A girder is a support 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 for strength, but may also have a box shape, Z shape and other forms. A girder is commonly used many times in the building of bridges, and planes.In traditional timber framing a girder is called a girt.The Warren type girder combines strength with economy of materials and can therefore be relatively light.