Truss
Definition
A truss is a structural framework that carries lighting, audio and video equipment overhead. The main types are box truss (four chords, widely used), triangle truss (three chords) and ladder truss (two chords). Its capacity depends on the span, on the position of point loads and on the number of supports, and manufacturers publish load tables for each span. A truss rated 500 kg as a uniformly distributed load (UDL) does not necessarily carry 500 kg as a single point load at the centre. In bending, a centre point load produces twice the maximum moment of the same total weight spread evenly, so it is limited to 50% of the UDL at most, and the manufacturer's table can give less when deflection or shear governs.
In the field
Point loads on a truss are checked by calculating the actual bending moment. A 200 kg point load at the centre of a 10 m span creates a peak bending moment of about 4,900 N·m at mid-span (M = F·L/4, with F = 200 kg × 9.81), twice the moment produced by the same weight spread along the span.
Related lessons
Sources
- ANSI E1.2-2021 (ESTA), Design, manufacture and use of aluminum trusses and towers : conception, fabrication et usage des trusses et tours en aluminium · webstore.ansi.org
- EN 17115:2018, Entertainment technology — Specifications for design and manufacture of aluminium and steel trusses : conception et fabrication des trusses en Europe · knowledge.bsigroup.com
- Structural Basics, Beam moment formulas, poutre sur deux appuis : M = F·L/4 (charge centrale), M = W·L/8 (charge répartie totale W) · structuralbasics.com