.

IStructE RC pcrmissiblc stress rccommcndations

.

.. . .

., -. . -

. .

.

..

~

. e

7 Protection, maintenance and repair 7.1 General Concretc structures that havc bccn propcrly dcsigncd and constructcd with good quality materials are gcnerally durable. Howcvcr, for ;I varicty o f rcasons, many unpredictable, damagc does occur. This may bc bccause of ii particularly aggrcssivc environmental condition that had not bccn anticipatcd, accidcntal impact loading, fire, settlement, carbonation of thc concrctc, unsuitablc matcrials, bad dcsign or construction, or other similar causc. T o avoid such happcnings, i t should bc thc aim of good dcsign to minimizc their chancc of occurring.

7.2 Protection Protection of a concrcte surfacc is somctimcs ncccssary particularly whcrc aggrcssivc chemicals are in contact with i t . Many protcctivc coatings arc available, and cxpcrt advice should bc sought from the nianufacturcr to dctcrminc thc corrcct coating for thc conditions which apply. Any coating used should be durablc and ablc to adjust itsclf to clastic and thcrmnl movement of thc structures and should bc rnaintaincd in good condition by rcncwcd applications during the lifc of thc building. Any concrctc paint iiscd should bc suitable for the alkaline charactcr of concrctc.

7.3 Maintenance The effect of unforcseen happcnings on a striicturc can bc minimizcd if thc problem is discovcrcd at an early stage. Periodic inspections should therefore bc carried out to detect early signs of cracking or spalling of the concrctc or dctcrioration of the surface. Remcdial action should be taken immcdiatcly damagc is discovered.

7.4 Repair The repair of reinforced concrete structurcs is a specialist mattcr end rcquircs a high level of skill and cxpericncc in determining thc cause of thc problem and the specification of the repair tcchniqucs to be adopted. The most common causc of damage to concrete structures is corrosion of the reinforcement causing staining of the surface or spalling or loosening of the concrete cover. I n such cases the whole of thc loose concrcte should be cut away back to sound concrete, the reinforcement complctcly exposed and thoroughly cleaned to remove rust or replaced if ncccssary and frcsh concrctc or mortar placed. The prccisc method adopted will depend o n the location and thc cxtcnt of thc repair required, and information should be sought from available technical literature.* Repairing concrete structures i s a skilled job, and unless the work i s done correctly and effectively by contractors experienced in the field furthcr repair work may become necessary i n a short time.

* See Allen. R. T. L.: The rcpuir of concww 106

.witctiircs, 4th ed.. C & C A . Wexham Springs 19x5.

IStructE RC pcrmissiblc strcss rccommcndations

Appendix A Nominal concrete mixes

..

A.1 Proportions A. I . 1 Cement and aggregates Thc finc aggrcgatc and thc coarsc aggrcgatc should bc mcasurcd scparatcly. Thc proportions of ccmcnt to finc aggrcgatc and coarsc aggrcgatc should bc as sct out in Tablc A I o r in any intcrmcdiatc proportions in which thc volumc o f coarsc aggrcgatc is twicc thc volumc o f finc aggrcgatc. Whcrc, howcvcr. a dcnscr or morc workablc concrctc can bc produccd by a variation in thc ratio of thc volumc of coarsc aggrcgatc t o that of finc aggrcgatc, this ratio may bc varicd within thc limits I ‘/2 to I and 3 to I . Thc sum of thc volumcs o f coarsc and finc aggrcgatc. cach mcasurcd scparatcly, should ncvcrthclcss cqual thc sum of thc volumcs of coarsc and finc aggrcgatc appropriatc to thc nominal mix (or intcrmcdiatc mix whcrc applicablc) in Tablc A I . I t is morcovcr dcsirablc to adjust thc ratio tosuit thc maximum sizc of thc coarsc aggrcgatc and also thc grading of thc finc aggrcgatc. For cxamplc. whcn thc finc aggrcgatc is within grading M of BS 882. ratios o f I ‘/2 to I . 2 to I and 3 to I may bc suitablc for maximum sizcs of 10 mm. I Y mm and 38 mm. rcspcctivcly. For finc aggrcgatcs within othcr grading zoncs. thc ratio should bc incrcascd as thc finc aggrcgatc gcts fincr. I n proportioning concrctc. thc quantity o f ccmcnt should bc dctcrmincd by wcight. Thc quantitics o f finc and coarsc aggrcgatcs may bc dctcrmincd by volumc. but this should prcfcrably bc donc by wcight. I n thc lattcr casc thc wcight should bc dctcrmincd from thc volumc rcquircd by thc Tablc and thc wcight pcr cubic mctrc o f thc aggrcgatc. Thc proportions givcn i n this clausc and in Tablc A l arc bascd on thc assumption that thc aggrcgatcs arc dry. I f thc finc aggrcgatc is moist. duc allowancc must bc madc for bulking.

