Design Of Abutments

Bridge Substructure Abutments MAB1053 Bridge Engineering Faculty of Civil Engineering, UTM azlanfka/utm05/mab1053

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Subtructures 

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Substructures may be classified as ‘end supports’ or ‘intermediate supports’, according to their position along a bridge. End supports can be abutment walls with associated wing walls for closed side spans, and either skeleton abutments or bank seats for bridges with open side spans. Intermediate supports are the piers and columns in all bridges with more than one span. azlanfka/utm05/mab1053

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Bridge Abutments 

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Current practice is to make decks integral with the abutments. The objective is to avoid the use of joints over abutments and piers. Expansion joints are prone to leak and allow the ingress of corrosion agents into the bridge deck and substructure. In general all bridges are made continuous over intermediate supports and decks under 6m long with skews not exceeding 30° are made integral with their abutments. azlanfka/utm05/mab1053

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Bridge Abutments 

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Usually the narrow bridge is cheaper in the open abutment form and the wide bridge is cheaper in the solid abutment form. The exact transition point between the two types depends very much on the geometry and the site of the particular bridge. In most cases the open abutment solution has a better appearance and is less intrusive on the general flow of the ground contours and for these reasons is to be preferred. It is the cost of the wing walls when related to the deck costs which swings the balance of cost in favour of the solid abutment solution for wider bridges. However the wider bridges with solid abutments produce a tunneling effect and costs have to be considered in conjunction with the proper functioning of the structure where fast traffic is passing beneath.

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Bridge Abutments 

Solid abutments for narrow bridges should only be adopted where the open abutment solution is not possible. In the case of wide bridges the open abutment solution is to be preferred, but there are many cases where economy must be the overriding consideration.

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Open Abutments 

A bridge constructed at existing ground level to span across a road in cutting may need only nominal bank seats if good foundation strata are available at shallow depths. This may give rise to problems where negative reactions are likely to develop.

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Open Abutments 

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Spill-through or skeleton abutments are suitable where spread footings are needed at a level well below a bank seat. It is often advantageous to design a footing to offset the foundations in relation to the bearings, because the permanent horizontal loading shifts the reaction.

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Various Types of Open Abutments

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Piled Foundation

Where load-bearing strata are at considerable depth below the bank seat level, piled foundations have to be used. azlanfka/utm05/mab1053

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Wall Abutments 

Mass Concrete

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Cantilever

Mass concrete is economic for small heights, such as where headroom is less than that needed for vehicular traffic. Cantilever is simple to form but demanding high concentration of reinforcement in the stem as height increases

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Wall Abutments

Counterfort 

Stub Counterfort

Counterfort and Stub Counterfort abutments. Reduces weight of reinforcement compared with cantilever, but calls for more complex shuttering. azlanfka/utm05/mab1053

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Hollow Abutment

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For high abutments on sloping ground, this construction offers advantages over heavy counterfort construction. azlanfka/utm05/mab1053

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Other Types of Wall Abutments

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Choice of Abutments Wall Abutments 

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These are normally designed as a reinforced concrete cantilever fixed along the base slab. Strutted abutments may be used for square bridges up to 12m span, where advantage is taken of the propping action of the deck to relieve the foundation pressure under the toe of the footing. Backfilling to these walls is generally selected granular material and earth pressures are often assessed on the basis of an equivalent fluid density. Typical details : a) Wall height – from 5m to 9m b) Wall thickness – 0.7m to 1.1m azlanfka/utm05/mab1053

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Choice of Abutments Skeleton Abutments This type of end support consists of transverse cill beam across one or more buried columns carrying the loads down to a base. It can be used where the road over a bridge is on embankment and a suitable foundation can be obtained near the previous existing ground level. Typical details :  Columns spaced at 3.5m center and directly under deck bearings where possible to avoid large bending moments in the cill beam.  Columns placed at ends of the cill beam since wing walls are cantilevered horizontally from each end.  The rear face of a column is usually vertical and the front face battered at 1:6 since each column is designed to act as a vertical cantilever from the continuous based slab and horizontal loads have a large effect. azlanfka/utm05/mab1053

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Choice of Abutments Bank Seats 

