Design of Abutments

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Bridge Substructure

Abutments

MAB1053 Bridge EngineeringFaculty of Civil Engineering, UTM


Subtructures

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.


Bridge Abutments

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.


Bridge Abutments

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.


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.


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.


Open Abutments

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.


Various Types of Open Abutments


Piled Foundation

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


Wall Abutments 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

Mass Concrete

Cantilever


Wall Abutments

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

Counterfort Stub Counterfort


Hollow Abutment

For high abutments on sloping ground, this construction offers advantages over heavy counterfort construction.


Other Types of Wall Abutments


Choice of AbutmentsWall Abutments

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 9mb) Wall thickness – 0.7m to 1.1m


Choice of AbutmentsSkeleton 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.


Choice of AbutmentsBank Seats

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.


Choice of AbutmentsWing WallsThese 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.


Wing Walls


Wing Walls


Wing Walls


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


Abutments – Modes of Failure

Sliding 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.



Foundation Yield Overturning



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.



Slip Circle Structural Failure



Slip Circle – Only a problem in cohesive soils.

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.


Basic Components of Abutment


Forces on an Abutment



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)



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.



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.


Forces on an Abutment 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.



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.



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.


Forces on the Abutment


WA

Abutment (Load Case 1)

Self Weight during construction


WA

DL + HA

Tr + Fstc + W

1/3 PSHB

Pa



WA

DL + HB

Tr + Fstc + W

1/3 PSHB

Pa



WA

DL

Fstc



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


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


Basic Design Considerations

Cantilever Wall Abutment


Cantilever Retaining Wall 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.


Cantilever Retaining Wall


Analysis & Design of CantileverRetaining Wall

Stability Analysis Design of Concrete Members


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


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


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.


Forces on Cantilever Wall


Lateral Earth Pressures

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.


Lateral Earth Pressures

Ka.w Ka.γH

H/2

Ka.wH

H/3

Pa = 1/2Ka.γH2

Due to surcharge Due to backfill soil


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.


Stability Analysis

1. Check factor of safety against overturning.

2. Check soil bearing pressure.

3. Check factor of safety against sliding.


Overturning 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.


Factor of Safety for Overturning

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.

The resisting moment is calculated as :

Overturning moment is calculated from :

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.


Factor of Safety for Overturning

The factor of safety against overturning is determined from :

FoS = Resisting Moment = MR

Overturning Moment Mo

FoS should be > 1.5


Bearing Pressure

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

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.


Checking for Bearing Pressure

B/2

e

X

Σ W

Eccentricity, e = B/2 –X

Either,

e ≤ B/6 or e > B/6


Bearing Pressure

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.


Bearing Pressure

If e > B/6, Qmin is zero,

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


Sliding 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.


Factor of Safety for Sliding

The driving force for sliding is calculated as

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.



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.



The factor of safety is calculated as

Resisting Force, ΣF > Sliding Force, μΣW


Forces on the Abutment


Design of RC Members

1. Check thickness of stem for shear stress.2. Design stem reinforcement for bending.3. Check thickness of heel for shear stress.4. Design heel reinforcement.5. Check shear stress for toe when the toe is long.6. Design toe reinforcement for bending.7. Check shear stress in key when key is deep

and narrow.8. Design key reinforcement for bending.


Design of Stem


Design of Heel

eu ≤ B/6

eu > B/6


Design of Toe

Documents

Design of Abutments