A.1.2 Waterkement ratio Thc quantity of watcr uscd for rcinforccd concrctc should bc sufficicnt. but not morc than sufficicnt. to producc a dcnsc concrctc of adcquatc workability for its purpose. which will surround and propcrly grip all thc rcinforccmcnt. So far as possiblc. thc workability o f thc concrctc should bc controllcd by maintaining a watcrkcmcnt ratio that is found to give a concrctc that is just sufficicntly wct to bc placcd and fully compactcd without difficulty, with thc mcans availablc.

A.1.3 Workability Thc workability should bc controllcd by dircct mcasurcmcnt of watcr content. making allowancc for any watcr in thc finc and coarsc aggrcgatcs. Thc slump tcst or thc compacting . bc uscd as a guidc. The lattcr factor tcst dcscribcd in BS 1881: Meih0d.y ofiesiing C ~ ~ C I ’ C I Cmay tcst is primarily dcsigncd for laboratory conditions and is particularly uscful for concrctc mixcs of vcry low workability.

A.2 Strength requirements for nominal concrete mixes A.2.1 Portland cement concrete or Portland-blastfurnace cement concrete with aggregates complying with BS 882 Concrctc madc with Portland ccmcnt or Portland-blastfurnacc ccmcnt should comply with the strcngth rcquircmcnts of Tablc A I . columns 3 and 4 (works tcst). Whcrc intermediate proportions o f aggrcgatc to ccmcnt arc uscd. as provided i n clausc A.1.1, the cubc strengths rcquircd should bc obtaincd by proportion from thc two nearest nominal mixcs. I f thc rcquircd cubc strcngths at 7 days, givcn in Table A I . column 4. arc not rcachcd. the concrctc may still bc acccptcd i f thc 28 day strcngths givcn in column 3 arc attained. IStructE RC pcrmissiblc stress rccommcndations

107

,-

Table A1 Proportions and strength requirements for nominal concrete mixes with Portland cement or Portland-blastfurnace cement and with aggregates complying with BS 882 (2)

(3)

(4)

Cubic metres aggrcgatc per 50 k g of ccmcnt

Cubc strcngth within 28 days aftcr mixing

Altcrnativc cubc strcngth within 7 days aftcr mixing

(1)

Mix proportions

l:1:2 l:lfi:3 1:2:4

Nlmm’

Nlmm?

Prcliminary

Works tCSt

Finc

Coarsc

tCSt

0.035 0.05 0.07

0.07 0.10 0. I4

40 34 28

30 25.5 21

Prcliminary tcst

Works tcst

26.7 22.7 18.7

20 17 14

A.3 Tests of concrete quality A.3.1 Preliminary cube strength tests Unless satisfactory cvidcncc of strcngth is produccd from rcliablc sourccs. prcliminary strcngth tcsts should bc madc in accordancc with BS 1881 both prior to thc commcnccmcnt o f thc constructional work and subscqucntly whcncvcr any important changc is to bc madc in thc matcrials or in thc proportions o f thc matcrials to bc uscd. Thc strcngths should comply with thc rcquircmcnts of subappcndix A.2 for prcliminary tcsts.

