Girder

A girder is a large horizontal structural member that carries loads over a span and transfers them to supports such as columns or walls. In buildings and bridges, girders act as primary bending members that support floors, roofs, or deck systems and resist bending and shear. In common engineering usage, a girder is the main beam between supports; smaller members rest on girders and transfer loads to them.

Girders are made in various materials and forms. Steel girders are often formed from rolled shapes such as I-beams or wide-flange sections, and may also be built up from plates or hollow sections (box girders). Concrete girders include cast-in-place or precast reinforced or prestressed members. Timber girders are used in traditional or light-frame construction. Bridges may employ plate girders or box girders for longer spans.

Design and detailing focus on resisting bending moments, shear forces, and deflection from loads such as dead load, live load, wind, and seismic effects. Ends are supported on bearings, and connections to flanges or adjacent members are designed to transfer forces safely. Design codes such as AISC in steel construction and Eurocode 3 provide rules for sizing, detailing, and tolerances.

Common applications include supporting floors in multistory buildings, spanning roadways or rivers in bridges, and carrying heavy equipment or loads in industrial facilities. Girders can be manufactured off-site and installed as part of a larger superstructure, sometimes combined with trusses, plate girders, or box girders to meet specific span and load requirements.