Truss

Historical detail of a steel truss with an actual revolute joint

A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure.^[1]

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".^[2] 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,^[3] or from graphic statics (see Cremona diagram) polygons.^[4]

Etymology[edit]

Truss derives from the Old French word trousse, from around 1200, which means "collection of things bound together".^[5]^[6] The term truss has often been used to describe any assembly of members such as a cruckframe^[7]^[8] or a couple of rafters.^[9]^[10] One engineering definition is: "A truss is a single plane framework of individual structural member [sic] connected at their ends of forms a series of triangle [sic] to span a large distance".^[11]

Characteristics[edit]

A truss consists of typically (but not necessarily) straight members connected at joints, traditionally termed panel points. Trusses are typically (but not necessarily^[12]) 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.^[13] 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.^{[citation needed]}

Simple truss[edit]

The simplest form of a truss is one single triangle. This type of truss is seen in a framed roof consisting of raftersand a ceiling joist,^[14] 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.^[15] 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.^[12] The traditional diamond-shape bicycle frame, which utilizes two conjoined triangles, is an example of a simple truss.^[16]

Planar truss[edit]

A planar truss lies in a single plane.^[15] Planar trusses are typically used in parallel to form roofs and bridges.^[17]

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.^[18]

Space frame truss[edit]

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.^[15] 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.

Simple tetrahedron
Diagram of a space frame such as used for a roof
This electrical pylon is a three-dimensional truss structure

Types[edit]

For more truss types, see truss types used in bridges.

There are two basic types of truss:

The pitched truss, or common truss, is characterized by its triangular shape. It is most often used for roof construction. Some common trusses are named according to their "web configuration". The chord size and web configuration are determined by span, load and spacing.
The parallel chord truss, or flat truss, gets its name from its parallel top and bottom chords. It is often used for floor construction.

A combination of the two is a truncated truss, used in hip roof construction. A metal plate-connected wood truss is a roof or floor truss whose wood members are connected with metal connector plates.

Warren truss[edit]

Truss members form a series of equilateral triangles, alternating up and down.

Octet truss[edit]

Truss members are made up of all equivalent equilateral triangles. The minimum composition is two regular tetrahedrons along with an octahedron. They fill up three dimensional space in a variety of configurations.

Pratt truss[edit]

The Pratt truss was patented in 1844 by two Boston railway engineers,^[19] Caleb Pratt and his son Thomas Willis Pratt.^[20] The design uses vertical members for compression and diagonal members to respond to tension. The Pratt truss design remained popular as bridge designers switched from wood to iron, and from iron to steel.^[21] This continued popularity of the Pratt truss is probably due to the fact that the configuration of the members means that longer diagonal members are only in tension for gravity load effects. This allows these members to be used more efficiently, as slenderness effects related to buckling under compression loads (which are compounded by the length of the member) will typically not control the design. Therefore, for given planar truss with a fixed depth, the Pratt configuration is usually the most efficient under static, vertical loading.

The Southern Pacific Railroad bridge in Tempe, Arizona is a 393 meter (1,291 foot) long truss bridge built in 1912.^[22]^[23] The structure is composed of nine Pratt truss spans of varying lengths. The bridge is still in use today.

The Wright Flyer used a Pratt truss in its wing construction, as the minimization of compression member lengths allowed for lower aerodynamic drag.^[24]

Bowstring truss[edit]

Named for their shape, bowstring trusses were first used for arched truss bridges, often confused with tied-arch bridges.

Thousands of bowstring trusses were used during World War II for holding up the curved roofs of aircraft hangars and other military buildings. Many variations exist in the arrangements of the members connecting the nodes of the upper arc with those of the lower, straight sequence of members, from nearly isosceles triangles to a variant of the Pratt truss.