This section sets out the testing methodology to evaluate the structural performance of sheet roof and wall cladding systems. Structural Testing covers uniformly distributed load (UDL) and concentrated (point load). These procedures apply to all metals and plastic sheeting.
The UDL test simulates wind uplift by subjecting test specimens to pressure from underneath.
UDL testing is typically carried out to simulate outward loads, where material is restrained by fasteners (or clips) at intermittent points; rather than inward loads where material is retrained linearly by purlins or girts. Even with asymmetrical trapezoidal profiles, failure adjacent to the fastener from the imposition of outward loads is inevitably cause of failure in typical applications.
Testing for inward load can also be conducted, if required.
17.1.1 Standards: AS Standards and COP Testing Standard
There is no specific New Zealand Standard for testing sheet roof and wall cladding. The MRM standard is broadly based on the Australian Standards AS 1562.1:2018 and the AS 4040 series. These Standards are inter-related; AS 1562.1 sets out the performance requirements to be achieved using the test methodology of AS 4040.
The MRM standard differs from the above standards in some significant aspects.
AS 1562.1 defines serviceability failure as a level of deflection under load.
In the COP standard, deflection testing is normative for wall cladding and informative for roof cladding. Additionally, the COP standard defines serviceability failure of pierce fastened roofing as permanent deformation, when permanent distortion or splitting occurs around the fastener head. The load at which permanent distortion occurs around the fastener head is defined as serviceability failure and the roof can still resist greater loads, but the damage would affect weather tightness.
AS 1562.1 tests point load only in the path of traffic specified by the manufacturer; i.e., the pans of the profile, and across 2 ribs on narrow profiles.
The COP tests both in the pan (or two ribs) and a single rib, the latter being the less resistant to load. This is to assess the profile strength when laypeople or careless roof traffic fail to adhere to manufacturers' recommendations.
Deflection under load, (span/120) + p/30 where p is the maximum distance between fasteners on the support structure. (Informative for roofing, Normative for cladding), or by
Permanent deformation or cracking around fastener head, or de-indexing or de-clipping of clip fastened profiles. Serviceability failure of clip fastened products may occur when a clip disengages from the central rib, but weathertightness of the lap is not affected and roof can continue to resist increased loads.
Strength tests. Strength tests are defined as: sheet unable to resist further loads i.e. pull-off from clips, buckling, tearing or pull-through of fasteners.
The design of the support system for wind load should consist of a rigid frame airbox, sealed on the bottom and four sides. A fan or blower is required, capable of controlling and maintaining the required test pressure, and pressure measuring devices. The deflection can be measured using several differential displacement transducers and traced and recorded coincidentally with the increasing load. Purlin and end wall positions can be changed to test for multiple spans of different dimensions.
The use of airbags rather than air pressure is not endorsed by the MRM COP
For new testing, at least two sets of specimens should be provided for each span-fastener combination. Three is preferred as it results in a higher confidence factor. At least three different spans must be tested, and four is preferred as it gives a more accurate gradient to load-span graphs. For confirmatory testing to review previous test results, smaller sample sizes are acceptable providing results are consistent with previous tests.
The width of a model or the test section of a lapped or interlocking system must be the width represented by at least two sheet side laps. The total width of the testing rig (2.1 m) must be covered by the test sample or other material.
For testing continuous spans, the test specimen should incorporate no fewer than five spans; i.e., two end and three intermediate spans. End spans should be two-thirds of the span of internal spans. Tests using four spans (two end spans and two internal spans) or three equal spans may be validated by comparing results against a five-span test of the same sample and adjusting the resultant load tables accordingly.
The samples are assembled on the testbed in accordance with how they are intended to be used, with the ends and sides sealed to minimise loss of air pressure. Measuring devices for deflection are placed at static points (fasteners) and pan centre at mid-span or as required to measure maximum deflection.
The air pressure is raised incrementally, and the point of deflection failure noted. With pierce-fastened products, signs of imminent deformation are observed. When product nears deformation failure, pressure may be periodically backed off to allow loosening of a fastener to inspect the integrity of cladding profile underneath. The pressure at these points should be noted. The maximum serviceability load test result is the recorded pressure level at the last observation before the onset of failure or the point before which the pressure level recorder showed a sudden small drop signifying profile movement.
More than one serviceability result may be recorded from separate fasteners during a single test if, at the discretion of the testing engineer, the loads imposed on subsequent failure points are equivalent to those imposed on the initial failure point
If lap screws are added for the purpose of attaining failure pressures by decreasing air leakage, these must not be positioned closer than 0.5 m to the nearest purlin.
