Trusses, used in framing structures like decks, roofs, and bridges, come in planar and space designs. Planar trusses use triangles, while space trusses have members extending in different directions. Analysis is performed at the joints, and graphical diagrams are used to calculate stresses.
The trellis is an engineering design that dates back thousands of years. A planar truss design is a simple triangle, or connected triangles, while a space truss has members extending in a different direction to establish a third dimension. Trusses are used to frame decks, roofs, floors, sub-floors, and many other structures. The best truss design typically depends on the application.
A planar truss design is used for frame decks and roofs. For a roof, the triangles connect the roof beams and ceiling beams; the load is distributed between the beams and joists. For a bridge structure, the top and bottom of the joist are parallel and are called chords. Solid beams to support the load on a bridge would greatly increase the weight and cost; the truss bears the load to varying degrees, depending on how far apart the strings are.
Proven planar truss designs include the Pratt truss – with boxes that hold one diagonal member per box – and the king post truss – with one upright member and two angled members connecting to the top of the upright. Queen post trusses have two vertical members and a top chord comprising a box that is supported by an angled member on each vertical. Lenticular trusses, commonly seen on bridges, have the upper chord of the truss running in a gentle arc, thus giving a lens shape to each truss structure.
For a space truss, a tetrahedral or pyramid shape is the most common truss design. More complicated space trusses will connect and configure tetrahedrons in various ways to distribute loading, torsion, compression and tensile forces. Space mast designs can be seen in structures, such as high-voltage power line pylons. Also, space trusses are used for roof structures of commercial buildings.
Analysis of truss designs is a complex process, but the analysis is usually performed at the joints, or hinges, of the structure rather than along the length of the members. Loading that is exerted by a passing car or train, for example, is considered to be a moment in which the stresses are predominantly at the joints and the stresses exerted along the lengths of the members are negligible. Graphical diagrams, such as the Cremona diagram or Culmann diagram, are used to calculate the stresses on the trusses themselves. To further examine these types of loads, formulas such as Ritter’s analytical method can be used.
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