COP v3.0:testing; commentary-part-151

16.2 Commentary On Part 15.1 

C1.1. GENERAL

Historically there has not been a specific New Zealand based Standard for testing sheet roof and wall cladding, so that most testing has been carried out using requirements and methods based on the Australian Standards AS 1562 and AS 4040. These two standards are closely interrelated as AS 1562 sets out the performance requirements while AS 4040 sets out the test methodology.

Although both of these standards have been the subject of a review by a joint Australian New Zealand standards committee for many years, major differences stemming from the respective National Building Codes as well as climatic conditions have been difficult to resolve. While the adoption of the joint Loadings Code AS/NZS1170 Structural design actions, has resolved many of the issues relating to the determination of wind forces on buildings, this has not assisted progress in adopting agreed testing methods and performance benchmarks. For these reasons, this New Zealand test procedure has been adopted by the NZMRM.

C.1.2. ALLOWANCE FOR VARIABILITY OF MATERIALS

Extensive testing has been carried out in New Zealand over many years, but for economic reasons,  only a single test has generally been carried out on a specific area of the roof profile for each span, material thickness, grade, and fastening pattern. Where replication of tests has been carried out, the variability due to materials has been negligible. Concentrated load results are very consistent as are those from top fixed steel roof and wall cladding and from the results of hundreds of previous tests the Coefficient of Variation (COV) can be assumed to be taken as less than 5%.

The same cannot be said for self-supporting and fully supported secret-fixed roof and wall cladding as the variability occurs from small but significant differences in profile or clip tolerances. It is necessary therefore to establish a variability factor from 16.1.1A Variability Factors that is acceptable for the metal and profile.

The assumption that where the mode of failure is the same but at varying structure spacings, the number of tests can be agglomerated, is taken from Australian practice. While this is permissible for pierce fixed roof cladding because of the greater variability of clip fixed roofs these results cannot be treated in the same manner.

Extensive testing both static and cyclic on standard profiles with varying fastening patterns have validated the variability values given in 16.1.1A Variability Factors. The use of static testing alone means that the 1.2 factor must be used, not the 1.15 factor provided for in 16.1.1A Variability Factors.

The factors in 16.1.1A Variability Factors are the same as those found in AS/NZS 1170 and AS/NZS 4600.

C.1.3. RESISTANCE TO CONCENTRATED LOAD

The requirements and general descriptions are taken from AS/NZS 1170.

The division of Type 2 is taken from the NZMRM Code of Practice.

C1.3.1.

The residual deformation criterion of L/400 has been set to allow for the use of different metals and plastic for roof and wall cladding. This is a limiting factor because permanent deformation is an indicator of potential stress failure and is also likely to cause ponding and visual distortion.

C.1.3.2.

Because the concentrated load is known and the number of tests is predetermined, it is possible to factorise the test load from 16.1.1A Variability Factors, unlike a UDL test where the test load in unknown and the results are factorised. Multiple concentrated load tests can be carried out on the same sheet provided that there is nil influence of any failure from a previous test.

C.1.4 RESISTANCE TO WIND PRESSURES

 

AS/NZS 1170.0:2002 defines Serviceability as Ability of a structure or structural element to perform adequately for normal use under all expected actions

The main function of a roof or wall is to provide a durable weathertight membrane and, therefore, serviceability is the ability to continue to provide a weathertight seal where the fixings penetrate through the cladding under maximum design load. For a Uniformly Distributed Load, this requires finding the applied load under which sufficient permanent deformation occurs that may cause loss of a weather seal at a fixing location. This is regarded as a serviceability failure.

Any buckling from point load that impairs the ability of the profile to be able to carry the same repeated load is also regarded as a serviceability failure.

From these definitions of serviceability for roof and wall cladding serviceability failure is considered the most important performance criterion, rather than Ultimate failure where the roof would blow off or a person would fall through it. Testing of pierce fastened profiled metal roofs has determined that failure under serviceability loads occurs at less than half the load at which strength load failure occurs. Hence serviceability load failure is considered the most important criterion for such roofs.