Serviceability and Ultimate Failure

Initial failure under wind uplift for pierce-fastened cladding is usually by local buckling of the rib crest adjacent to the fastener. While the cladding can still resist a load, this permanent deformation is liable to cause leakage at that point; therefore, it is a serviceability failure and is often the limit state for trapezoidal profiles. Medium and high rib trapezoidals may have ultimate loads as the determining factor.

Failure under wind load for clip-fastened cladding is usually by the clips de-indexing and the cladding sheets blowing off. This is an ultimate failure.

If a standard corrugate or low rib trapezoidal product passes UDL for serviceability it will comfortably exceed ultimate design load requirements. For medium and high ribbed trapezoidal, both serviceability and strength loads should be checked. Clip-fastened products are typically restricted by strength load only.

Point load resistance should also be checked against the expected roof traffic load of the finished structure.  (See Roof Traffic)