Clip fastened sections that de-clip at the central rib or de-index on a lap but still resist increased pressure, represents a serviceability failure.
Load is then increased to establish the strength load result. This may be when pierce-fastened cladding pulls fasteners through the profile or clip fastened profile de-clips. Both these actions result in sudden and permanent loss of air pressure and terminate the test. The load at which this occurs, or maximum load achievable due to air leakage, is recorded.
If strength load limits cannot be attained by the testing apparatus, a load of at least 2.0 times the serviceability load must be imposed to demonstrate that the sample failure load is governed by serviceability rather than by strength. If serviceability/strength ratios exceed 2.0 for at least two different spans, it may be assumed that the ratio will be similar on all spans.
Fastener pull-out from testbed purlins is not recorded as a failure as this is independent of the profile strength and is relevant only to the condition of the purlin material on the testbed. Such fasteners may be replaced or repositioned and the test recommenced.
Concentrated load must be applied in a direction perpendicular to the roof through a circular steel disc of 100 mm diameter faced with a 20 mm thick rubber pad, with a Shore A Durometer hardness of 30-70, to the part of the profile judged to be least resistant to load. See 3.6.1A Point Load Test at the MRM.
Where the dimension of the sheeting profile does not allow the use of a 100 (±2) mm diameter pad, the shape must be spread over two ribs by way of a disc and pad of 100 mm width.
The pressure cell and deflection sensor are positioned above the profile pan or rib at mid-span. For Type A (unrestricted access) and B roofs (restricted access), the load is increased to 1.32 kN, the pressure is released and residual deflection measured after 1 minute. Residual deflection must be less than S/1000 or 1.5 mm, whichever is higher. The pressure is then increased to failure (or at least 2.41 kN) and noted.
For roofs only intended for Type C (non-trafficable) applications, the initial pressure requirement is 0.60 kN, and secondary load is 1.1 kN.
Test results must not be published as load-span data. Before they can be offered as a design guide they must be checked and interpreted by an engineer and have suitable factors applied to the test result figures. It is highly desirable that this person has witnessed at least some of the tests being conducted. As fastener pull-out is not included in testing, published load-span tables should encourage the designer to check for pull out values of specified fasteners when fastening into timber or ply less than 30 mm thick, into composite materials, or into steel sections less than 1.2 mm thick.
Coefficient of Variation (COV) is derived from Standard Dev / Av for a group of tests. The COV to be applied to a series of tests is the average overall spans measured during a test schedule conducted over a continuous period using identical material.
For serviceability loads, a coefficient of 5% may be assumed unless there is evidence that a higher figure is warranted. For strength loads, a coefficient of 10% may be assumed unless there is evidence that a higher figure is warranted. For clip-fastened products, the applied depreciation factor may not be less than 1.3.
The number of units tested relates to the number of valid results derived from a given fastener configuration and purlin spacing.
For values between those listed in the table, interpolation may be used.
When using data from testing for producing graphs, tables, or other design aids, the following conditions apply:
Data must not be extrapolated except where a minimum of four span-combinations within the limitations of (b) below can be shown to provide a statistically reliable load span graph. In such cases, an extrapolation of a further ±10% at either end may be calculated.
Interpolation of data between different spans of a specific type of test is acceptable only in the following circumstances:
Where the data is taken from tests for a single type, size, and profile of cladding and type and spacing of fastener.
Where at least three different spans or support spacings have been tested for the same type, size, and profile of cladding and fastener details, and that in all three tests the failure mode was the same.
Where test loads were derived from the same test criteria.
Point load requirement for trafficable roofs is defined in NZS 1170 as 1.1 kN, which approximates a 100 kg person on one foot. (Note: many workers carrying tools or equipment exceed this limit.) To allow for statistical variation this is factored by 20% and a test load of 1.32 kN is applied to give a pass/fail.
Type A roof (unrestricted access roofs) is required to withstand a 1.32 kN test load on the rib without deformation or excessive deflection, (serviceability load) and a 2.41 kN strength load.
Type B roof (Restricted access roofs) is required to withstand a 1.32 kN test load in the pan or over two ribs without deformation or excessive deflection, (serviceability load) and a 2.41 kN strength load.
Type C Roof (non-trafficable) is required to withstand a 0.6 kN test load to the pan or over two ribs, and a 1.1 kN strength load. See 3.7.4 Roof Traffic.