- Q.What happens to trussplate
connectors as the wood truss members expand and contract
with temperature and moisture changes? The prongs on the plate
don’t seem to penetrate very deeply, and it seems as if
they could work loose over time.
A.There is no question that
the wood shrinks away from the prongs on truss plates as the
lumber dries, and this does reduce the strength of the truss
somewhat. But trusses are designed with a large safety factor.
For example, most trusses are designed for a roof load of 40
pounds per square foot, yet joist-and-rafter roof construction
usually fails at less than 15 psf, and such failures are rare.
Most of the loosening of the plates would occur during the
initial drying period rather than from repeated moisture
A greater problem is the lateral loads often placed on
trusses as they bend and twist during handling and erection.
Trusses are designed to resist vertical loads. The lateral
loads that occur when they are taken off the truck, and when
they are swung from an inverted position on the wall plates to
the upright position, can loosen the plates, or even pop them
off one of the members being joined.