A.3.2 Works cube strength tests Work cube strength tcsts should bc made in accordancc with BS 1881 as may bc ncccssary and particularly whcncvcr matcrials or proportions arc changcd. to provc thc quality o f thc concrctc. A rccord of such tcsts. identifying thcm with thc part of thc work cxccutcd. should bc kcpt on thc works.

A.3.3 Standard of acceptance for cube strength tests Three tcst cubcs should bc madc for cach agc at which tcsts arc rcquircd. Thc cubc strcngth should be calculated for cach agc at which tcsts arc rcquircd. Thc cubc strcngth should bc calculatcd from thc maximum load sustaincd by thc cubc at failure. Thc appropriatc strcngth rcquircmcnt may bc considcrcd to bc satisfied i f nonc o f thc strcngths of thc thrcc cubcs is bclow thc spccificd cubc strcngth. o r i f thc avcragc strcngth o f thc thrcc cubcs is not lcss than thc spccificd cubc strcngth and thc diffcrcncc bctwccn thc grcatcst and thc lcast strcngths is not more than 20% o f that avcragc.

108

IStructE R C pcrmissiblc strcss rccommcndations

.

.

..*

. .

.~

. _.

.

Appendix B Column design charts B . l Charts The following design charts cover (U)

rectangular columns reinforced with four equal bars and

(h) circular columns with evenly distributed bars. The lines give values of r -

P\C

215

8.25 ~ r c

so that for high-tensile steel and 30Nlmm’ concrete (P,~= 215, pur = 8.25). the required reinforcement may be read off directly. Reinforcement for other values of pcc and psc may be determined pro rata.

B.2 Rectangular columns with bars in side faces Where a rectangular column has some of its reinforcement placed in its side faces, this reduces its moment resistance compared with the 4-bar columns considered in the charts. However the charts may still be used for the design of such columns if the actual reinforcement arrangement is translated into an equivalent 4-bar column of reduced effective depth. The Table and Figure show equivalent effective depths for various proportions of side steel. Equivalent effective depth ratios for multibar connections

095

Actual dlh

090

0.75

0.80 0.85

0.90 0.95

0.72 0.70 0.68 0.64

0.76 0.74 0.71 0.67

0.83 0.79 0.75 0.71

TU

0.2 0.4 0.6 0.8

0.79 0.76 0.73 0.69

0.87 0.83 0.78 0.73

0.85

f

U

.-5 0 8 0

w” 075 0.70 0.65 075

IStructE RC permissible stress recommendations

080

0.85 dlh

090

095

109

U

9

0

110

2

0

IStructE RC permissible strcss rccommcndations

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,

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.-

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- -. . .

.

,

.

.

.

.

I

..

.

I

. I

....

II L

IStructE RC pcrmissiblc stress recommendations

111

E

112

IStructE RC permissible stress rccommendations

. .

T

i

7 4 /

7 4 /

IStructE RC permissible stress recommendations

113

U YI

a

I 0

114

U

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IStructE RC pcrmissiblc strcss rccommcndations

IStructE RC pcrmissiblc strcss rccommcndations

,

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.

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-

-

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IStructE RC pcrmissiblc strcss rccommcndations

.

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,

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8

IStructE RC pcrmissiblc stress rccommcndations

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IStructE RC pcrmissiblc stress rccommcndations

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.

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i IStructE RC pcrmissiblc strcss rccommcndations

119

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IStructE RC permissible stress recommendations

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--.

B.3 Worked example The following worked example is based o n the design principles in the recommendations and illu5trates the use of the charts Exurtzple In a braced structure, a column 6m long is to resist the loads shown. Try 300mm X 300mm column in 30N/mm’ concrete with 40mm cover to main bars.

