Structural implications of cladding systems

Here the stuctural implictions of each cladding system is explored.

The structural strength of any cladding system is only as good as the fixings holding it. The recommendations of the cladding manufacturers on type and frequency of fixings should be followed to ensure the structural performance of the system.

The stiffness and spanning capability of the prefinished steel cladding is generated by, and dependent upon, the external sheet profile and gauge. The cavity between the liner sheet and the external cladding is maintained by a spacer bar and bracket system which is secured through the liner sheet into the purlins or sheeting rails.

The spacer bar system transmits the load from the external sheet to the secondary structure, provides lateral restraint to the cladding and is designed to resist localised compression from point loads. It is, therefore, integral to the structural performance of the system.The action forces including self-weight, on the spacer bar system creates an applied torque on the purlins or sheeting rails. It is therefore particularly important that these loads are taken into consideration in the design process. The cladding system or steelwork supplier will be able to give further guidance on this and provide design load calculations.

Panels are secured to a framework, which is bolted tothe existing structure. There is usually a form of interlocking mechanism, which secures the panel to theframe. The backing frame transmits the imposed loads to the primary structure.

The secondary structure and the internal liner tray are replaced with a structural liner tray system which is able to span the primary frame. The spanning capability is achieved by using a deep tray profile and increased material gauge typically up to 1.5 mm. Profiled external cladding which provides stiffness and spanning capability is fixed across the liner tray, as for the builtup system.

The relevant British Standards (BS) give calculation methods for loading and there are now also several software packages available to assist with these. Note that for agricultural buildings, BS 5502-22 should be followed for calculation of loading and design rules. In designing a metal-clad building, the following loads should be considered.

Panels are designed to fix to a secondary steel framework, although they can also be secured to structural blockwork. The panels are secured using through fasteners in an overlap joint or external extruded aluminium sections, then through-fixed to the support structure. Gasket seals are usually required around the joints. Particular attention must be paid to the sealing gasket design and application to ensure seal retention, particularly in areas of high wind suction loading. Panels can be fabricated for use in curtain walling systems, in which case, interaction with the curtain wall structure will be similar to a glass panel.