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Multi celled box girders : Components

Girders are the most essential part of the bridge. They support the deck slab for the traffic passing above the deck. They can be likened to the beams constructed in buildings. They are simply supported or, or even multiple spans continuing for more than one span.

A bridge is typically parceled into 3 main parts:

1. Foundation

2. Sub-structure

3. Super-structure

The Superstructure is the entire viaduct structure above the piercap. This includes the the rails, rail pedestals, parapets, blisters, etcetera

Box girders are a part of the bridge superstructure and are a type of girder used as the main bridge viaduct.

The material of construction is either of:

· PSC Concrete

· Pre-Fabricated Concrete

· Steel Plates

· Steel Truss

Based on the nature of supporting condition of girders, bridges are either simply supported, cantilevered, fixed, etc. the orientation of the girders is along the longitudinal axis of the structure. These box girder spans are simply supported on both sides.

Types of girders

· I section girders

· Box girders

· U section girders

· Steel girders



Labels for various components
Components sketch

The geometry of the box girder can be briefly classified into 4 parts-

· Top flange

The top flange is the upper part of the box girder that double serves as a deck slab. The top flange is the surface of interaction between the vehicles and the girder. The top of the flange is provided with a cross slope to drain off water to the pipes.

· Bottom flange

The bottom flange is the lower horizontal portion of the box girder. This part , as you can see, has most of the post tensioning cables fixed to it via an anchorage block. The bottom flange becomes thicker at the ends of the viaduct and remains constant throughout the mid segments of the span.

· Webs

The webs are the vertical members of the box girders. They may also be inclined and made up of composite material.

· Cantilevered flange

The cantilevered flange portion is the outwardly protruding member of the box girder. Their section gradually reduces towards the ends.

This video explains all the components in 3D-


The flat slab surface that allows vehicles and pedestrians to cross highways is a deck slab. It is an orthotropic slab member supported by the girders in composite bridges. For bridges with post-tensioned box girder viaducts, the top flange acts as the deck slab.


A diaphragm member is a structural wall constructed at certain intervals laterally along a bridge girder system.

Diaphragms in box girder spans are placed at the end of the girders and are called as the ‘end diaphragms’.

Their main function is to augment the overall stiffness of the entire structure and enhance the structural stiffness of the end segment.


Rail plinths are the elongated precast concrete members that support the steel rails and their attachment accessories.

In order to avoid rebuilding rail plinth frequently and having to disrupt the operational traffic, they have a design service life of about 100 years. A higher grade of concrete is used along with an sufficient reinforcement.


Steel rails are the long welded I section steel rails that are placed upon the rail plinth. They serve as the primary interface of contact between the bogie wheels and the bridge structure.


Anchorage block is a load-bearing and transferring structure component in that is socketed out of the box girder to provide a strong sitting for Poste tensioning cables and transfers prestressed force from steel strand to concrete member directly. They are also provided to prevent concrete bursting or crushing under extreme loads.


The cables are arranged in a grid pattern at the bottom flange ends. The anchorage block ensures the safety of the anchorage group by providing additional strength during its service life. Each cable has multiple strands that are stressed at different stages of construction and some left for future stressing should the need arise.


Blisters are longitudinal structural members monolithically constructed with the girders at the bottom flange. These members provide the necessary future provision of post tensioning

3D model indicating the Shear keys, Anchorage blocks, blisters, etc.


Shear keys placed on the box girders in bridges to provide lateral restraints to the bridge superstructures under service and earthquake conditions. In the event of an earthquake, shear keys function as structural fuses to prevent the transmission of large seismic forces to the seismic arrestor and thereby to the foundation


Vertical bearings are the interface between the concrete surfaces of the seismic arrestor and the box girder. They are placed at the notches in the end segment and vital in preventing concrete rupture during seismic or lateral impact with the arrestors.


Crash parapets are the protective walls on wither sides of the bridge girder. They are positioned as a safety and protective measure and may vary in shape as per the aesthetic or streamlined requirements. Most of these members carry utility trays that carry fixtures, ropes, rails, fences, or other conduits.

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