,,,iN

Check erid section, no slenderness reduction.

p/p,,bh = 450 X ’ 10” = 0.606 M/p,,bh’ = 80 8.25 x 300’ Try 4-bar coli(t?zti Assume d = 300 - 40 - Y

2

X

10‘/8.25

X

300’ = 0.36

= 242Ymm = 0.808h

40 kN

Check cohrrm slrufi

Effective length I = (say) 0.85L = 5 . l m I/b = 17 Maximum moment at %L from end = 80 - 30 = 50 kNm Table 17 gives k

=

0.724.

p = 450/0.724 = 622 kN

Design section for M = 50/0.724 = 69.1 kNm

p = 622 X 10’ = 0.837 pobh 8.25 x 300’ p&i’ d = O.8h : r = 3.4% = o.85/l : = 3,0yo

= 69.1

X

106 = 0.31

8.25 x 300’

d = 0.808L : r = 3.3% use 4T32 = 3220mm’

=

2970mm’

Try alrertiurii~e8-bur colirttiri

Assume d = 300 - 40 - Y?= 244mm = 0.813h if bars are equal, r4 = 0.25 giving equivalent d = 0.7611 (see sub appendix B.2) d = 0.7512 : r = 4.0% = 0,8011: I’ = 3,4yo

d = 0.7612 : r = 3.9% = 3510mm’

use 8T25 = 3927mm‘

IStructE RC permissible stress recommendations

121

...

Technical notes These notes do not form part of the Recommendations for the pert~zissiblestress design of reinforced concrete bidding structures. The notes are meant to show where the Recorntnendations differ technically from the requirements of BS 8110 (other than those due to the permissible stress format) and to explain the differences.

General Overall safety factors are generally conservative or similar to BS 8110. Principal values are as follows, with BS 8110 average values (Y = 1.5) in parentheses: steel tension, compression - 1.8 (1.72): concrete compression - 2.4 (2.25): concrete shear - 1.98 (1.88): bond - 2.08 (2.1).

Subsection 3B.2 Deflection and stiffness of members The recommendations are based on a revised presentation of the spanleffective depth rules of C P 110: 1972 as are those in BS 8110.’.’

Section 3C Flat slab construction Recommendations are generally similar t o BS 8110 except for design moments. In subsection 3C.11 the critical section for hogging moments has been taken as D/3 from the column centre rather than D/2, making it more consistent with the general design moments and because of a concern that strict application of the BS 8110 rules could lead to unrealistically low hogging moments in some situations. BS 8110 introduces major changes in design moments from traditional ‘empirical’ values, but no published research evidence to justify the change has been presented. Because of concern that: the use of low design hogging moments may lead to cracking and shear problems at columns, and 0 BS 8110’s increases in sagging moments make the method needlessly conservative, particularly where column heads are large. Moment coefficients close to the well-established ‘empirical’ values have been retained. However these have been slightly adjusted to improve performance under ‘alternate spans’ imposed loading. Limitations on column stiffness have also been introduced: these have been calculated so that the standard design moments and reinforcement details will combine with them to give adequate resistance to ‘alternate spans’ loading, as described. 0

Clause 3C.6.3 Shear reinforcement In CP 110, the effectiveness of shear links in flat slabs was taken to be 0.75 x those in beams, but BS 8110 gives new recommendations which effectively double this, making links in slabs apparently 50% stronger than those in beams. No research or theoretical findings have been presented in support of this change and there is doubt as to whether it is justified. In clause 3C.6.3 it is therefore recommended that links in slabs are designed similarly to those in beams, giving consistency in design for different elements and avoiding the possibility of under-design of this critical reinforcement.

Subsection 3E.2 Permissible loads on columns The treatment of slender columns in BS 8110 is complicated to use and has been criticized on theoretical grounds. The method in subsection 3E.2 is based closely on the results of a rigorous theoretical analysis but is relatively simple to use. The results are similar to BS 8110 in many cases but tend to be more conservative for Ilb ratios between 10 and 20.3 1. Beal. A. N.: ‘Spanldepth ratios for concrete beams and slabs’, Srrucr. Eng., 61A, no. 4. April 1983. p. 121. 2. Venulam: ‘Control of deflection in reinforced concrete‘. Srrrrcr. Eng., 62A, no. 3. March 1984, p. 101. 2. Beal. A. N.: ‘The design of slender columns’, Proc. ICE, 81, Part 2. September 1986. p. 397.

