A curtain walling specification can go awry if the wrong means of attaching it to the building structure are chosen. Barbour Index and Scott Brownrigg run through the fixing options
A specifier has to Understand the principles behind the fixing of curtain walling to a building structure. Here are some fundamental points.
Usually the structural frame and, in particular, the edge of the floor slab is used as the attachment point for the cladding system. Some systems rely on loads being taken down to ground floor slab level independently of the building structure, but typically curtain walling is fixed - hence the term "curtain wall".
The standard arrangement is for each cladding unit to have its base next to the floor slab and be hung from the fixing above. Each fixing comprises a bracket attached to the cladding panel and a structural fixing attached to the building structure. The structural fixing can be cast into the floor slab, but drill fixing is preferable because wrongly cast fixings are difficult to rectify.
The fixing design varies according to the type of curtain walling system and most makers have a range of fixings to suit most applications and building types. The most common types of curtain walling are the so-called "stick system", assembled from individual components, and the unitised system, in which complete panels are factory-assembled before delivery to site. It is important to establish which type is required early on and develop the fixings to suit.
1. Regulations and standards
Approved document A of the Building Regulations, which covers structures, is not clear about the types of fixing material that can be used. Section 3: Wall Cladding requires structural fixings that cannot be easily checked be made of "suitably durable material".
In practice, this means they should last as long as the cladding and building frame. Additionally, fixings should be corrosion-resistant and of a type of material appropriate for the local environment. For most designers this means they should be made of stainless steel; however, some will interpret this as being galvanized steel. Future regulations promise more clarity on this issue and for the moment designers have to argue their case on the basis of good practice.
The Centre for Window and Cladding Technology requires that fixings comply with Part A and be designed to cope with wind pressures as defined in the code of practice BS 6399 pt 2 2001. This should be tested to CWCT parameters as set out in their document Test Methods for Curtain Walling.
The brackets have to co-ordinate with the building structure so the specifier must liaise with the structural engineer. Fixing points that can take both lateral and dead loads must be identified. Typically, fixings will be situated at the edge of the slab. Structural engineers should be consulted at an early stage, especially if something unusual is required. The exact detail of the fixings should be reviewed to ensure co-ordination with components nearby, such as raised floor supports.
It is common for brises-soleil, signs and perhaps lighting to be fixed to the curtain walling. These must be identified early on and approximate loadings established in order that the fixings can carry this additional load.
Bi-metallic corrosion should be considered. Consult the British Standards Institute's professional document PD 6484 to assess whether this is potentially a problem. The most likely risk is the use of stainless steel next to aluminium. A separation membrane should be used where the proportion of stainless steel is high.
It is very important that the fixings' tolerances can be adjusted. The structure will typically be constructed to a tolerance of plus or minus 15 mm and the curtain wall positioning tolerance will typically be plus or minus 3 mm, sometimes less. Adjustment has to be possible in all three planes to ensure a quality installation.
The best fixings achieves this without the use of shims as the curtain wall section can be held securely while it is correctly positioned. Once the curtain wall section is precisely in place, the fixing can be locked off in its final position.
In some installations, notably those that use structural glazing, the fixings will be visible, so must be considered as finished items. This will sometimes require a completely different type of fixing, which may be more costly. It is usually quite difficult to cover fixings in an acceptable manner.
6. Cold bridging
As buildings are required to have a better thermal performance, greater environmental control and lower air infiltration rates, there is a corresponding need for the performance of fixings to improve. There are now specialist fixings that can reduce thermal bridging considerably. These reduce the metal connection to a minimum by using high-strength, stainless-steel bolts in tension and an insulated block in compression.
7. Vapour control
There may also be a need to co-ordinate the fixings with a vapour control barrier on the inside of the walling. A smooth fixing point may be required to allow the membrane to be attached around the fixing.
Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification services. For further information, contact Barbour Index on 01344-899280 or visit www.barbour-index.co.uk