COP:structure; fastener-performance

3.12 Fastener Performance 

Most fastener failures happen due to negative load, or uplift conditions, and testing procedures for fasteners are designed to closely simulate these conditions.

If every rib has a fastener, the only way to improve performance is to use load-spreading washers under the fastener heads, or place purlins closer together.

3.12.1 Screw Fasteners 

A fastener penetration of three threads through the steel member is sufficient for the fastener to meet its full design capacity.  Pull out failure is not normally a risk with high tensile steel purlins over 1.1 mm in thickness.

3.12.1A Steel Purlins Thinner Than 1 mm

The pull-out values of screws into light gauge steel battens or purlins varies greatly with thread design, pitch and drill point shape. Pull out can be the mode of failure of light gauge steel battens depending on the cladding profile and the fastener design. When fastening into battens less than 1.1 mm in thickness, 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, it is important to avoid stripping out the formed screw thread, therefore a depth setting screw gun is recommended.

3.12.1B Timber Purlins

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.


3.12.2 Pull-over Values 

When metal cladding is subjected to uplift wind loads within the withdrawal capacity of the fastener, the failure mode will be pull-over or pull-through. The pull-over value is determined by the thickness and strength of the metal and the area over which the load is spread. See 3.12.2A Typical Pull over values for crest fixing (serviceability failure).

If the pull-over load is likely to be exceeded, the options are to increase the metal thickness or use a load-spreading washer. The pull-over value when using 0.70 mm aluminium is approximately the same as 0.40 mm steel.

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.

The pull over values for pan fixing cladding are higher than those obtained by crest fixing by a factor greater than 2, depending on the screw’s position in the pan. See 13.5.2 Pan Fixing.

3.12.2A Typical Pull over values for crest fixing (serviceability failure)

Thickness (mm)Screw Only (kN)Load Spreading Washer (kN)