Bridge Components Explained : Backbone of Bridge Structures
A bridge is built to provide rail or road connectivity between two lands separated by a natural barrier that might span from a few meters to even kilometers. Not just in length, the bridges vary widely in their designs too. But despite their large differences, the fundamental bridge components remain the same. In this blog, we’ll look at the different sections of a bridge and the common bridge components specific to each section.
The Principal Sections of a Bridge
A bridge is designed to carry passing loads of traffic up to a specified limit. Moreover, the structure of the bridge itself creates a static load. The number of footings may vary depending on various factors. Taking everything into account, the design of a bridge is mainly divided into the following sections:
- Superstructure
- Bearings
- Substructure
- Foundation
Superstructure
The main parts of the superstructure of a bridge consist of a deck slab, girder, and truss. These components vary based on the type of bridge (whether concrete, steel, or composite). The superstructure of the bridge bears the load passing over it. This helps in transmitting the forces formed by the loads to the below-listed substructures.
Deck
The deck or deck slab is formed by the road, rail, or pedestrian walkway over the obstacle, supported by the girders or the huge beams placed on the piers. The whole arrangement is supported by a deep foundation, mainly piles and cap arrangements.
Girders
The girders are long horizontal beams usually made of steel or prestressed concrete spanning two consecutive piers. The girders support smaller beams that, in turn, support the deck.
Truss
A truss is an array of triangulated structures of interconnected linear axial members, connected at pin joints. Since a triangle cannot be deformed under stress, a truss provides a sturdy structure that can sustain heavy external loads across a wide area. Trusses are often used in bridge construction because, in relation to the weight they can sustain, they require very little material.
Bearings
The bearings are applied at the contact points between the girders and piers. The use of bearings prevents cracks on the piers due to the movement of the girders. Due to moving loads of traffic on the deck and variations in temperature, longitudinal forces are exerted on the girders.
The bearings on bridges allow the longitudinal movement of the girders without affecting the piers. So the other components of the loads from the superstructure are evenly transmitted to the substructure.
Substructure
The substructure of a bridge receives the loads from the superstructure and transmits to the foundation. The main components of the substructure include:
- Piers
- Abutments
- Wing Walls and the Returns
Piers
Piers are critical substructure components that support the superstructure and transmit their loads to the foundations. Piers are widely used to span bridges with deck widths of up to 8 metres (equal to two traffic lanes). The size and design of piers, which are usually built in situ, vary depending on site-specific factors along with geographical, economic, and aesthetic considerations.
Piers are typically constructed to withstand only vertical loads. However, in seismic zones, it is advised that the pier be designed for lateral stresses as well. The pier, being a vertical element, resists lateral forces via a shear mechanism.
Most of the piers are made of concrete. The use of composite columns, which are steel columns filled with concrete, is a new technology in pier construction. The pier, made up of many columns, is referred to as bent.
Abutment
Abutments are robust pillar-to- wall constructions on both ends of a bridge, supported by earth on their backs. The bridge abutments absorb both the dead and live loads from the superstructure of the bridge. The design specifications for the abutments are comparable to those for retaining walls and pier construction.
These serve as the structural connection between the bridge’s structure and the road. They support the bridge girders on one hand and the launch apron, or one edge of an approach slab, on the other.
The main purpose of the abutments is to prevent overturning and sliding by providing stability to the whole system. The foundations of abutments require special consideration. The issues of differential settlement and excessive movements brought on by lateral loads or stresses must be resolved by the abutment foundation.
Wing Walls
In a bridge, the wing walls serve as retaining walls and are located next to the abutments. They are often made of the same material as abutment construction. The wing walls may be independent of the abutment or linked to it.
In order to maintain earth pressure in the approach bank, wing walls are erected at both ends of the abutments. Their design is determined by the embankment’s characteristics rather than the kind or components of the bridge.
Foundation
Foundations are the crucial parts of a bridge structure that transmit the aggregate loads from the superstructure to the underlying bearing strata. They are necessary to support portal frames, bridge towers, and piers. There are multiple varieties of bridge foundations, including but not limited to open, raft, pile, well, caisson, and inverted arch foundations. Bridge foundations frequently use piles, also known as H-piles, bore piles, pipe piles, or precast concrete piles.
Bridge Construction Methods
Modern bridge constructions typically employ repetitive use of segments and components.
The following are the different methods:
- Segmental construction
- Cantilever construction
- Incremental Launching Method
- Accelerated construction
The selection of the appropriate method depends on various factors, such as:
- Regularity of the span lengths
- Horizontal and vertical profiles of the bridge decks
- Soil strata
- Materials
- Accessibility for the equipment
- The time allowed for construction
- Traffic impact
What is Segmental Bridge Construction?
Unlike traditional methods that build a bridge in large sections from one side to the other, a segmental bridge is built in short parts (called segments), extending from the opposite sides to the centre using the balanced cantilevering method.
Currently, segmental construction is the predominantly used bridge construction method for both concrete and steel bridges. The concrete segmental bridges can be built using either precast concrete, manufactured somewhere else and then moved to the bridge’s site, or cast-in-place concrete.
For lengthy spans (greater than 100 metres or 330 feet), the segmental method of bridge construction offers a cost-effective solution. Their visual appeal is another factor in their selection.
Despite the wide variety of bridge designs, their basic structures and principal components remain the same, as presented in this blog. Write to us at [email protected] for more on bridge components and construction.