This Architectural Drawing is AutoCAD 2d drawing of Girder cross section in AutoCAD, dwg file. A girder (/????rd?r/) 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 composed of two load-bearing flanges separated by a stabilizing web, but may also have a box shape, Z shape, or other forms.