Section Properties
Various attributes or effects can be identified by looking at individual shapes or profiles of metal cladding, but their performance cannot be accurately determined by their section properties as their profile shape, and therefore the sectional properties, change during deflection.
Corrugated iron is a common profile traditionally used in roof and wall cladding. It has a symmetrical sinusoidal profile and is equally strong under positive or negative loading. Its ‘neutral axis’’ runs through the centre, or halfway-depth, of the profile. Symmetrical trapezoidal profiles have similar attributes.
Symmetrical corrugate or trapezoidal profiles have the advantage of being more easily curved around a radius. Because the ribs are necessarily close together, they have the disadvantage of roof traffic loads having to be spread over two crests or along the purlin line, and they have a lower run-off capacity.
Most profiles fixed on non-residential roofs in New Zealand are asymmetrical trapezoidal or ribbed profiles, with ribs formed at various spacings and different heights. The angle and height of the trapezoid rib determines the profiles performance under compression. A steeper angle generally gives improved performance, but raises cost per square metre as it lowers sheet effective coverage.
The rib spacing determines the position of the neutral axis, which is the point of zero stress. Bigger spacing between the ribs lowers the neutral axis.
Roof cladding should be designed to withstand both positive and negative design loads. Profile designers use computer software to compare the "moment of inertia" (deflection) and "section modulus" (strength) to find a compromise in different profiles.
Strength and deflection are interrelated but not interdependent, and the design strength is determined by the stress (f) at which permanent deformation occurs. Stress is determined by the section property known as the Modulus of Section (Z). Although it is possible to calculate the sheeting performance from the section properties of the profile, only physical testing can prove the actual capabilities of the profile.
Deflection under load depends on the profile section dimensions or section properties, and it is determined by the Moment of Inertia (I) and Young's Modulus (E). Various metals have different E-values, which means the same cladding profile made in aluminium will deflect more than steel. See General Methods Of Testing Sheet Roof And Wall Cladding.
The strength or grade of the metal does not affect the deflection. A high-strength steel profile will deflect to the same extent as low-strength steel profile, but using high-strength steel will increase the point at which yielding or permanent damage occurs. See Walking On Roofs.
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