The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
Calculation of performance is considered invalid because it assumes the section properties remain static under load.
Although in the past much of the testing of N.Z. roof and wall cladding has been done with a three equal span arrangement, it evidenced awareness that end spans should be approximately 2/3 of intermediate spans, which means that a three span test which consists of two end spans has no true intermediate span. Because the reaction forces are not the same it is not possible to test a true intermediate span without using a four span test arrangement.
A further complicating factor in the determination of performance loads is the use of varying or different fixing patterns. Many pierce-fixed roof cladding profiles are not fixed on every rib which gives rise to variability in the serviceability results, depending whether the fixings on the missed ribs are co-linear or staggered.
When using a four span test configuration, there is only one fully loaded central purlin. If the penultimate purlins have a staggered fixing pattern in relation to the central purlin, the rib that is fastened at the central purlin will fail at approximately 10 -15% less load than had the fastening pattern be fully linear. Linear fastening is where the same rib is fastened at each purlin and the adjoining ribs are not fixed, except at the ends. Where a staggered fixing pattern is to be tested a minimum of five spans must be used.
The most common serviceability limit state failure mode is permanent local deformation, which commonly precedes fracture or buckling.
Current research indicates that small changes in load can significantly affect the fatigue performance of the cladding.