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If the road over a bridge is at or near to existing ground level, then a bank seat may be sited at ground level after either a s a simple base or carried on piles. A bank seat carried on piles driven through fill is usually preferable to a skeleton abutment carried on piles at a lower level. The height of a bank seat is often only 2-4 metres so that it is possible to employ mass concrete wall sections. Where the foundation level is above the level of a nearby open surface, a slip circle analysis should be made to check the stability of the bank slope. azlanfka/utm05/mab1053

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Choice of Abutments Wing Walls These walls are included at all end supports in order to contain the immediate areas of back-fill. There are two basic types to be considered and the choice is normally made on purely structural or economic reasons.  Horizontal cantilevered wall – this type is very economic since it requires a minimum amount of material and saves on excavation for additional footings.  Vertical cantilever free-standing wall – this type is similar to a normal retaining wall except that horizontal cantilever extensions are often used. They are suitable beyond the lengths and skew angles at which horizontal cantilevered walls become unpractical. The main disadvantage is the large height of these walls and the amount of buried structure which causes the cost to become excessive. azlanfka/utm05/mab1053

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Wing Walls

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Wing Walls

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Wing Walls

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Modes of Failure The stability of an abutment should be checked for several modes of failure :  Sliding failure  Overturning  Foundation yield  Slip Circle  Structural failure azlanfka/utm05/mab1053

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Abutments – Modes of Failure

Sliding Failure azlanfka/utm05/mab1053

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Abutments – Modes of Failure Sliding Failure  Resisted by friction in granular soils or adhesion in cohesive soils, aided by the passive resistance of the soil in front of the toe.  If public utilities are to install services in front of the wall, the location or depth of the trenches may invalidate the passive resistance.  Sliding resistance can be increased by incorporating a heel below the foundations. Factor of safety = 2.0 considering passive resistance.  JKR use f.o.s = 1.5 not considering passive resistance. azlanfka/utm05/mab1053

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Abutments – Modes of Failure

Foundation Yield azlanfka/utm05/mab1053

Overturning 24

Abutments – Modes of Failure 

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Foundation yield (bearing failure) – produces similar effect to overturning Overturning – In practice overturning is usually associated with some yielding of the foundation, since this produces very high pressures under the front of the footing. azlanfka/utm05/mab1053

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Abutments – Modes of Failure

Slip Circle

Structural Failure azlanfka/utm05/mab1053

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Abutments – Modes of Failure 

Slip Circle – Only a problem in cohesive soils.

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Structural failure – Failure can occur in the stem of the footing if an inadequate section is provided (design fault). Factor of safety for reinforcement is provided in code. Substructure : nominal f.o.s. = 1.0 (piles). Use partial safety factors for material. azlanfka/utm05/mab1053

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Basic Components of Abutment

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Forces on an Abutment

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Forces on an Abutment 

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Dead load due to the superstructure. Proper dead load include self-weight of beams and deck. Superimposed dead load include premix, surfacing, services and railings etc. Live load on the superstructure.   

BS 5400 – HA UDL and HD KEL BS 5400 – HB (45 units) abnormal vehicle load JKR Standard – special vehicle (SV)

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Forces on an Abutment 

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Self-weight of the abutment – Components of the abutment include main body, wing walls and approach slab. Traction force – Horizontal forces due to braking and acceleration of vehicles. BS 5400 specifies maximum traction force. JKR puts a maximum value of 253 kN.

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Forces on an Abutment 

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Temperature variations – Expansion and contraction due to temperature variation will induce force in the substructure. Substantial movements occur in steel bridges. The temperature induced movements or deflections give rise to forces which will be transferred to the abutments. Creep and shrinkage – These are time dependent properties of concrete. For both creep and shrinkage, it is assumed (JKR practice) that about 50% occurs after 3 months and about ¾ has taken place after 6 months.

