A main horizontal beam made of steel, reinforced steel or wood upon which floor joists rest and used to support other structural members or concentrated loads at isolated points along its length.
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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 composed of two load-bearing flanges separated by a stabilizing web, but may also have a box shape, Z shape and other forms. A girder is commonly used to build bridges.
The Warren type girder replaces the solid web with an open latticework between the flanges. This truss arrangement combines strength with economy of materials and can therefore be relatively light. Patented in 1848 by its designers James Warren and Willoughby Theobald Monzani, its structure consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression. It is an improvement over the Neville truss which uses a spacing configuration of isosceles triangles.