Various components and parts of a bridge such as superstructure, substructure, bearings of concrete and steel bridges, their types, importance, functions are discussed.
Basic Components and Parts of Bridge Structures
The bridge structure consists of the following components:
- Superstructure or decking component
- 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.
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:
- Wing Walls and the Returns
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 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.