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Trusses are designed to carry load in their own plane and are unstable laterally until braced. Temporary bracing during erection is the contractor's responsibility, and permanent bracing is part of the design.
A truss standing alone is a tall thin thing that falls over. In a braced roof it is a structural element. The gap between those two states is where collapses happen. Brace it properly. Diagonal too.
Why trusses are unstable during erection
They are designed to carry load in their own plane. Out of plane they have almost no stiffness, and a row of them can fold like dominoes.
That is not a defect. It is the consequence of using material efficiently, and it is why bracing is not optional.
Whose responsibility
Temporary bracing during erection is the contractor's. The truss designer provides the truss and the permanent bracing requirements; getting it standing safely is the erector's job.
A scenario describes a truss collapse during erection and asks who was responsible. The answer is the contractor, because temporary bracing sits with them and not with the truss designer or the supplier.
The three bracing types
- Ground bracing - stabilizing the first truss, from which the rest are braced
- Lateral bracing - holding trusses at spacing along their length
- Diagonal bracing - preventing the whole assembly folding over
Lateral without diagonal is the classic mistake. It holds spacing and does nothing about the row folding.
Do not modify a truss
Cutting, notching or drilling a truss member without the designer's approval voids the design. A plumber running a pipe through a web member has structurally altered it.
Coordination is a general contractor responsibility, and this is where it bites.
Engineered wood generally
Laminated veneer lumber, I-joists and glulam all come with manufacturer requirements on holes, notches and bearing. Those requirements are the design.
Common questions
Why are trusses unstable during erection?
They carry load in their own plane and have almost no stiffness out of it, so a row can fold over until braced.
Who is responsible for temporary bracing?
The contractor. The truss designer supplies the truss and permanent bracing requirements.
What are the three bracing types?
Ground bracing, lateral bracing and diagonal bracing. Lateral without diagonal is the classic mistake.
Can a truss be modified on site?
Not without the designer's approval. Cutting or drilling a member voids the design.
What about engineered wood generally?
Manufacturer requirements on holes, notches and bearing are the design and must be followed.