girder

thumb|Ceiling of Hinkle Fieldhouse in [[Indianapolis, Indiana, is constructed of large trusses built of riveted girders]]

A girder () is a beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing flanges separated by a stabilizing web, but may also have a box shape, Z shape, or other forms. Girders are commonly used to build bridges.