Curtain walling

Cladding systems must be able to tolerate expansion and contraction in the glass and framing members together with normal movements, deflections and long-term axial shrinkages in the structure to which it is fixed. The effects of wind sway, differential settlements and elastic deformations also need to be factored into the design of both framing and fixing systems.

Thermal movements in frames must be accommodated. If there is no provision for movement in the frame design, stresses will build up in the profile. The less the provision for movement, the greater the stress and the greater the propensity for deflection or buckling. For northern climes, it is usual to apply a rule of thumb of 1mm movement provision for every 1m of length of component and to calculate stress with this in mind. The typical range of UK temperatures for design purposes could be -25°C to +80°C. If the stress in the element in question is allowed to exceed its permissible stress, failure can occur.

Curtain wall systems are usually fixed by way of brackets back to the floor slab. Deflections at the edge of the floor slab are usually calculated at design stage - possibly span ÷ 360 or span ÷ 500, giving 18-25mm movement for a span of 9,000mm. The system must accommodate this movement, but to achieve this it may be necessary to incorporate split transoms as well as sliding mullion joints. A split transom usually incorporates twin overlapping pressings and an internal concertina gasket.

Concrete structures suffer from long term creep - axial shrinkage such that movement of as much as 0.1% can be expected, for example, 30mm for every 30m in height (Wall cladding defects and their diagnosis, BRE Digest 217, 1978). Movement will become progressively greater towards the top of the building and must be accommodated in the cladding and glazing systems to prevent stress formation.