Most fastener failures happen due to negative load (or uplift) resulting in pull out of the screw from the support, or the screw pulling through the cladding or causing permanent deformation. Testing procedures are designed to closely simulate these conditions.
Product testing does not include pull-out of the screw, this should be checked independently.
Fastener failure by fatigue can occur when fasteners embedded in timber or hot rolled steel, are subject to frequent bending action by repeated thermal expansion forces. This is unlikely to happen to cold rolled steel purlins up to 2.5 mm in thickness as the thinner depth of connection of the screw into the purlin allows it to rotate, rather than bend.
A fastener penetration of three threads through the steel member is sufficient for the fastener to meet its full design capacity. Pull-out failure of screws must be checked when assessing a profile's load/span capacity.
The pull-out values of screws into light gauge steel battens or purlins varies greatly with thread design, pitch, and drill point shape. When fastening into light gauge steel, the pull-out values of the specified screw must be considered, and the installation must be completed with that type, gauge, and brand of screw. In light gauge steel under 1mm in thickness, it is also important to avoid stripping out the formed screw thread
Timber purlins generally require a fastener penetration of 30 mm. With this level of embedment, a screw equipped with a profiled washer though 0.55 material will fail by pull-through of the cladding before it fails by fastener withdrawal from the timber. Greater thicknesses of cladding may require specific design. For fastening into sarking or rigid air barrier less than 30 mm thick, the pull-out values for the specific screw and sarking material should be obtained from the supplier and required fastener pattern calculated.
Pull-over values for medium and high rib trapezoidal profiles must be checked against strength loads provided by testing.
Pull-over load depends on the head or washer size. For example, as 12# and 14# screw heads have approximately the same diameter, these screw sizes have the same design load value for pull-over. If the pullover load is likely to be exceeded, the options are to increase the metal thickness or use a load-spreading washer.
Profiled load-spreading washers spread high wind uplift-loads over a larger area around the fastener head. Using load spreading washers under the fastener can increase the load resistance of each fastener by up to 50%.
The type, size and stiffness of washers are critical for performance. Where performance data incorporating load-spreading washers is used, the specification of the washer must be quoted with the fastener.
In general, load-spreading washers should have a minimum thickness of 0.95 mm for steel and 1.2 mm for non-ferrous metal.
Where oversized holes are used to accommodate thermal movement of the sheeting, load-spreading washers should be used with sealing washers to ensure weather tightness.