122

IStructE R C permissible stress recommendations

Index This index supplements, and does not repeat, the contents list Accelerators, 18, 101 Acidic conditions, 80 Admixtures, 79 air-entraining, 18, 77-78 calcium chloride, 77 retarders, 18, 101 specifying, 19 using, 17-18 water-reducing, 101 Aggregates, basalt, 34 chemical attack, 80 coefficients of expansion, 34 fire resistance, 80, 82 flint, 34, 82 foamed slag, 76 granite, 34, 82 limestone, 34, 80 measuring, 107 quartzite, 34, 82 size, 18, 46 testing, 104 Air entrainers, 18, 77-78 Basements, cracking, 34 drying shrinkage, 34 thermal movements. 34 Beams, bending moments, 35 beams supporting slabs, 58 box beams, 51 concentrated loads, 49, 58 continuous, 39 exposure to fire, 80 flanged, 35 framing openings, 58 hollow beams, 51 L-beams, 35. 51 flanges, 36 shear stress, 53 torsion reinforcement, 53 webs, 36 non rectangular, 51 rectangular, 51 reinforcement, cover, 29 for torsion, 52-56 links, anchorage, 32, 49 bond length, 32 IStructE R C permissible stress recommendations

spacing, 29, 49 resistance to bending, 39 shear reinforcement, 47, 49 shear stress, 47 size, 53 small sections, 52 strength testing, 104 T-beams, flanges, 36 shear stress, 53 torsion reinforcement, 53 webs, 36 Bearing stress, hooks, 31-32 Blocks, elastic modulus, 46 hollow, 45 Blockwork, movement joints, 34 Bond stress, 30 Braced structures, corner columns, 57 edge columns, 57 internal columns, 56 Brickwork, movement joints, 34 Buckling effect, 24

-

Cantilevers, 21. 35, 37. 49, 64 Casing, to steel members, 45 Castellated joints, 97 Casting sequences. 34 Cement, hydration, 79, 101 proportioning. 107 specifying, 18-19 Chamfers. precast. 93 nibs, 94 Characteristic strength. 13 concrete, 25 reinforcement, 17 Churches, see public buildings Coatings. 78. 80, 106 formwork, 103 Cold-weather curing, 100 Columns. asymmetrically loaded. 71 buckling, 70 circular. 64. 70 concrete density, 76 corner, 57, 87 eccentrically loaded, 69. 71 edge, 57 123

heads, 58 effective length, 71 exposure to fire, 80, 82 external, 58, 64, 71, 87 fire resistance, 83 internal, 56, 64, 71, 72 heads, 58 lightweight concrete, 82 moment of inertia, 60 rectangular, 60, 64, 68, 70 reduction in direct load, 72 reinforcement, cover, 29 transverse ties, 32 resistance to bending, 39 shear, 72 square, 60, 70 stiffness, 72 strength testing, concrete 104 unreinforced bases, 74 Compacting concrete, 56, 100, 101 Concentrated loads, beams, shear enhancement, 49 supporting slabs, 58 walls, 73 Constuction work, compacting, 66, 100, 101 composite construction, 97 construction joints, 100 floor toppings, 46 inspecting work, 104 precast units, 95 stability of buildings, 83 Contaminated land, 80 Corbels, 49 Corrosive environments, 79-80 tensile stress, 24 Cracking, anti-crack reinforcement, beams, 40, 50 slabs, 47 columns, 68 load tests, 105 openings, corners, 44 plastic, 101 tensile stress, 24 thermal, 101 torsion, 50 Creep, precast members, 94 Crushing strength, hollow blocks, 45 Cube strength, 103, 107-108 columns, 68 compressive strength, 23 Curing, 25, 79, 103

Damage to concrete, 106 Deep beams, 29 Density. normal-weight concrete, 22 lightweight concrete, 76 Detailing, reinforcement spacing, 29 Disproportionate collapse, 13, 20, 86 composite construction, 89 precast concrete, 89 Durability, 29. 34, 101 connections, precast concrete, 89 Effective span, 35, 38 Erection, precast units, 89 Explosions, 20 Factories, loads, 88 Fats, 79 Finishes, composite construction, 98 joints transmitting shear, 96 mosaics, 34 movement joints, 34 reinforcement cover, 29 Fire protection, 29 resin adhesives, 96 Fire resistance, 34 Frames, beams, 39 braced, 21, 57 columns, 39 continuous, 39 subframes, definition, 21 unbraced, 21, 60 Frost, 77-79 curing concrete, 100 damage, 101 deicing salts, 78, 79

ggbs, 77. 80 curing, 100 Grades, concrete, 18 defini:ions, 19 Grouting, pockets, 74 Halls, see public buildings High-alumina cement, 13 Insulation, sprayed, 82

