Box girder bridges are quite common .They are usually manufactured from prefabricated steel girders. This means that the girders are manufactured in a factory and transported to the location of the bridge, where they are fixed together. The steel girders of a bridge are fixed together normally with large nuts and bolts. Sometimes rivets are used although these are seen on older structures.

Specification:

Each side of the bridge is composed of three triangles. Each triangle is made up of three steel girders bolted together. This is called triangulation and produces a structure of great strength.

An example of girders bolted together can be seen below. The girders in diagram ‘A’ have been made slightly transparent so that the bolts penetrating both girders can be seen. Diagram ‘B’ shows the girders and bolts as they are normally seen.

Normally bridges and other structures made from prefabricated steel are bolted together even though it is expected that they will be permanent structures. Fixing girders together with bolts is faster, more efficient and safer than using ‘red’ hot steel rivets (See construction of the Empire State Building).