Trusses
Why trusses suit this technology
Section titled “Why trusses suit this technology”In a truss the members work mainly in tension and compression, not in bending. That suits a thin-walled section: a large span can be crossed with little material.
The other reason is manufacturing: every member of the web arrives from the machine cut to length and prepared. No hand cutting, no welding, no fitting on site.
Geometry is not a limit
Section titled “Geometry is not a limit”Because the machine cuts the members, the angle and the web layout are a design question, not a manufacturing obstacle. Geometries can be built that would be disproportionately expensive in conventional steelwork because of the hand labour: skewed planes, trusses of varying depth, approximations to curves.
Whatever can be issued to the machine controller as a program can be made — the complexity adds design work rather than limiting what is buildable.
What has to be designed
Section titled “What has to be designed”The connections
Section titled “The connections”The capacity of a truss is decided at the connections: how many screws, in what pattern, and what tear-out resistance the sheet has. Self-drilling screw connections.
Buckling of the compression web members
Section titled “Buckling of the compression web members”Short but slender members — buckling and twisting count here too.
Continuity and splices in the chords
Section titled “Continuity and splices in the chords”The position and detail of the splice is a design decision.
Stability out of the plane of the truss
Section titled “Stability out of the plane of the truss”A truss is stiff in its own plane and not out of it. Hence the need for bracing in the roof plane and restraint between the trusses — which also determines the sequence of erection.
Bearing and support
Section titled “Bearing and support”The bearing and anchorage at the ends of the truss: uplift from wind can appear here.
Erection
Section titled “Erection”Trusses are unstable out of plane while being lifted, so temporary bracing is not a step that can be dropped. It is part of the erection design.
What else they are good for
Section titled “What else they are good for”Not only for roofs: longer-span floors, transfer members and cross beams can all be answered economically with a truss.
The point
Section titled “The point”A truss is at its strongest where the geometry is unusual. Machine cutting means a complicated shape costs no more — it only takes more design work.