Abutments Piers

Design of Bridge Substructure

Abutment & Piers

Components of Bridge Substructure:  Abutment / Pier and its Cap  Wells/Piles and their caps

Design of Pier/Abutment Caps (Bed Block)

Design aspects of Pier/Abutment Caps or Bed Block • A block resting at top of pier/abutment to disperse the loads from the bearings to the pier/abutment more evenly • Covers the entire surface of the pier/abutment projecting 75 mm (off-set) from face of the pier/abutment – to prevent rain water to wet the pier/abutment. – To increase the aesthetic appearance of the pier/ abutment

• Suitable slope (except near bearing) is provided to pass rain water smoothly

Design aspects of Pier/Abutment Caps or Bed Block Cont….. • Grade of concrete used M20 • Minimum thickness 225 mm up to span 25 m and 300 mm for longer spans • 1% steel is provided and distributed equally at top and bottom and provided in two directions both at top and bottom (In shorter direction steel is provided in the form of hoops) • Under the bearings additional steel is provided in the form of mesh • Meshes are provided in two layers one at 20 mm and other at 100 mm from top • Each mesh consist 6 mm bars at 75 mm C/C in both directions

Design of Piers

Types of Piers • Solid Piers with Rectangular Section

Elevation

Plan

• Solid Piers with Cut ease waters – Generally, Pier ends are shaped for easy passage of water

– It may be semi-circle or triangular.

• Trestle Type Pier

• Hammer Head Type Pier

Cellular RC Pier

RCC Framed Type Pier

Design of Solid Piers

Materials for Pier – Stone Masonry: Good quality stone in cement mortar (1:4) – Mass concrete: 1:3:6 Mix (by volume) with large size of aggregate – Reinforced concrete : For RC piers M25-M35 concrete Shape of Pier – First Shape is assumed and then, stresses are checked – Preliminary dimensions of Piers may be adopted as Height:

– 1 to 1.5m above the flood level, depending upon the depth of water on the upstream side Width:

– top width sufficient to accommodate bearing. – Generally 600 mm more than outer dimensions of bearing Batter:

– 1 in 12 to 1 in 24. More bottom width is provided to reduce the stresses under pier

Forces / Loads on the Piers

• • • • • •

• • • • •

Dead Load, Live load with impact load Moment due to eccentricity of loads, if any Active earth pressure Surcharge due to live load, Longitudinal force due to – tractive efforts of vehicle – braking of vehicles Wind force, Water current force, Buoyancy Force, Earthquake Force Forces due to collision of boat/steamer to piers

Longitudinal Force due to Tractive Efforts • Due to tractive efforts = 20% of live load • Reduction in Longitudinal Effect on Bridges Accommodating More than Two Traffic Lanes: – Since all lanes may not be occupied simultaneously, the longitudinal loads (tractive efforts) may be reduced as No. of Lanes Two Lanes Three Lanes Four lanes Five or More than Five Lanes

Reduction in Longitudinal Effect 0% 10% 20% 30%

Longitudinal Forces due to Friction at Bearing • For the multi-span simply supported bridges, each pier supports two bearings (one fixed and one free) (for other than elastomeric bearings) on stiff supports, horizontal forces at the bearing level in the longitudinal direction is determined as per Clause 706.2.1.1. • If the friction coefficient for both the bearings is  and the horizontal force due to Tractive effort is Fh, then the horizontal reactions at Fixed and Free bearings may be calculated as Fixed Bearing

Free Bearing

Non-Seismic Combinations Greater of the following two values:

Case (i) Fh   Rg  Rq 

Case (ii ) Fh 2   Rg  Rq 

 Rg  Rq 

 Rg  Rq 

Seismic Combinations Fh For the worst load combination, live load will be considered on one span only.

• In case of simply supported small spans up to 10 m, where No bearings are provided, horizontal force in longitudinal direction at each bearing is taken as (Clause 706.2.1.2) Fh/2 or Rg whichever greater

Horizontal Force on Bearing due to Temperature Change •Determination of Long. force (due to Temperature change) – For worst condition

– assume different Coefficients on left and right bearings on a pier say 0.25 and 0.225 respectively – Assume Live Load on one side of pier only – Due to friction at left bearing = left (= 0.25)(DL+LL) – Due to friction at right bearing bearings = right (=0.225)(DL) – Unbalance force = Difference of above two

= 0.25(DL+LL) - 0.225(DL) •Calculate bending stress at base due to this unbalanced force.

