CVS 517E: Design of Bridges 2017/2018 Academic Year Chapter 3: Bridge Design

3.5.2 Bridge Abutment Design

Loads transmitted by the bridge deck onto the abutment are:

i. Vertical loads from self-weight of deck.
ii. Vertical loads from live loading conditions.
iii. Horizontal loads from temperature, creep movements etc. and wind.
iv. Horizontal loads from braking and skidding effects of vehicles.
These loads are carried by the bearings which are seated on the abutment bearing
platform. The horizontal loads may be reduced by depending on the coefficient of
friction of the bearings at the movement joint in the structure.
However, the full braking effect is to be taken, in either direction, on top of the abutment
at carriageway level.
In addition to the structure loads, horizontal pressures exerted by the fill material
against the abutment walls is to be considered. Also a vertical loading from the weight of
the fill acts on the footing.
Vehicle loads at the rear of the abutments are considered by applying a surcharge load
on the rear of the wall.
For certain short single span structures it is possible to use the bridge deck to prop the
two abutments apart. This entails the abutment wall being designed as a propped
cantilever.

a) Earth Pressures
• Active earth pressures (Kaγ h) are considered to ensure that the abutment is
stable.
• At rest earth pressures (Koγ h) are considered to ensure that the structural
elements are adequate.
• Passive earth pressures (Kpγ h) are only considered for integral abutments or
where shear keys are provided.

At rest pressures are initially developed on the back of the abutment wall during
construction and whilst the backfill is compacting. Consequently the structural elements
have to be designed to resist the effects of these pressures.

Any movements in the structure caused by the at rest pressure, either through rotation
or deflection will reduce the pressure on the back of the wall; a state of equilibrium is
reached when the pressure reduces to the active earth pressure value. Consequently the
stability of the structure can be checked by using active earth pressures.
Passive pressures are developed when the structure pushes against the soil. Since
movements required to develop passive pressures are considerably greater than that for
active pressures, and the structure is designed to ensure that the foundations do not
slide under active pressures, then it is unlikely that passive pressures will be developed
in front of the abutment. There is also the chance that, at some time in the future, the soil
in front of the abutment may be removed temporarily. This could happen if services,
such as drainage pipes, water or gas mains, are installed or repaired in front of the
abutment. Consequently the structure needs to be designed to be stable with no soil in
front of the concrete footings.
If shear keys are required to prevent sliding then the key should be located under the
rear half of the base and a factored value of passive pressure is used.

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CVS 517E: Design of Bridges 2017/2018 Academic Year Chapter 3: Bridge Design

Integral bridges experience passive pressures on the back of the abutment wall when
the deck expands.

b) Abutment Construction
Departmental Standard BD 30 gives recommendations for the layout of backfilled
cantilever retaining walls with spread footings or piled foundations. The layout of the
abutment will have implications on the design which need to be considered.

The provision of a drainage layer will allow pore water pressures to be ignored (unless
there is a possibility of a large water main bursting). However the drainage layer
separates the backfill soil from the wall so back of wall friction should not be included.
Traffic vibration will also affect any vertical friction effects on the back of the wall.

Foundation level is usually set at least one meter below ground level to avoid
deterioration of the foundation material through frost action. If services, such as gas
pipes, water mains, electricity cables etc., may be installed in front of the abutment wall
then the depth to foundation level may need to be increased to allow the services to be
installed above the concrete footing.

It is usual to provide granular backfill to the back of the wall which limits the material to
Class 6N or 6P. A typical value for the effective angle of internal friction (ϕ') for Class 6N
or 6P material is 35o. This equates to serviceability limit state values of:
Ka = (1-Sinϕ') / (1+Sinϕ') = 0.27
Ko = (1-Sinϕ') = 0.43

c) Loading
Loading from the deck is applied to the abutment through the bearings. Maximum
vertical bearing loads are obtained from the deck analysis; these loads, together with the
type of restraint required to support the deck, will dictate the type of bearing provided.

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CVS 517E: Design of Bridges 2017/2018 Academic Year Chapter 3: Bridge Design

Horizontal loads from the deck are produced by wind loading, temperature effects, creep
movements, traction, braking and skidding loads, collision loads when high level of
containment parapets are used, and centrifugal loads if the horizontal radius of
curvature of the carriageway is less than 1000 metres.
Longitudinal loads from temperature effects in the deck will be determined according to
the type of bearing used. Elastomeric bearings are effectively 'glued' in place between
the deck soffit and the abutment bearing plinth so that the bearing has to distort when
the deck expands and contracts. The longitudinal force produced by this distortion is
proportional to the shear stiffness of the bearing and the magnitude of the movement.
Sliding bearings, on the other hand, produce a longitudinal load which is proportional to
the dead(permanent) load reaction and the coefficient of friction between the sliding
surfaces. The cofficient of friction (μ) varies between 0.01 and 0.08 depending on the
type of bearing and bearing stress (see BS 5400 Part 9:1, Tables 2 and 3).

The longitudinal load from the temperature effect will act equally on both abutments. If
sliding bearings are used then the load transmitted is equal to the friction at the bearing
under dead and superimposed dead loads (permanent actions). If elastomeric bearings
are used then the load transmitted is equal to the force required to distort the bearing
by the distance the deck expands or contracts.

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CVS 517E: Design of Bridges 2017/2018 Academic Year Chapter 3: Bridge Design

The deck is very stiff in the axial direction so horizontal loads will have negligible effect
on the length of the deck. Hence longitudinal loads due to traction, braking and skidding
are assumed to be transmitted to the fixed abutment only. If only elastomeric bearings
are used, i.e. there is no fixed abutment, then the loads due to traction, braking and
skidding are shared between the two abutments.

Transverse loads on the deck will be transmitted to the abutment through the fixed and
sliding-guided bearings only. These loads are unlikely to have an effect on the stability of
a full height abutment, but the bearing plinths need to be designed to resist the loads.
The stability of small abutments, such as bank seats, may need to be checked for these
loads.
Live loading at the rear of the abutment is represented by a surcharge loading. The
curtain wall (also called up stand wall or ballast wall) does however need to be designed
for braking forces.

Vehicle collision on abutments need not normally be considered as they are assumed to
have sufficient mass to withstand the collision loads for global purposes.

d) Stability
Stability of the abutment is determined by considering:
• Sliding
• Overturning
• Failure of the foundation soil
• Slip failure of the surrounding soil
A comprehensive Ground Investigation Report is essential for the design of the bridge
structure. Boreholes need to provide information about the nature of the ground below
the foundations. Adequate sampling and testing also need to be carried out to obtain
design parameters for allowable bearing pressures, together with friction and cohesion
values of the soil at foundation level.
When using BD 30 sliding and overturning effects are calculated using nominal loads
and active earth pressures. A factor of safety of 2.0 is used to ensure that the abutment is
stable against sliding and overturning.
Several load cases need to be considered to ensure all loading conditions are catered for.

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