COP v3.0:testing; wind-and-point-load-testing

17.7 Wind and Point Load Testing 

Profiled steel metal cladding is manufactured from high-strength and low-strength steel. Non-ferrous claddings are classified as low-strength.

High-strength steel is the predominant material used for roof cladding in NZ.  When roof sheeting is made from low strength steels, deflection under load is more important; it can yield progressively or is deformed by constant traffic, which in turn lowers its performance.

Metal roof tiles are usually made from low strength steels and are not intended for roof traffic. (see section 10 roof tiles) .

Wind and traffic imposed loads are dynamic, moving or fluctuating loads, but most test regimes are static load tests. 

The Loadings Code (AS/NZS 1170) requirements consist of serviceability loads and ultimate loads. However, serviceability loads are more relevant to roof cladding because:

  • Any permanent deformation of the ribs of profiled metal cladding — caused by metal expansion or repeated walking, or deformation at the fasteners — will eventually lead to leakage or failure.
  • Ultimate failure due to a point load on any metal cladding profiles manufactured in NZ has not been known to happen unless corrosion has affected the profile's structural integrity.
  • The UDL at which ultimate failure would occur on all metal cladding profiles manufactured in New Zealand is caused by structural failure or unsuitable fasteners, not of the material under test.
  • In the context of testing metal cladding, serviceability loads determine weather resistance. The publishing of strength criteria is irrelevant to pierce fastened profiles. Strength criteria are, however, the determining factors in clip fastened profiles.

The citing of AS/NZS 1170 in the NZ Building Code meant that a reassessment of the testing load parameters was necessary. Test ing methods had been under review for a considerable time, and the joint AS/NZS committee made considerable progress in reassessing a cyclic test regime, suitable to New Zealand's climate, to be used as a verification for static UDL loads.

During the testing using the new criteria, it became evident that in assessing roof and wall cladding performance a much greater emphasis should be given to the fastening pattern.

The effective area of the profile affected by one fastener is known as the 'tributary area'.

This means that, although the previous calculations for this parameter have been confirmed, all load/span graphs should be qualified by the fastener type and pattern.

The full text of the new regime, including the performance and test requirements, is contained in 17 Testing in this document.

 

17.7.1 Commentary On Section 15.5 

C6.2.

The reason for the required 50% load is that it is used as a 'preload' which settles the sheeting to a stable position before proceeding with the test.

C6.3.

It is suggested that the increments be at 10% intervals.

C6.4.

Although deflection under load is not regarded as a design criterion, it is an important indicator of stress leading to permanent strain.

Residual deflection, after all load has been removed is regarded as the only deflection criterion that will affect the performance of the sheeting.