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Forces on an Abutment 

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Earth pressures – The equivalent fluid concept (Rankine’s or Coulomb’s theory) is normally used for calculating the earth pressures on an abutment, but the selection of the appropriate intensity depends on the degree of restraint offered by the wall and the particular calculation being considered. In a situation where a wall can move by tilting or sliding and the backfill is a free draining granular material, active pressures are assumed. A common design approach is to use an equivalent fluid pressure of 5H kN/m2, where the active coefficient, Ka is normally 0.25. Modern compaction technique for placing the backfill material and the use of more rigid type of construction have caused many designers to estimate design pressures for the at-rest condition. The value of the earth pressure coefficient at-rest, Ko is often taken to be 1.5-2.0 times the active coefficient, Ka. azlanfka/utm05/mab1053

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Forces on an Abutment 

Surcharge pressure – The effect of HA and HB loadings on the carriageway behind the abutment is arbitrarily treated as an additional surcharge loading. The nominal values suggested in BS 5400 for live load surcharge are 10kN/m2 for HA loading and 20kN/m2 for HB loading. The weight of granular material is assumed to be 19kN/m3.

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Forces on an Abutment 

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Wind loading – must be considered only for bridges with spans greater than 20m. A typical value for wind speed of 40 mph is assumed for 30m span. Seismic loading – There was only one case so far in 1960 of medium size disturbance. Long span bridges such as Penang Bridge include seismic loading consideration in the design.

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Forces on the Abutment

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Abutment (Load Case 1)

WA Self Weight during construction azlanfka/utm05/mab1053

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Abutment (Load Case 2) DL + HA 1/3 PSHB Tr + Fstc + W Pa

WA azlanfka/utm05/mab1053

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Abutment (Load Case 3) DL + HB 1/3 PSHB Tr + Fstc + W Pa

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Abutment (Load Case 4) DL Fstc

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Design Standards for Abutments British Standards  BS 5400: Part 2: Specification for Loads  BS 5400: Part 4: Code of Practice for the Design of Concrete Bridges  BS 8002: Code of Practice for Earth Retaining Structures  BS 8006: Strengthened/Reinforced Soils and Other Fills azlanfka/utm05/mab1053

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Design Standards for Abutments Design Manuals  BD30: Backfilled Retaining Walls and Bridge Abutments  BD37: Loads for Highway Bridges  BA41: The Design and Appearance of Bridges  BA42: The Design of Integral Bridges  BD42: Design of Embedded Retaining Walls and Bridge Abutments  BD57 and BA57: Design for Durability  BD70: Strengthened/Reinforced Soils and Other Fills for Retaining Walls and Bridge Abutments azlanfka/utm05/mab1053

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Basic Design Considerations Cantilever Wall Abutment

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Cantilever Retaining Wall 

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The CONCRETE CANTILEVER RETAINING WALL is constructed of reinforced concrete and it supports backfill soil by a cantilever action. The cantilevered stem portion is fixed at the bottom and is free at the top. The base slab serves as a fixed support and prevents against sliding and overturning. There is an option to install a key at the bottom of the base slab to ensure further safety against sliding. These walls provide prolonged durability and serviceability. They are widely used due to their ease in construction and cost-effectiveness. azlanfka/utm05/mab1053

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Cantilever Retaining Wall

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Analysis & Design of Cantilever Retaining Wall Stability Analysis  Design of Concrete Members 

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Modes of Failure Overturning  Sliding/Translation  Bearing capacity  Bending or shear failure of stem  Bending or shear failure of heel  Bending or shear failure of toe  Bending or shear failure of key 

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Design Considerations The design of the wall must:  Resist sliding along its base  Resist overturning  Not exceed the bearing capacity of the soil beneath the base  Avoid excessive settlement.  Built structurally strong to resist failure from the build up of internal stresses produced by external forces azlanfka/utm05/mab1053

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Forces and Pressures on Retaining Walls 

The basic objective is to apply the conditions for static equilibrium, which are:

1. All the forces in the horizontal direction must add to zero. 2. All the forces in the vertical direction must add to zero. 3. The clockwise moments (or torques) must equal the counter-clockwise moments.

