Girder
A girder is a large and deep type of beam that is used in construction. It is typically capable of longer spans and taking greater loads than a normal beam, and is often used as a main horizontal structural support for smaller beams, such as in bridge construction.
There are several different types of girder available depending on circumstances, and the load they are required to support:
- Smaller steel girders can be 'rolled' into the required shape. When girders become larger however, a standard rolled shape may not be available and a plate girder may have to be fabricated instead.
- A plate girder is typically an I-beam cross-section made up of separate structural steel plates which are welded, bolted or riveted together to form the deeper vertical web and narrower horizontal flanges of the beam. Plate girders are commonly-used for spans of up to 15m.
- A gantry girder is used for a gantry crane - typically consisting of two ‘A’ frames connected by a lattice cross member which straddles the work area. The lifting gear is suspended from the horizontal girder and can move along it on rails. For more information, see Types of crane.
- A box girder is fabricated from steel plates used to form a rectangular box. This resists torsion better than a plate girder and can be used when depth constraints mean a plate girder cannot be made deeper.
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