Joggles, 100 Jointing materials, precast units, 95,96 Levelling devices, precast units, 95

I

I

124

IStructE RC permissible stress recommendations

-

...._-,..I

,

.

,

Lightweight concrete, 18 fire resistance, 80, 82 modular ratio, 23 weight of concrete, 22 Lintels, 73 Load factor, 25 columns, 68 Loads, accidental, 20, 86, 106 compressive, walls, 72-73 concentrated, beams, 49, 58 . walls, 73 constrqction, 20, 22 cyclical, 32 dead, 21, 22 impact, 20, 106 lateral, 21 partitions, 90 plant, 88 slabs, 22 stairs, 22 uniform, beams, 49 wind, 20, 21, 23, 24, 88 Masonry, loadbearing, 34 Mass concrete, strength testing, 104 Monolithic structures, 39 Mortar, as fire protection, 82 Movements, drying shrinkage, 34 movement joints, 34 precast members, 91, 94 settlement, 20, 34 shrinkage, 34 thermal, 20, 34 coefficients of expansion. 34 Negative moments, columns, 64 reinforcement, beams, 40 slabs, 40, 42, 62-63 slabs, 60, 62 Nibs, 95 Oils, vegetable, 79 Padding, bearings, 91 Permeability, concrete, 79 pfa, 77, 80 curing, 100 Pigments, 18 Placing, concrete, 25 columns, 66 Plaster. 82 IStructE R C permissible stress recommendations

_,

Positive moments, reinforcement, beams, 40 slabs, 40, 42 slabs, 60, 62 Prestressed concrete, 13, 77 Public buildings, stability, 87-88 Punching shear, column bases, 73, 74 wall bases, 73 Quality control, mixes, 19 Radius of gyration, columns, 70 Ready mixed concrete, 100 Redistribution of moments, 38, 39 Reinforcement, anchorage, 31-32, 35, 49, 73 beams, 49 length, 32 precast concrete, 89 nibs, 94 anticrack, beams. 40 bars, bundles, 29, 34 deformed, 23 bond stress. 24 high-yield, 31, 35 inclined, 49 plain, 23, 32 beams, anchorages, 49 anti-crack, 40 cover, 29 shear reinforcement, 49, 52 bond stress, 24, 13 lightweight concrete, 76 Suckling, 66. 67. 68, 72 casing to steel members, 45 columns, 29 compression, 28, 35, 72 laps. 33 compressive stress, 24, 26 congestion, 53 corbels. 49. 89. 94 corners, 32 corrosion, 106 cover, 83, 101 chemical attack. 80 laps, 32 sleeves, 95. 102 fabric, 23, 45 laps, 32 helical, 66-67, 70 pitch, 67 joining, 32 125

I

laps, 32-33 columns, 66 lightweight concrete, 76 precast concrete, 89 tension, 32 lateral ties, columns, 67 links, 23, 49, 52-53, 57, 82, 89, 98 columns, 67 corbels, 49, 94 fire, 82 shape, 53 spacing, 53 loadbearing walls, 72, 73 longitudinal, 53, 66, 67, 68 buckling, 66 mesh, 47, 82 movement, 94 precast concrete, 89 nibs, 89, 94 quantity, 35-36, 52 rebending, 101 shear, 49, 73, 94 precast units, joints, 96-97 & torsion, 52 slabs, 49, 66 anti-crack, 47 cover, 29 torsion, 40-42 spacing, 29, 49, 53, 73, 98, 101 stainless steel, 79 steel binding wire, 45 tensile stress, 24-25 tension, 35 laps, 32 pile caps, 75 tests, couplers, 102 welded joints, 32 Restaurants, 88 Restoring moment, 21 Retaining walls, 34 Retarders, 18, 101 Rotation, bearings, 91 S t Venant value, 50, 51 Schools, see public buildings Serviceability, 36 Settlement, differential, 20 Shear forces, beams, 39 columns, 72 slabs, 39 Shear resistance, 20 Shear stress, composite construction, 99 joints transmitting, 96-97

