Structural design rules

Design and test procedures for metal cladding are specified in BS 5950-6 (1995). Eurocode 3 will cover the same area, and will in due course replace the BS, but the part relevant to metal cladding is not, at the time of writing, in publication. BS 5427-1 (1996) gives further design guidance for profiled steel sheeting. Design rules for steel cladding are specified in European recommendations, The Design of Profiled Sheeting, and for composite panels in European recommendations for sandwich panels. A Euro-norm for composite panels, EN14509, is currently in draft form. Note that for agricultural buildings, BS 5502-22 should be followed for design rules.

Practically, the design rules are considered by the individual roof and wall cladding system manufacturers when designing profiles, and form the basis of published load-span tables.

In general terms, the strength of a profiled sheet depends on the thickness or gauge and yield strength of the steel and the stiffening effect of the profile. More severe profiling generally gives greater spans. However, the failure mode is not necessarily the same for different profile types, so it is important to consult the system manufacturer if special spanning needs are to be considered.

It is usually appropriate to design on the basis of strength under ultimate loading, and then to check that the deflection is not excessive and cladding performance is optimised under serviceability loading.

Design codes for profiled claddings have traditionally been based on elastic design principles in which the upper limit on loading is determined by limiting the maximum stresses in the profile to the yield stress, divided by a safety factor.

Ultimate limit design, as it sounds, sets upper boundaries. It is used to determine the failure limits, thus setting an upper loading limit by comparing actual stresses related to some proportion of the yield stress of the steel. In determining the ultimate or collapse load limit, the following failure modes are considered:

• Tensile fracture. In practice, tensile failure is extremely unlikely, since with thin plate elements (such as steel sheet cladding) the compressive instability of the thin flanges is more critical.
• Compressive buckling This form of failure is particularly relevant to wide compressive flanges, which are usually stiffened by longitudinal ribs. The stress levels at which buckling occurs increases with the material yield stress. Where this failure mode is critical, span can be affected by steel grade.
• Shear failure Shear failure is unlikely, except in the webs of very deep profiles when they are used over short spans. For this reason, very deep profiles often contain web stiffener elements.
• Web buckling can be a limiting factor in profiled sheeting, particularly when the sheets are used in double span or multi-span conditions. In these circumstances, failure would occur over intermediate purlins, as a result of an interaction between compressive buckling of the flanges and buckling of the webs.
• Composite panels.A sandwich panel, consisting of a core material with bonded steel skins, acts as a composite beam and offers some significant benefits. First, the prefinished steel sheets are held apart by the core, which increases the stiffness of the panel. Second, the core material provides support for the prefinishedsteel sheets, increasing the resistance to failure by compressive buckling and web buckling. In general, shear failure in the foam or interface is the most relevant failure mode for composite panels.

The forces acting upon the cladding which are used to calculate the ultimate load limit are factored to ensure that the total load does not exceed the maximum tolerable load for the selected claddings and structure. For composite panels, the calculation is complicated by the need to factor in the temperature loading, as the temperature differential between the internal and external surface of the composite panel will cause a very slight natural bow in the cladding. The serviceability limit is now used to check that the deflection is within acceptable limits.

Consideration of the serviceability limit load is necessary to avoid the possibility of problems with the cladding, such as:

• Inadequate drainage and localised ponding.
• Broken seals on fixings at overlaps.
• Undue strain on fixings at overlaps.
• Damage to, or excessive strain between other connecting components or lining materials.
• Surface distortions on flat panels.

The allowable deflection which gives the effective serviceability limit is specified as a fraction of the span dependent on the loading condition (e.g. L/200 where L is the span). This deflection limit should be specified by the cladding manufacturers for specific load-span tables and allows comparability between tables. BS 5427-1:1996 gives recommendations for the deflection limits to be used in specific cases, although more conservative limits are sometimes adopted.

The deflection limits stated by BS 5950 (Part 6) for steel profiles.