Restraint and restraint points
The basic relationship
Section titled “The basic relationship”The stability of a compressed or bent member depends on the length over which it can deflect. Prevent the deflection and capacity rises.
With a thin-walled section this effect is especially strong, because stability is the bottleneck. The same section with closer restraint can carry an order of magnitude more — with no extra material.
What provides restraint in practice
Section titled “What provides restraint in practice”The lining
Section titled “The lining”Board screw-fixed to the flange of the section restrains the member laterally. The condition is that the lining is actually there, properly fixed — so the screw pattern is structural data.
It also means the structure is weaker in the unlined, under-construction state: temporary bracing is not optional.
Horizontal restraint members
Section titled “Horizontal restraint members”The strap or profile threaded through the studs of a wall panel, tying them together and preventing them from rotating. Their spacing is a design decision.
The bracing system
Section titled “The bracing system”Diagonal bracing and shear walls provide global stability. Bracing with steel straps.
The connections
Section titled “The connections”The track, the floor connection and the hold-down are restraint points too.
What the design has to state
Section titled “What the design has to state”It is not enough to specify which section is needed — it must also state where and how it is restrained:
- the type, spacing and fixing of the restraint members,
- the type of lining and its screw pattern, where the restraint comes from it,
- the quality and spacing of the screw fixing.
If this is not on the drawings, the design assumption cannot be verified — and the contractor will build a different structure in good faith.
The most common mistake
Section titled “The most common mistake”When restraint is omitted or altered: a strap is left out, the screw spacing is opened up, or a layer of lining is dropped “because it is only cosmetic”.
The mistake is invisible, and under normal loading it does not show — only under extreme loading, when it is too late.
The point
Section titled “The point”The capacity of a thin-walled member belongs to the system, not the member. Leaving out restraint is not a saving, it is a breach of the design assumption.