Throughout history, man has built to keep out the elements. Today, the prime functional purpose of the building envelope is still to enclose the interior of the building and protect it from the elements.
The envelope does not, in most cases, provide primary support to the building, but the ability to withstand the worst of the weather can have significant implications for the structural performance of the cladding. Before looking at the kinds of loading which cladding can accept, it is important to understand how the structural frame of the building supports the cladding, and how the various parts interact.
The primary structure of the building provides a framework. The external cladding can be attached to this framework, usually via secondary steelwork. It is the primary structure which transmits the loads exerted on the cladding (wind, snow etc.) to the ground.
This primary structure could be:
Structural steelwork is usually fabricated off-site and then bolted together during construction. One of the benefits of this is that it allows complex shapes and designs to be fabricated.
The primary structure consists of columns and beams. Fabricated sections may also be used to optimise load-bearing capabilities. These include cellular or castellated beams, truss girder structures, etc. Connection plates are welded onto the steel sections to allow them to be bolted together. Additional fillets or haunched beams are often fitted to increase the strength of the structure.
The materials used for the primary structure will depend on the design and end application of the building. Steel-based cladding systems can be attached, usually through some secondary components, to most types of primary framework.
Some typical examples of building applications range from portal frame industrial buildings and sports stadia to commercial offices and domestic dwellings. As a cladding material, pre-finished steel gives the designer the flexibility to design the form of the building in the secure knowledge that the cladding will follow this form.
The tolerances on structural steelwork need to be such as to be acceptable for the cladding system. In particular, the sagging of purlins and sheeting rails can cause problems with fixings. It is common practice to include ties and stays to minimise sagging and twisting of the secondary structure.