Under wind uplift loading, when all spans of the roof sheet are under upward loads, the crest of the profile is placed under tension and the pan under compression. The deflection and stress patterns are the reverse of those for point load.
Failure under wind load for a clip fastened cladding is usually by the clips de-indexing and the cladding sheets blowing off. This is an ultimate failure.
Initial failure under wind uplift for pierce-fastened cladding is usually 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.
The design load relates to a specific building and is calculated by the engineer. The failure load relates to a specific product or system and is supplied by the manufacturer. Engineers calculate both the Serviceability load and the Ultimate Load. They compare these values with the maximum failure loads of the products and systems they are considering.
- In design, the serviceability load is no more than 0.72 of the ultimate load.
- In testing, the serviceability load is about 0.5 of the ultimate load for a pierce fastened standard corrugate or low rib trapezoidal products.
Therefore, if a standard corrugate or low rid trapezoidal product passes for serviceability it will comfortably exceed ultimate design load requirements. In the above example, the product has failed against serviceability but still exceeds ultimate 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.
Refer to manufacturer's load/span tables for all other profiles, which should give the maximum recommended load for end and continuous spans when tested as described in 17.7 Wind and Point Load Testing.
To determine the performance of corrugated and low rib trapezoidal profiles, see 3.17 Steel Cladding Wind Load Span Graphs.