Wind Force on Deck and Piers Calculate stresses due to wind (in transverse direction) Wind force on Deck type structure = area (as seen in elev. and railing)average wind pressure Wind force on Through/half through type bridge = (Area of wind ward Truss + ½ of elevation above the deck level of other trusses)  average wind pressure wind force acting at the pier = wind pressure  Average Projected area Moment at base of pier = wind load on pier  (h/2) (=distance of point of application of wind force from base)

Wind force on the Live load •

Calculated at 1.5 m height from the carriage way

• Moment at base of pier (Mxx) = wind load  (h+1.5) (distance of point of application from base) • When wind speed > 130 kN/m  Assume No vehicle on bridge • Wind force on vehicle not less than 3 kN/m •Overall wind force not less than 4.5 kN/m •If above load combination gives large stresses, assume wind pressure 2.4 kN/m2 on unloaded structure •Calculate Total Moment and stresses due to this moment as

1 = My/ Ixx

;

2 = My/ Ixx

Water current Force Water current force = water pressure (=0.5kV2) Average area Exposed to water current Where, K = 1.5 for square ended piers = 0.66 for circular piers or for piers with semicircular cut waters = 0.5 for to 0.9 for triangular cutwaters = 1.25 for trestle type

Moment at the base of the pier = Force  distance 1 = My Ixx ; 2 = My/ Ixx

Design of Bridges for Earthquake (seismic Effect)

Design of Bridges for Earthquake (seismic Effect)

Earthquake Force Check for EQ is made at different levels, (i) Bearing level, (ii) Foundation level (above scour level) For bridge span up to 150m, EQ force is determined by following equation

Feq     G

• Stresses due LL during EQ = Stresses due to static LL + Stresses due to EQ on LL • LL considered for calculation of static Stresses due to LL =50% for Railways bridges =100% for Road bridges • No EQ force on LL in direction of Traffic

Total Stresses Finally calculate total stresses due above considered forces and check whether these stresses are with in permissible stress rang of concrete. The following cases will be considered: (1) Dry condition, and (2) Flood Condition (i.e. max comp stress and Max Tensile stress

Stress Check: Dead Load (+) Calculate stress due to dead load at the base of the pier(+) Weight of pier = Average area (i.e. at mid-height)  height of pier  density of concrete Compressive Stress at base = Weight of pier/Area at Base Live Load (+ or -) Live load reaction may act at some eccentricity, calculate stresses at the edges of pier/abutment P Pe . y   A I

Buoyancy Force (-) Buoyancy Force = Equals to weight of submerged volume × Density of water

Longitudinal Force Due to tractive efforts = 20% of live load Bending Moment at Base of Pier (supporting hinged bearing) = Longitudinal Force × Height of Pier



H .h. y I

Results in bending stresses at base of pier/abutments

Design of Abutments

ABUTMENTS • Structure upon which the ends of a Bridge rests  Referred as Abutment Functions of Abutment: • Abutment serves the following functions: • Distributes the loads from Bridge Ends to the ground • Withstands any loads that are directly imposed on it • Provides vehicular and pedestrian access to the bridge

Components of Abutments Breast Wall – Directly supports DL and LL of super structure – Retains filling of embankment in its rear

• Wing wall – An extension of breast wall in transverse direction – Retains the backfill in transverse direction – Does not takes any load from super structure

• Back wall or Dirt wall – A small retaining wall behind the bridge – Prevents the flow of backfill material towards the bridge seat

Components of an Abutment Approach slab Back or dirt wall

Wing wall

backfill

wing wall

Breast wall

Types of Abutments • • • • • • •

Mass Concrete/Masonry Type Reinforced Concrete Cantilever Abutment Reinforced Concrete Counterfort Abutments U type Abutment Counterfort abutments Spill Through type Abutments Pile bent type Abutments