126

lightweight concrete, 75 pile caps, 75 resin adhesives, 96 slabs, 36, 5 5 , 57 Shear transfer, composite construction, 97 joints, 90 precast concrete, 90, 96 Shops, 88 Shrinkage , aggregates, 37 differential, 97 precast members, 94, 96 walls, 72 Shrinkage bays, 34 Slabs, bending moments, 39,40-41,44, 58 columns strips, 54, 62, 63 openings, 58 continuous, 38, 39 corner bays, 38 corners, 40, 41, 42 drops, 62, 63 edge strips, 41, 62 torsion, 58 effective stress, 55 effective width, 44 end spans, 38, 62, 63 exposure to fire, 80 finishes, 36 flat, 38, 71 definition, 53 hollow, 82 horizontal movements, 34 joints with columns, 60 landings, stairs, 64 loads, 22 middle strips, 41, 42, 54, 62 openings, 58 moment of inertia, 60 reinforcement, 56, 62 cover, 29 spacing, 29, 49 ribbed, 38, 53, 82 shear reinforcement, 47, 49 shear stress, 36, 47, 55-56 simply-supported, 40, 44, 47 spaddepth ratio, 55 lightweight concrete, 76 strength testing, concrete, 104 supporting beams, walls, 42, 58 tensile, 47 torsion, 40, 41, 42, 58 two-way, 38, 40-42 Specifying concrete, 18, 100 IStructE RC permissible stress recommendations

I

I I

I

Stability, construction work, 95 movement joints, 34 precast units, 95 unbraced frames, 21 Stairs, loads, 22 Stiffness, composite materials, 97 supports, beams or slabs, 39 structural members, 21 Storage, precast units, 89 Strength. concrete. design, 23 connections, precast. 89 floors, 46 in fire, 80 lightweight concrete, 76 precast, connections, 89 S t r e s s k a i n relationship, 25 Strip footings. 74 Sugar solutions, 79 Sulphate resistance, 77, 80 Sulphates, 80 Superplasticizers, 18 Supersulphated cement, 17. 18, 80 Sway. frames, 39 unbraced. 21 Sway resistance, 60 Temperature effects. precast members, 94 walls. 72 Tensile stresses. 20 composite construction, 97 Test cubes. strength 23. 25. 107-108 Tests. aggregates. I04 alkali-silica reaction, 79 concrete. 100 load tests. 104-105 reinforcement couplings. 102 workability, 107 Theatres. see public buildings Ties. composite construction, 89

IStructE RC permissible stress recommendations

~

horizontal, columns & wall, 87 internal, 86 peripheral, 87 precast concrete, 89, 97 spacing, 86, 89 vertical, 87 Toppings, composite construction, 89, 97 hollow block slabs, 46 precast concrete, 89 reinforcement, 47 thickness, 46, 97 Torsional rigidity, beams 50-51 Torsional stresses, precast units, 95 Transporting, precast units, 89 variable depth members, 30 Truss analogy, pilecaps, 75 Truss theory, 50 Uniform loads, beams, 49 Vermiculite slabs. 82 Walls. crosswalls, 73, 87 definition, 72 effective height, 72-73 exposure to fire, 80 fire resistance, 83 openings, 83 precast concrete. joints, 96 slenderness ratio, 73 thickness, 72 ties. 86-87 Warehouses, 88 Water-reducing admixtures, 18. 101 Water-retaining structures, 13, 35 Waterproofing, 34 admixtures, 18 Wear on surfaces, 79 Wedges. columns, 74 Weight of materials, 22 Workability, concrete. 107 Workshops, loads. 88 Yield strength. reinforcement, 17

127