Basic Components and
Parts of Bridge Structures
The
bridge structure consists of the following components:
- Superstructure
or decking component
- Bearings
- Substructure Components
Fig:
Semi-Through Section of a Concrete Slab Road Bridge
Superstructure Components
of Bridges
The
superstructure of the bridge structure consists of deck slab, girder, truss
etc. These components vary based on the type of bridge (whether concrete or
steel or composite). Superstructure of the bridge bears the load passing over
it. This helps in transmitting the forces formed by the loads to the below
substructures.
Decks
The
decking is considered as the road or the rail surface of the bridge. The decks
are supported by the girders or the huge beams that is in turn supported by the
piers. The whole arrangement is supported with a deep foundation mainly piles
and cap arrangement.
Bearings in Bridges
The loads
received by the decks are properly and safely transmitted to the substructure
with the help of bearings. These are components of bridge that enables even
distribution of load on the substructure material. This transmission is very
essential in situations where the substructure is not designed to take the load
action directly.
The
bearings in bridges allows the longitudinal movement of the girders. This
movement is created due to the forces acting on the longitudinal direction. The
forces due to the moving loads and the variation in temperature are the main
causes for longitudinal forces.
The
selection of bearing is dependent on certain parameters, which are: Loads
acting, the geometry, the extent of maintenance, the clearance available, the
displacement, rotation and deflection policy, availability, preference of the
designer, the construction tolerances, and the cost criteria.
For the
bridge design, all the above-mentioned aspect is considered for the design and
the choice of bearings. The designer must consider the bearing arrangement in
the bridge construction as a separate system.
In most
of construction practice, the bearing is selected or the decision for bearing
is done in the last moment. This results in increase of maintenance in the
future, which must be avoided.
Substructure Components
of Bridges
The
components involved in substructure of bridges are:
- Piers
- Abutments
- Wing
Walls and the Returns
- Foundation
Piers
The piers
are vertical structures used to support deck or the bearings provided for load
transmission to underground soil through foundation. These structures serve as
supports for the bridge spans at intermediate points.
The pier structure has mainly two functions:
- Load
transmission to the Foundation
- Resistance to the horizontal
forces
Most of
the cases, piers are designed to resist the vertical loads alone. In areas
which lie in the seismic zone, it is recommended to design the pier for lateral
loads also.
Most of
the piers are constructed using concrete. Steel for the construction of pier is
used in very few cases till now. Use of composite columns i.e. steel columns
filled with concrete is used as new technology of pier construction.
The pier is a vertical member that resist the forces by means of
shear mechanism. These forces are mainly lateral forces. The pier that consist
of multiple columns are called as bent.
Types of Piers in Bridge
Construction
There are
different types of piers based on the structural connectivity, the shape of the
section and the framing configuration.
- Based on the structural
connectivity, the pier can be classified as monolithic or cantilevered.
- Based on the shape of the section
pier can be classified as solid or hollow, hexagonal, round or
octagonal or rectangular.
- Based on the framing configuration
the pier can be classified as single or multiple column bent, hammerhead
or pier wall type.
Abutments
Abutments
are vertical structures used to retain the earth behind the structure. The dead
and the live loads from the bridge superstructure is supported by the bridge
abutments.
The
abutments are also subjected to lateral pressures mainly from the approach
embankment. The design loads on the abutment is mainly dependent on the:
- Type
of abutment selected
- The sequence of construction
The
figure below shows the primary functions carried out by an abutment.
Fig:
Abutments in Bridge Construction- Primary Functions
As seen
from the above figure, the abutments have the design requirements similar to
retaining walls as well as in pier construction. The abutments are primarily designed
to resists the overturning and sliding. More focus is on the stability of the
whole system.
The
special care has to be provided for the foundations of abutments. The abutment
foundation must overcome the problems of differential settlement and excessive
movements caused due to lateral forces or loads.
The below
figure shows the components of abutments.
Fig:
Abutments Components
Wing Walls and Returns
Structures
constructed as an extension of the abutments to retain the earth present in the
approach bank are called wing walls. This portion will otherwise have a natural
angle of repose. These are retaining walls constructed adjacent to the
abutments. This wall can be constructed either integrally or independent with
the abutment wall.
The rear
of the wall must consider three design loads while designing. This includes:
- The
earth pressure from the backfill
- The
surcharge from the live loads or the compacting plant
- The hydraulic loads from the
saturated soil conditions
The
stability of the wing wall is mainly based on its resistance against the active
earth pressures. The structural elements of the bridges are hereby designed and
constructed to resist the earth pressures at rest.
Parapets and Handrails/
Guard Rails or Curbs
These
components of bridges are not of structural importance, but provided for the
safety concerns. These are provided above the decks. This will help in
prevention of the vehicle from falling off the bridge into the water body below
or as a means for the separation of traffic streams.
Foundation of Bridges
Foundation
are structures constructed to transmit the load from the piers, abutments, wing
walls and the returns evenly on the strata.
The
foundation provided for bridge structures are deep in sufficient manner to
avoid scouring due to the water movement or to reduce the chances of undermining.
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