Secondary fixings - cladding etc
Hot rolled structural sections are less precise than smaller scale components and attachments should therefore provide for some site adjustment.
Connections for fixing cladding and other finishes pose similar problems to those associated with connecting to concrete. Curtain walling, ceiling tiles and similar relatively small scale components can often be manufactured to a greater degree of precision than the steel frame. In certain respects the setting out of the steel frame will be very accurate. For example modern fabrication methods mean that cut member lengths, drilled holes etc. are very precise. Site erection too can be completed to tight tolerances so that, for example, the top level of a beam is likely to be established very accurately. However the soffit level, the straightness along the length, and some other dimensions, will be less closely controlled because they will depend on inevitable variations in manufacture. Tolerance therefore has to be provided for fixing items such as factory extruded aluminium sections which are capable of being fabricated far more accurately than the main frame can be erected.
Fixings should avoid transferring significant stresses into non-structural elements and provide for some movement where necessary.
The fixings must be designed so that they do not transfer load from the main frame to the secondary elements. This is a case where the difference between strength and stiffness is important. For example, a steel beam may deflect by, say, 20mm at midspan due to live loads, yet still be quite strong enough to carry the loads. This is not an excessive amount of deflection in structural terms, but it may cause damage to a block wall or pane of glass which would crack if it were built tight up against the beam. This is because of the brittle nature of materials such as blockwork and glass. It is therefore necessary to allow the beam to deflect without bearing on the cladding.
Differential movements may also need to be considered. Being exposed directly to the atmosphere, the curtain walling will be subjected to far wider variations in temperature than the steel beam which supports it. The jointing system must allow the skin to ‘breathe’ relative to the frame rather than break rigid fixings.
For these reasons, it is usual to fix secondary elements at one or two points only, and restrain the other corners in one direction only.
Secondary connections often use corrosion resistant materials which may need to be separated from the steelwork by suitable washers.
Fixings for claddings are often made of corrosion resistant materials - stainless steel or copper alloys - and to reduce the risk of electrolytic corrosion, these materials must usually be separated from the steel frame (e.g. by neoprene washers or gaskets).
