Serviceability limit is a state when a building, or any part of it, becomes unfit for its intended use due to deformation or deflection.
Ultimate limit is a state associated with collapse or failure, or when a building or any part of it becomes unstable or unsafe.
Initial failure under wind uplift for pierce-fastened cladding is usually by local buckling of the rib crest adjacent to the fastener. While the cladding can still resist a load, this permanent deformation is liable to cause leakage at that point; therefore, it is a serviceability failure and is often the limit state for trapezoidal profiles. Medium and high rib trapezoidals may have ultimate loads as the determining factor.
Failure under wind load for clip-fastened cladding is usually by the clips de-indexing and the cladding sheets blowing off. This is an ultimate failure.
If a standard corrugate or low rib trapezoidal product passes UDL for serviceability it will comfortably exceed ultimate design load requirements. For medium and high ribbed trapezoidal, both serviceability and strength loads should be checked. Clip-fastened products are typically restricted by strength load only.
Point load resistance should also be checked against the expected roof traffic load of the finished structure. (See 3.7.4 Roof Traffic)
Deflection is the temporary bending of roof or wall cladding that occurs under load when the material is subjected to forces within its elastic limits.
Deflection under distributed load is a voluntary limit of wall cladding, deformation causing residual deflection after application of point load is a serviceability failure of roofing.
Deformation is a permanent alteration to the profile's shape that occurs when the material is taken beyond its elastic limits. This is most often seen as deformation under the fastener head after high outwards loads, or buckling of the rib after the imposition of a point load
With high-strength claddings buckling failure is often quite sudden and severe once the point of yield is reached, while progressive deflection under repeated loads within the material’s elastic limit is minimal.
Low-strength (G300) steel or non-ferrous metals can progressively yield under repeated heavy foot traffic, and are more prone to point load damage. This should be considered at the design stage. Machine roll-curved, crimp-curved cladding, and metal roof tiles are usually made from low-strength steels.
It is not acceptable for G300 steel to be substituted for G550 steel in products that are traditionally run from high-tensile strength material.
The designer must consider the degree and type of foot traffic that may be expected on a roof. The following requirements are subjective standards and must be considered in line with customer expectations, and building use and type.
Any permanent load added to the roof cladding or structure is termed imposed; that includes air-conditioning equipment, solar installations, television aerials, anchor points, and walkways.
Construction loads on a building include the wind load on a partially clad or braced roof or building. Depending on the method and sequence of construction, it can be greater than the load on a completed building. Construction loads also include temporary localised loads from material storage, scaffolding, and tradespeople.
Other forces contributing to construction load include:
All metals may be subjected to fatigue under repeated heavy load conditions.
While high-strength steel is susceptible to fatigue, this seldom happens in practice. Aluminium brackets and clips have been known to fail by fatigue and this should be considered in designing such items.
Other metals, such as lead and copper, are restricted in length or overall panel size to avoid cracking by fatigue. Sharply folded corners should be avoided on these materials and the minimum radii requirements should be followed.
Hydrogen embrittlement can occur when folded items such as external gutter brackets are subjected to post-forming hot dip galvanizing. This can be avoided by forming to radii appropriate to the material grade and thickness being formed.