• A significant component of overall cost of bridge is involved in abutments. • Improper selection of abutment of bridge may increase the cost of bridge drastically especially in short span bridges

Mass concrete Type Abutment • Most common type of Abutment Structure • Preferred due to ease in construction • May be adopted when headroom requirement is approximately 5 m. • preferred in case of slab culverts and small span bridges • Consists of a bridge seat • In addition to loads from the bridge super structure, subjected to lateral earth pressure • Stability of abutment is achieved by the weight (mass) of abutment. • Checks to be performed: • bearing capacity • sliding resistance of the foundation materials  • overturning stability

Reinforced Concrete Cantilever Type Abutment • Most common type of Abutment Structure • Consists of a bridge seat, wing walls, back wall, and footing. • Used to hold back an earth embankment • Checks to be performed: • bearing capacity • sliding resistance of the foundation materials • overturning stability

Failure Modes for Retaining Wall Type Abutments: Abutments are subject to various types of failure, Failures can occur within soils or the structural members. Sliding failure Occurs when the lateral earth pressure exerted on the abutment exceeds the frictional sliding capacity of the foundation.

Failure Modes for Abutments: Bearing Failure: • Occurs when the bearing pressure under the abutment/pier foundation is larger than the capacity of the foundation soil or rock • Associated with overturning

Structural failure Occurs when Bending moment/stresses in the pier/abutment are greater than moment/ capacity of pier/abutment

Reinforced Concrete Abutments with Wing walls

U Type Abutment It is an abutment whose, wing walls are perpendicular to the bridge seat

• Wing walls of a gravity abutment are placed at right angles to the back wall • Name 'U-abutment' is due to the shape of this abutment in plan. • Wing walls are typically cast monolithically with the abutment back wall and cantilevered both vertically and horizontally

counterfort abutments • A counterfort abutment is very much similar to a counterfort retaining wall. • In counterfort abutments, a thin wall called counterfort connects the breast wall to the footing. • These counterforts are spaced at regular intervals so that the breast wall is designed as a supported slab rather than as a cantilever.

counterfort abutment

Spill Through Abutment It consists of • a beam that supports the bridge seat, • two or more columns supports the beam, and • a footing supports the columns

Beam Columns

Pile Bent Abutment • A pile-bent abutment with stub wings is another

type of spill-through abutment, • a row of driven piles supports the beam

SELECTION OF ABUTMENTS: The type of abutments is selected based on the following consideration:

• Construction and maintenance cost • Cut or fill earthwork situation • Traffic maintenance during construction

• Construction period • Safety of construction workers • Availability and cost of backfill material

SELECTION OF ABUTMENTS Cont…. • Superstructure depth • Size of abutment

• Horizontal and vertical alignment changes • Area of excavation • Aesthetics and similarity to adjacent structures • Previous experience with the type of abutment • Ease of access for inspection and maintenance. • Anticipated life, loading condition, and acceptability of deformations.

STEPS FOR DESIGN OF ABUTMENTS: A series of steps must be followed to obtain a satisfactory design. STEP 1: SELECT PRELIMINARY PROPORTIONS OF THE WALL STEP 2: DETERMINE LOADS AND EARTH PRESSURES STEP 3: CALCULATE MAGNITUDE OF REACTION FORCES ON BASE STEP 4: CHECK STABILITY AND SAFETY CRITERIA a. Location of normal component of reactions b. Adequacy of bearing pressure

c. Safety against sliding STEP 5: REVISE PROPORTIONS OF WALL AND REPEAT STEPS 2-4 UNTIL STABILITY CRITERIA IS SATISFIED AND THEN CHECK FOR SETTLEMENT

Design of Retaining Wall Type (Gravity) Abutments

Materials for Abutment • Masonry

• Good quality stone in cement mortar (1:4) • Mass concrete

• 1:3:6 Mix (by volume) with large size of aggregate • Reinforced concrete • For RC abutment M20 concrete with nominal reinforcement at face,

