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A steel truss is an arrangement of steel pieces connected to form a structure intended to span or bridge a gap of some sort. A steel truss transfers weight, or pressure applied to it, to weight-bearing structures on either side of the gap. Steels trusses come in many types and are commonly used for large roofs and bridges. While any truss can be made of steel to improve its load-carrying capabilities, and many often are, there are several types of steel truss that are more common than others.
Post trusses are the oldest and simplest forms of truss and come in two basic designs: king post and queen post. The king post truss is a simple triangle shape with a single vertical post in its center. The queen post truss is composed of two horizontal pieces with the lower member being longer than the upper. At each end of the upper member, there is one vertical post connecting the upper and lower members and a diagonal member that connects the ends of both horizontal members to form a triangle. Post trusses were originally made of wood to support bridges; however, today they are made of both steel and wood for roofs as well as bridges.
A Brown’s truss is a form of truss developed in the 1860s. This truss has two horizontal pieces that are connected with a series of diagonal members forming X's across the length of the truss. While not in heavy use, Brown’s trusses are often made of steel and used for heavy-duty spans such as railroad bridges.
Pratt trusses are made of two parallel horizontal members with a number of vertical members creating what appears to be a series of squares. Each square then has a diagonal member, dividing it into two, with the lower end pointing toward the center of the truss. Originally made of wood, this form of steel truss is commonly used to roof over large areas and for bridges. A variation of the Pratt truss is the Howe truss, which has its diagonals pointing down toward the ends of the truss instead of toward the center, and it is also used for heavy-duty bridges and large-scale roofing systems. Another variation is the Pengram truss, which is identical to the Howe truss except that its upper horizontal member is curved instead of straight.
Space frame trusses are another common type of steel truss used for large-scale roofing systems. Each section consists of a three-dimensional, point-up, three-sided pyramid. When several of these structures are joined together, their tips can be joined to create a series of point-down pyramids. These structures can be continued and interconnected as far as needed in all directions, unlike most trusses that only travel in a single plane.