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)
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.
More robust design than specified below (reducing purlin spacing or adding protection from mechanical action), is required for roofs that are regularly accessed, roofs used as staging by subsequent trades, or areas that are adjacent to access points, particularly step down access.
Type A – Unrestricted Access
Type A are roofs:
that need to be regularly traversed by the roofer for access during installation;
that will be accessed regularly by sub-trades;
that butt on to walls or windows that may require maintenance;
that have plant, chimneys, or solar installations requiring regular maintenance; or
that require regular access for clearing gutters or spouting of debris.
For Type A roofs, the cladding must resist the load of 1.1 kN applied to the pan or a single rib, and a strength load of 2 kPa applied to the weakest point.
Type B – Restricted Access
Type B are roofs:
that are simple in design and do not have to be regularly traversed by the installer;
which are infrequently accessed by qualified trades people for maintenance; or
with a pitch of more than 35°.
For Type B, roofs the cladding must resist the load of 1.1 kN applied to the pan or over two ribs and a 2 kPa load applied to the weakest point.
Type C – Non-Trafficable
Type C are roofs:
where supports are required to be laid to support roof traffic, or traffic should take place within 300 mm of purlin lines,
which have a pitch of 60° or greater; and
including non-trafficable translucent roof sheeting.
For Type C roofs, the cladding must resist the minimum load of 0.5 kN applied to the pan or over two ribs.
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:
The intensity of internal wind pressures due to a temporary absence of ceilings, walls and glazing.
Storage of roof cladding on the structure. Bundles of roof cladding should be placed so they do not cause overstress in purlins.
Any scaffolding above an existing roof must be designed to avoid damage to the roof structure or coatings, and be designed or sequenced to avoid wet storage corrosion. It is far better practice to stage jobs so that lower roofs are not used as a working platform for work on higher levels
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.