A bridge is the civil engineering structure that spans horizontally between the supports primarily designed for withstanding the imposed vertical loads.
The basic objective of bridge construction is to overcome various obstacles or establishing the linkage between two locations intervened by some obstructions. The obstructions usually include water bodies such as rivers, streams, valley, road etc.
The bridge is constituted of three major categories namely the superstructure components, the bearings and the substructure components which have been further discussed below.
COMPONENTS OF BRIDGE
1. Superstructure Components of Bridge
The superstructure components of bridge refer to the components that lie above bearings.
The superstructure components mainly include the beams or girders, arches, cables, handrails, parapet walls, and decks. However, the type of superstructure components may vary based on the type of bridge i.e. the superstructure components can vary for steel, concrete and composite bridges.
The primary objective of the superstructure components is to withstand the load directly imposed on it and thus transferring the load to the substructure components. The superstructure components of the bridge have been further explained below.
a. Beams and Girders:
The beams and girders are provided to support the road pavement thereby preventing it from bending.
Beams are mostly used in the concrete bridges.
The beams consisting of the rectangular cross-section are commonly used.
The girders are also a form of beam used in the girder bridges.
In the case of girder bridges, girders consisting of I-shaped cross-sections and two load-bearing flanges and webs are mostly used.
b. Arches and Cables:
Arches are provided in the arch bridges whereas cables are provided in the suspension and cable-stayed bridges.
The arches in case of the arch bridges help to resist the forces acting on the bridge.
The cables in the suspension bridge serve as the tensile element that provides support to the deck as well as transfer the loads to the supporting towers and anchorages.
c. Parapet Walls and Handrails:
The parapets and the handrails serve as the protective safety elements of the bridge.
A parapet prevents the vehicles from falling off. Apart from this, the parapet also functions as the noise barrier and also prevent waste and rubbish from passing below. The parapet also functions as the means of separation of traffic streams.
The handrails also serve as the safety component of the bridge and can also be used to improve the aesthetic appearance of the bridge.
The deck consists of the roadway or the rail surface that facilitate the passage of traffic in the bridge. Mostly, the deck is supported by the beams or the girders that are further supported by the piers or deep foundations with caps.
Bearings lie between the superstructure and the substructure components of the bridge. In other words, bearing transfers the load coming from the superstructure to the substructure components of the bridge.
The primary objective of providing the bearings is to ensure the even distribution of the loads on the underlying substructure.
The bearings also stabilize and permit translational and rotational movements.
The rotational movements are induced by the moments whereas the translational movements include the displacements in both the vertical and horizontal directions due to in-plane or out-of-plane forces like wind and self-weight.
The bearings are highly important for the transmission of the imposed loads particularly when the substructure components of the bridge have not been constructed to withstand the loads directly.
A wide variety of bearings have been developed. Usually, the selection of the type of bearing is done based on several factors such as types of loadings, geometry and dimensions of the bridge, the displacement and deflection, availability of the materials, financial resources available, clearance available, maintenance criteria etc.
The different types of bearings in bridges have been further explained below:
i. Pin Bearing:
The pin bearing allows the rotational movements utilizing steel members. This bearing essentially consists of a pin that is composed of upper and lower semicircularly recessed surfaces with a solid circular pin placed between them. Mostly, caps are also provided at both the ends of the pin. These caps prevent the sliding off of the pin as well as withstand the uplifts. The pin bearings do not permit the translational and lateral movements.
ii. Roller Type Bearing:
The roller type bearing essentially consists of cylindrical rollers and balls. It is the form of the fixed bearing. It is capable of resisting the service movements as well as damping. This type of bearing does not allow rotational and lateral movements. However, it permits longitudinal movements. The major disadvantage of using this bearing is that it relatively accumulates more dust and debris.
iii. Rocker Type Bearing:
It consists of a pin at the upper portion and a curved surface at the bottom portion. The primary objective of the pin is to permit the rotations while the objective of the curved surface is to permit the translational movements. This type of bearing is a form of expansion bearing and is highly desirable in steel bridges.
iv. Sliding Bearing
Sliding Bearing consists of two metal plates that are capable of sliding against each other. Such sliding of the metal plates thus permits the translational movements. Mostly, this type of bridge bearing is used in combination with other types of bearing. It alone can be used as bridge bearing in the cases where the rotational movements at the supports due to the deflection is negligible.
v. Pot Bearing
Pot bearing constitutes of a shallow steel cylinder attached to a neoprene disk. The steel cylinder is commonly referred to as the pot. The pot is fitted on the disk on a vertical axis.
3. Substructure Components of Bridge
The substructure components of bridge refer to the components of bridge that lie below the bearings.
The primary objective of the substructure components is to transfer the load coming from the superstructure to the underlying soil layer.
The major substructure components include the piers, the abutments, wing walls & returns and the foundation which have been further described below.
The piers consist of vertical structures designed to provide support to the deck or the bearings.
They act as a medium to transfer the incoming load to the soil layer below utilizing the foundation.
Piers are also capable of resisting the horizontal forces.
Mostly, piers are made up of concrete and are provided at intermediate points throughout the bridge.
The abutments are constituted of vertical structures designed to retain the earth behind the bridge structure.
The abutments also provide additional support to the dead loads and the live loads imposed on the bridge.
c. Wing Walls and Returns:
The wing walls are the substructure components that simply refer to the extension of the abutments to retain the earth in the approach bank.
The wing walls are the additional retaining walls constructed adjacent to the abutments.
They transfer the load on the underlying strata evenly.
The bridge foundation must be sufficiently deep to prevent the scouring due to water currents.