Preliminary Dimensions of Gravity Abutments • Abutment Height Approach slab – From hydrological data based on HFL Back or dirt wall • Abutment Batter: – On water face – 1 in 24 to 1 in 12 Wing wall – On earth face – 1 in 3 to 1 in 6 • Abutment Width – Top width – enough to put bearing – Bottom Width – 0.4 to 0.5 times the height of thebackfill abutment • Abutment Length – at least equal to width of the bridge wing wall

Breast wall

Loads/Forces acting on Abutments • Loads: – Same as in Piers but also lateral Earth pressure due to soil and LL surcharge

• Water Pressure – All structures will be designed for a fluid pressure of not less than 480 kg/m3 (for river at HFL and no soil in the embankment)

• LL Surcharge – All structures will be designed for a live load surcharge equivalent to 1.2 m height of earth fill

Earth Pressure on Abutments • Forces exerted on Abutments due to Earth pressures can be classified according to the direction and the magnitude of the abutment movement – Earth Pressure At-rest • When the wall is fixed rigidly and does not move, • pressure exerted by the soil on the wall is called Earth pressure at-rest – Active Earth Pressure • When a wall moves away from the backfill, • Earth pressure decreases referred as active earth pressure – Passive Earth Pressure

• When wall moves toward the backfill, • earth pressure increases referred as passive earth pressure

Determination of Earth Pressure on Abutments – Coulomb’s theory is used with a modification

– Center of the pressure is assumed at 0.42 (instead of 0.33) height of wall from base for dry soil

Design Aspects of Gravity Abutments • Since, the abutments are subjected to large horizontal forces, need to check the stability of the abutment against sliding, overturning in addition to stresses at the base of the abutment – Factor of safety against Sliding > 2.0 – Factor of safety against overturning > 1.5 – For No tension condition, the resultant of forces must lie with in B/6 – Maximum stress not greater than safe soil capacity – Friction coefficient between soil and concrete = 0.5 – Friction coefficient between rock and concrete = 0.8 for good rock = 0.7 for fissured rock

Steps for Design of Abutments • Calculate stress due to dead load at the base of the abutment Stress due to dead load at the base of the abutment(+) = Average area (i.e. at mid-height)  height of abutment  density of concrete • Calculate stresses due to live load Calculate stresses due to live load as 1 = P/A + Pey/Iyy

;

2 = P/A - Pey/ Iyy

Steps for Design of Abutment Cont… • Calculate the stresses due to longitudinal force – Determine braking/tractive force – Calculate the moment due to these forces at the base of the abutment (pined) – Calculate stresses due to this moment at base of the abutment as 1 = My Iyy ; 2 = M/ Iyy

Steps for Design of Abutment Cont… • Calculate stresses due to wind (in transverse direction) – Calculate wind force acting at the abutment ( = wind pressure  Average Projected area) – Moment at base of pier = wind load abutment  (h/2) (distance of point of application from base) – Calculate the wind force on the Live load at 1.2m height from the carriage way – Moment at base of pier = wind load  (h+1.2) (distance of point of application from base) – Calculate Total Moment and stresses due to this moment as 1 = My Ixx ; 2 = M/ Ixx

Steps for Design of Abutment Cont… • Stresses due to water current – Water current force = water pressure (=0.5kV2)Ave. area Exposed to water current – Moment at the base of the pier = Force  distance

1 = My Ixx ;

2 = M/ Ixx

• Calculate the stress at base of the abutment due to friction at abutment – Calculate friction force as maximum of two values (as mentioned earlier) – Calculate moment at base of pier and finally stresses

Steps for Design of Abutment Cont… • Check for Stresses – If Moment developed about point ‘A’ of abutment = M – It may be represented as the total vertical force acting at distance ‘Z’ from ‘B’ – ‘Z’ may be calculated as,

M Z W – If ‘b’ is width of the abutment, eccentricity of vertical load – Measured from Center of base of abutment,

b e  z 2

Steps for Design of Abutment Cont… • Stresses at edge of abutment,

W My W W  e  b  W 6We W  6 e      3    2  1   A I bl lb 12  2  bl lb lb  b • Considering, length perpendicular to paper, as unity, i.e. l = 1 W  6e   1   b  b

Steps for Design of Abutment Cont… • Check for Friction:

