In by Mark Goodman

A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. The space between each truss is known as a bay.[1]

Timber roof trusses were a medieval development. Earlier roofs had been supported by coupled rafters – pairs of rafters linked by horizontal beams. But such roofs were structurally weak, and lacking any longitudinal support were prone to racking, a collapse resulting from horizontal movement.[2]

Two King post trusses linked to support a roof.
Key:1: ridge board, 2:purlins, 3: common rafters. This is an example of a “double roof” with principal rafters and common rafters.
From Wikipedia, the free encyclopedia
Merriam-Webster Online Dictionary
truss (verb)
transitive verb
a) to secure tightly - bind
b) to arrange for cooking by binding close the wings or legs of (a fowl)
to support, strengthen, or stiffen by or as if by a truss
truss (noun)
an iron band around a lower mast with an attachment by which a yard is secured to the mast
a) - bracket
b) an assemblage of members (as beams) forming a rigid framework
a device worn to reduce a hernia by pressure
a compact flower or fruit cluster
Truss (Wikipedia)
Truss bridge for a single-track railway, converted to pedestrian use and pipeline support
An Egyptian ship with a rope truss, the oldest known use of trusses. Trusses did not come into common use until the Roman era.
Typical detail of a steel truss, which is considered as a revolute joint
Historical detail of a steel truss with an actual revolute joint

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.

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