truss

truss.
1. Rigid structural framework of timbers bridging a space, each end resting on supports at regular intervals (often defining bays), to provide support for the longitudinal timbers (e.g. purlins) that carry the common rafters and the roof-covering. Its stability, dependent on e.g. triangulation, also prevents the roof from spreading. Types of truss or roof-structure include:aisle: in timber-framed work a complete aisled structure set over the tie-beams;Belfast or bowstring: of timber, for spans of up to 15 metres, with a segmental top member joined to a horizontal lower chord, string, or tie (sometimes slightly cambered) by inclined lattice-members;box-framed: complete cross-frame the entire height of the building in a box-framed structure;closed: with the spaces between its members filled in (e.g. between rooms or at gable-ends);common rafter: type of roof constructed of pairs of common rafters. If common rafters are held together with collars or tie-beams, the resulting structure is called a coupled rafter roof or a trussed rafter roof, to emphasize the presence of additional components;compass or compass-headed roof: one in which the braces, rafters, and collar-beams of each truss are arranged and shaped in the form of an arch, thus creating a half-cylindrical underside to the roof-structure; coupled rafter roof: a common rafter roof, but with the rafters connected by collars; cradle: where the tie from the foot of one rafter is attached to the opposite rafter at a considerable height from its foot, or the structure has collar-beams and braces as well, thus forming a shape like part of a polygon which, if upside-down, could resemble a cradle, the result is called a cradle-roof; cruck: pair of cruck blades with transverse members (e.g. tie-beam, collar, saddle, yoke, or spur);cut: truncated, with the part of a truss over the collar-beams flattened off;double arch-braced: with two pairs of arch-braces forming a continuous curve from where the braces are supported to where they join in the middle of the collar;double-framed roof: with principals or principal rafters supporting horizontal members (e.g. purlins) which carry the common rafters: the principal rafters divide the length of the roof into bays; double hammer-beam: as a hammer-beam truss, but with upper hammer-beams carrying upper hammer-posts (e.g. Church of Sts Peter & Paul, Knapton, Norfolk);false hammer-beam: with a transverse timber like a hammer-beam, but braced to a principal or collar without a hammer-post;hammer-beam: with transverse timbers, like a tie-beam from which the middle section has been removed, supported on braces and carrying hammer-posts and braces that carry the open structure of the roof;intermediate or secondary: truss of relatively light construction between the main trusses (defining the bays) and carried on horizontal plates spanning between the main trusses rather than on a main structure rising from the ground;kerb-principal: with two curved kerb-principals rising from a tie-beam to a collar on either side of a crown strut;king-post: with an upright post set on a tie-beam or collar rising to the apex to support a ridge-piece;open: with spaces between timbers unfilled (e.g. in a hall of two bays when one truss supports the structure half-way along its length, the trusses at the ends of the hall being closed);post-and-rafter: with principal rafters and wall-posts strengthened by knee- or sling-braces, but no tie-beams;principal rafter roof: type of structure in which common rafters are supported on plates and purlins, the latter carried on principal rafters forming part of a truss;queen-post: with paired vertical posts set on the tie-beam and supporting plates or purlins;scissor-truss: with braces crossing and fixed to each other, thus tying pairs of rafters together;single-framed roof: constructed with no main trusses, the rafters being fixed to a wall-plate and ridge, or with horizontal members entirely omitted, so the roof consists only of common rafters butting together at the apex of the roof;spere: set at the lower end of a hall dividing the cross-entry or screens passage from the hall itself.

2. Element projecting from the naked of a wall, e.g. a console, corbel, modillion, etc.

Bibliography

Alcock,, Barley,, Dixon,, & and Meeson (1996);
Brandon (1860);
Gwilt (1903);
W. McKay (1957);
W. Papworth (1892);
Sturgis et al. (1901–2)