W H

 2.0

– If not safe in Friction  Provide Shear Key – Check for Overturning

M stab  1.5 M overt

Steps for Design of Abutment Cont… • Calculate stresses due to EQ – Calculate EQ force on Super structure and finally stresses at base (in both directions) – Calculate EQ force on Live load and finally stresses at base (in both directions) • Finally calculate total stresses due above considered forces and check whether these stresses are with in permissible stress rang of concrete

Steps for Design of Abutment Cont… • Approach slabs: – Minimum straight length = 15 m on either side of bridge – Approach slab straight not curved – Slope must be gradual, such that the vehicles on other side must be visible • Weep holes: – to drain out the water accumulated at back of the abutment – Size of weep hole: 150 mm deep and 75 mm wide – Spacing of weep holes = not greater than 1 m in horizontal as well as vertical direction

Example on Analysis of Abutment asg asg

sahfbch

asg Approach Slab

asg

Bridge Deck

asg

asg

asg

asg

Abutment asg

asg

asg

A

1.8m 1.8m

0.9m

0.5m

7.2m

A

0.9m

Plan at Top A 8m 7m 2.8m

1.4m 2.8m Section at AA

7.2m

A

Plan at Bottom

1.4m

A 1.8m

1.8 m 0.5m HFL

0.9 m

7.2m

0.9 m

A

Plan at Top

8m

A

7m 2.8m

2.8m Section at AA

1.4m

7.2m

A

Plan at Bottom

1.4m

Limit State Design of Abutments LIMIT STATES • When abutments fail to satisfy their intended design function, they are considered to reach “limit states” • Limit states can be categorized into two types: – Ultimate Limit States – Serviceability Limit States

ULTIMATE LIMIT STATES • An abutment reaches an ultimate limit state when: – Strength of a least one of its components is fully mobilized or – Structure becomes unstable. • In the ultimate limit state an abutment may experience serious distress and structural damage, both local and global. • In addition, various failure modes in the soil that supports the abutment can also be identified. • These are also called ultimate limit states, they include bearing capacity failure, sliding, overturning, and overall instability.

SERVICEABILITY LIMIT STATES • An abutment experiences a serviceability limit state when it fails to perform its intended design function fully, due to excessive deformation or deterioration. • Serviceability limit states include excessive total or differential settlement, lateral movement, fatigue, vibration, and cracking.

Dynamic Analysis of Bridges: When load passes over the bridge  Span deflects quickly After the passing the load  Bridge comes to its original position due elasticity This loading and unloading causes vibrations in the bridge Effects of vibrations: • Unpleasant physiological and psychological reaction of human • Structural damage • Structural damage occurs due to additional stresses in the bridge elements

• Earlier studies have been made to determine additional load using the impact factor to avoid the damage due to additional stresses • Impact factor  does not guarantee to control the vibrations

• To control the vibrations in the bridges Several thumb rules are used by codal provisions • Limiting the Span to depth ratio • Limiting the deflection to span ratio • Due to above limitations  Bridge becomes more rigid  Bridge is less prone to vibrations • Since above provisions not based on frequencies  No guarantee against occurrence of vibrations

Factors affecting the vibrations in bridges:

• Rigidity of bridge • First-mode natural frequency of vibration of bridge deck • Natural frequencies of vehicle system and the suspension systems • Vehicle speed • Ratio of vehicle weight to bridge weight

• Irregularities in the bridge deck and approaches • ill functioning of expansion joints • Damping characteristics of bridge and vehicle system • Many studies have been made, but these are case specific and can not be generalized since they focus on few parameters only not all at a time

• Codal Provisions: • Codes of several countries recommend the use of Impact factor • Due to the use of impact factor  additional design static load • Needs more Section strength of section  No Failure of structural elements due to dynamic effects • Indirectly reduces the vibrations in bridge due to more rigid sections • But no guarantee

Impact factors by different countries: British Standards: 25% is added to the axel load or pair of adjacent wheels which produce greatest bending moment or shear force as the case may be All other countries consider the span length as a parameter to decide the impact factor In British, Germany and Italy  Impact factors are ignored for span > 30m, 50m and 100m respectively (other countries use certain minimum impact factor for large span bridges) In different countries the Maximum impact factors may vary from 25% to 65% In India, Japan and Austria  In addition to span, the bridge material is also considered as additional parameter Material is considered due to the fact that steel bridges are more prone to vibrations due to light weight as compared to concrete.