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Forces on Cantilever Wall

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Lateral Earth Pressures  

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Lateral earth pressure is normally calculated based on Rankine or Coulomb’s theories. Lateral earth pressure is assumed distributed triangularly. The location of resultant is at 1/3 of height. If there is surcharge, lateral earth pressure from surcharge is distributed uniformly. The resultant is at ½ of height. The lateral earth pressure is calculated at the edge of heel. azlanfka/utm05/mab1053

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Lateral Earth Pressures

Ka.wH Pa = 1/2Ka.γH2 H/2 H/3

Ka.w

Ka.γH

Due to surcharge

Due to backfill soil

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Pressure Coefficients 

The Rankine active earth pressure coefficient Ka for the specific condition of a horizontal backfill surface is calculated as follows:

Ka = (1 – sin(φ)) / (1 + sin(φ))  φ is the angle of internal friction of soil backfill. 

The equation is modified if the backfill surface is sloped. azlanfka/utm05/mab1053

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Stability Analysis 1. Check factor of safety against overturning. 2. Check soil bearing pressure. 3. Check factor of safety against sliding.

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Overturning 

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The rotating point for overturning is normally assumed at bottom of toe. The height of soil used to calculate lateral earth pressure should be from top of earth to the bottom of footing. Elements that resist overturning are weight of stem, weight of footing, weight of soil above footing. If there is a surcharge, the weight of surcharge can also be considered. The factor of safety against overturning is resisting moment divided by overturning moment. Acceptable factor of safety is between 1.5 to 2. azlanfka/utm05/mab1053

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Factor of Safety for Overturning 

Overturning moment is calculated from :

Where γ is unit weight of soil, Ka is active pressure coefficient, and H is the height from top of earth to bottom of footing, q is surcharge.

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The resisting moment is calculated as :

where Ws,Wf,We,Wk,Wq are weight of stem, footing, earth, key and surcharge, Xs,Xf,Xe,Xk,Xq are distances from the center of stem, footing, earth, key, and surcharge to the rotation point at toe.

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Factor of Safety for Overturning 

The factor of safety against overturning is determined from : FoS = Resisting Moment = MR Overturning Moment

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Mo

FoS should be > 1.5 azlanfka/utm05/mab1053

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Bearing Pressure 

The centre of the total weight from the edge of toe is

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Where W is total weight of retaining wall including stem, footing, earth and surcharge. The eccentricity, e = B/2-X, where B is width of base footing.

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Checking for Bearing Pressure Σ W

e Eccentricity, e = B/2 –X B/2 X

Either, e ≤ B/6 or e > B/6

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Bearing Pressure 

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If e ≤ B/6, the maximum and minimum footing pressure is calculated as:

Where, Qmax, Qmin are maximum and minimum footing pressure, B is the width of footing.

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Bearing Pressure 

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If e > B/6, Qmin is zero,

Qmax should be less than allowable soil bearing capacity of footing soil.

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Sliding 

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The driving force that causes retaining wall to slide is the lateral earth pressure from soil and surcharge. The forces that resist sliding are passive pressure at toe, the friction at the base of the footing; and the passive pressure against the key if used. The factor of safety against sliding is the total resisting force divided by total driving force. Acceptable factor of safety is between 1.5 to 2. azlanfka/utm05/mab1053

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Factor of Safety for Sliding 

The driving force for sliding is calculated as

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The friction resisting force at the base of footing is calculated as where µ is friction coefficient between concrete and soil. µ is often taken as tan(2/3 φ). φ is internal friction of the soil.

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Factor of Safety for Sliding 

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The passive resistance (if any) at the toe of retaining wall is calculated as

Where Kp is passive earth pressure coefficient, h is the height from top of soil to bottom of footing at toe. If a key is used to help resist sliding, h is the height from top of soil to the bottom of the key. azlanfka/utm05/mab1053

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Factor of Safety for Sliding 

The factor of safety is calculated as

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Resisting Force, ΣF > Sliding Force, μΣW

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Forces on the Abutment

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Design of RC Members 1. 2. 3. 4. 5. 6. 7.

Check thickness of stem for shear stress. Design stem reinforcement for bending. Check thickness of heel for shear stress. Design heel reinforcement. Check shear stress for toe when the toe is long. Design toe reinforcement for bending. Check shear stress in key when key is deep and narrow. 8. Design key reinforcement for bending. azlanfka/utm05/mab1053

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Design of Stem

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Design of Heel eu ≤ B/6

eu > B/6

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Design of Toe

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