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Perils of glazed curtain walling

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Buildings flex much more than some people think. Knowing the technical and the legal facts may keep you in the clear

Imagine the situation; an architect drafts a performance specification describing a stick system curtain wall with 50mm-wide framing members together with drawings showing a conventional curtain wall grid secured to the floors of a building. The engineer separately inputs a specification in which deflections in floor slab edges might be span/360 or span/500.

Unfortunately, this is an all too common situation. This article sets out the conflict between these specifications, which can potentially give rise to the breakage of glass infill panels in curtain walls.

The problem is that a curtain wall system using extrusions with a face width of 50mm may typically only accommodate 5mm of downwards deflection in each storey of a building.

Conversely, a floor spanning 9000mm, for example, could theoretically have a mid-span deflection of just 18 to 25mm. Unless these tolerances have been allowed for, the result is inevitable curtain wall failure.

Whether or not a building experiences problems one must take into account the weight distribution on floor slabs. One should consider that floor slab deflections are likely to occur in two stages. The first stage will be a dead load deflection and some if not all of this movement could take place before a cladding attachment is made. It is possible for this movement to be accommodated by the cladding fixings as a normal construction tolerance. After the cladding is attached to the structure, some further deflection might occur owing to the dead weight of the cladding. The second stage deflection will be as a result of live loads applied to the floors.

Unfortunately, cladding fabricators can find it confusing if a specification states a cumulative deflection rather than making a distinction between dead and live load deflections.

The amount of movement that can be safely accommodated by a curtain wall must be less than the gap at the head of infill panels used where one curtain wall storey ends and another begins. This is because the framing above these panels can move downwards whereas the framing below may remain static (fig 1). This gap is typically 5mm, and while it is acceptable for such joints to narrow occasionally, the degree of movement should not result in the gap being closed permanently or infill panels being pinched by the transom members above, especially if they are made of glass.

On a drained and ventilated glazing system, BS6262: Glass and Glazing of Buildings stipulates a minimum recommended edge gap of 5mm at the sides and head of double-glazed units and 6mm at the bottom. This is in part necessary to allow drainage and ventilation around the unit edge seals but, also to allow differential movement between the unit and its framing.

To prevent such occurrences, fabricators might elect to provide a movement joint consisting of two closely spaced transom rails designed to span continuously around each storey of a building. Typically, the gap between each transom is closed by two overlapping pressings and an internal concertina gasket that expands and contracts with movements in the building frame (fig 2). Alternatively, one may specify a much wider transom at the bottom of each storey with glazing rebates sized to accommodate anticipated movements. Clearly, these alternatives must accord with the aesthetic requirements for the building.

The nub of the problem is that in some cases, architects and engineers can inadvertently produce specifications and drawings that are not mutually compatible. An architect's drawings can show a conventional 50mm wide curtain wall grid without either double or extra-wide transoms at each storey of a building. The design intent drawings could also be those used to obtain planning consent and, in conjunction with the curtain wall specification, form the basis on which prospective fabricators price the works (having failed to notice the incompatibility). Problems might not become apparent until the curtain wall is built and the table below looks at how failings might come about and shows the parties who might be responsible.

There are a variety of courses of action to remedy discrepancies, depending on the time of discovery:

If conflicting specifications result in a defect, an aggrieved main contractor might feel that an employer's architect or engineer could be held culpable but the wording of the Contractor's Design Portion Supplement (CDPS) suggests not.

The main contractor has an obligation to deal with discrepancies under the CDPS. Assuming the architect and the engineer's specifications are part of the employer's requirements, it seems that under CDPS clause 2.3.6, the contractor is obliged to give the architect written notice of any discrepancies or divergence and the architect shall issue instructions to cover it.

However, clause 2.5.1.2 states that where the discrepancy to which clause 2.3 refers is a discrepancy within the employer's requirements, the contractor's proposals shall prevail without any adjustment of the contract sum.

In effect, the problem becomes something the contractor must rectify at his own cost, meaning that if the contractor is left to his own devices, an employer might receive a visually unacceptable solution.

Whether the main contractor can make the fabrication company bear the remedial costs depends or the wording of their contract and, to some extent, on whether both the architect and engineer's specifications were contract documents.

If conflicting documentation is discovered at the design stage, it will be necessary to either stiffen the floor edges on for the architect to vary the curtain wall design to include either a double band of transoms at each storey as described on a single transom much wider than its neighbours. Clearly, the appearance of the chosen solution must be acceptable to the employer, architect and probably the planners.

Main contractors can safeguard their position by ensuring that curtain wall fabricators receive both the curtain wall and engineering specifications at the time of tender while drawing their attention to this potential failing.

If it is too late in a project to make any design changes it will be necessary to carry out a risk assessment to identify whether a problem is likely to occur. This should take into account:

The specified floor edge deflections and the spacing of structural columns at the floor edges Actual floor deflections, given that the original design calculations should have included a factor of safety The design and composition of the affected infill panels If a problem is identified it will be necessary to execute remedial works although, under the CDPS provisions the architect and employer might have limited control over the appearance of the remedial solution. The best course is to ask the question early in the design process and to coordinate specification requirements accordingly.

Brendan Donaghy is a cladding consultant and chartered building surveyor, Cladding Consultancy Services, Hove, tel 01273 325 300

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