Some countries like France and Belgium  also consider the vehicle speed Austria specifies different Impact factors for the directly loaded girder and indirectly loaded girder

Human Aspect of Vibrations • Normal range of the fundamental frequency of bridge system varies from 1 to 20 cps • If this frequency coincides with the frequency of vehicle  Resonance  Vibrations • Human Physiological and Psychological discomfort to human • The human physiological and Psychological discomfort depends on frequency 0.25 to – 1 cps  Associated with motion sickness 2 cps  Associated with motion sickness and head resonance (for horizontal movement) 4 – 6 cps  major resonance of the whole body 7-9 cps  abdominal resonance 10-12 cps  unspecified trunk resonance • Few studies have been made to study the reaction of passengers on vibrations

Practical Approach for Vibration analysis: British Standard (CP117 Part 2-1967) and Lenzen’s criterion may be adopted to check the vibration problem in bridges:

Estimate the maximum deflection () due to a single 20 tone hypothetical load placed at center of span as well as at mid of width of bridge (in inches) using the fully composite flexural rigidity of deck Estimate the fundamental frequency of the bridge Nf using the relation

2 Nf  2 L

EI g cycle per sec ond wd

Where, L = span of the bridge; (ft) g = 32.2 ft/sec2

EI = Flexural rigidity of the full width section of the bridge deck expressed in ft tone units wd = dead load of deck including the finishes expressed in kN/m of the bridge span Calculate the parameter  = 0.4   = 0.75 

if Nf > 4 Cps if Nf < 4 Cps

Estimate the maximum acceleration A as A = 40  (Nf)2 For bridge to be safe for vibrations the parameter (A ) Not greater than 5 in2/sec2

Lenzen’s criterion for Vibrations

Long span Bridge Construction (i) Cast in Situ construction (ii) Construction using the precast elements Construction Methods using the Precast Elements – Segmental Construction Advantages: Proper construction and curing, Small elements easy to transport, Faster Construction Cantilever method economical

Balanced Cantilever method of Bridge Construction: — Chosen where a bridge has few spans which range from 50 to 250m — Construction begins at each bridge pier

— Formwork is positioned and cast-in-situ pier segment is begun — Other segments are constructed by subsequently moving the formwork — Cast-in-situ segments range between 3m to 5m in length — Segment construction is continued until the joining midpoint

— Stability of the end cantilever is maintained by using temporary pier supports as the end span is begun —Length of the end spans is equal to between 0.55 and 0.65 times the length of the typical span in the bridge — Since, the segments are attached to the cantilever ends one at a time, an overturning moment is created in pier — a temporary supports is provided on either side of the pier

Incremental launching — a technique where segments are cast at the end of the crossing and pushed — most useful when the piers can be easily located at regular intervals — Temporary support bents may or may not be required at midspans depending on the span length

— A steel launching nose is attached at end of segments to control erection stresses — The segments are usually 15 to 30 m) in length — Incremental launching is best adopted to bridge lengths of 300 to 600 m

Span by Span — most economical technique for erecting segmental bridges for medium spans(<75 m) — utilizes an assembly truss spanning between permanent piers to support precast segments prior to installation and stressing of post-tensioning tendons — Segments are placed on assembly truss by a crane in required position — After all segments comprising a span are assembled, the post-tensioning is done

Truss Bridge Construction

Or 2 -3 beams can often be lifted in, singly or joined in pairs, by cranes on each bank

Roller launch method: The bridge is constructed in situ and then jacked across the span using rollers and cantilever technique.

A temporary nose section is used this is removed once the bridge is in position

The bridge is then simply lowered into place

FULL SPAN CONSTRUCTION

Preliminary Dimensions of the abutments

1 in 24 to 1 in 12)

1 in 3 to 1 in 6

Various Shapes of stress Distribution and Maximum bearing Stress