Truss

A truss is an assembly of beams or other elements that creates a rigid structure. In engineering, a truss is a structure that 'consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object'. A 'two-force member' is a structural component where force is applied to only two points. Although this rigorous definition allows the members to have any shape connected in any stable configuration, trusses typically comprise five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes.Simple tetrahedronDiagram of a planar space frame such as used for a roofThis electrical pylon is a three-dimensional truss structureThe Hong Kong Bank of China Tower has an externally visible truss structureThe HSBC Main Building, Hong Kong has an externally visible truss structureDouble chorded heavy timber truss with 80 foot clear spanSupport structure under the Auckland Harbour BridgeThe Auckland Harbour Bridge seen from Watchman Island to its westLittle Belt: a truss bridge in DenmarkPre-fabricated steel bow string roof trusses built in 1942 for war department properties in Northern AustraliaRoof truss in a side building of Cluny Abbey, FranceA section through a queen post timber roof trussA space truss carrying a floor in The Woodlands MallElectricity pylonTimber roof trussModern temporary bridge made of Bailey bridge truss panels in Montreal Québec A truss is an assembly of beams or other elements that creates a rigid structure. In engineering, a truss is a structure that 'consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object'. A 'two-force member' is a structural component where force is applied to only two points. Although this rigorous definition allows the members to have any shape connected in any stable configuration, trusses typically comprise five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. In this typical context, external forces and reactions to those forces are considered to act only at the nodes and result in forces in the members that are either tensile or compressive. For straight members, moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes, as is necessary for the links to be two-force members. A planar truss is one where all members and nodes lie within a two-dimensional plane, while a space truss has members and nodes that extend into three dimensions. The top beams in a truss are called top chords and are typically in compression, the bottom beams are called bottom chords, and are typically in tension. The interior beams are called webs, and the areas inside the webs are called panels, or from graphic statics (see Cremona diagram) polygons. Truss derives from the Old French word trousse, from around 1200, which means 'collection of things bound together'. The term truss has often been used to describe any assembly of members such as a cruck frame or a couple of rafters. One engineering definition is: 'A truss is a single plane framework of individual structural member connected at their ends of forms a series of triangle to span a large distance'. A truss consists of typically (but not necessarily) straight members connected at joints, traditionally termed panel points. Trusses are typically (but not necessarily) composed of triangles because of the structural stability of that shape and design. A triangle is the simplest geometric figure that will not change shape when the lengths of the sides are fixed. In comparison, both the angles and the lengths of a four-sided figure must be fixed for it to retain its shape. The joint at which a truss is designed to be supported is commonly referred to as the Munter Point. The simplest form of a truss is one single triangle. This type of truss is seen in a framed roof consisting of rafters and a ceiling joist, and in other mechanical structures such as bicycles and aircraft. Because of the stability of this shape and the methods of analysis used to calculate the forces within it, a truss composed entirely of triangles is known as a simple truss. However, a simple truss is often defined more restrictively by demanding that it can be constructed through successive addition of pairs of members, each connected to two existing joints and to each other to form a new joint, and this definition does not require a simple truss to comprise only triangles. The traditional diamond-shape bicycle frame, which utilizes two conjoined triangles, is an example of a simple truss. A planar truss lies in a single plane. Planar trusses are typically used in parallel to form roofs and bridges. The depth of a truss, or the height between the upper and lower chords, is what makes it an efficient structural form. A solid girder or beam of equal strength would have substantial weight and material cost as compared to a truss. For a given span, a deeper truss will require less material in the chords and greater material in the verticals and diagonals. An optimum depth of the truss will maximize the efficiency. A space frame truss is a three-dimensional framework of members pinned at their ends. A tetrahedron shape is the simplest space truss, consisting of six members that meet at four joints. Large planar structures may be composed from tetrahedrons with common edges, and they are also employed in the base structures of large free-standing power line pylons.