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Rendered ceiling failure

Report ID: cross149

Published: Newsletter No 17 - January 2010

Source: CROSS

Report Content

A report (similar to report 148) relates to a failure of a false ceiling below a pre-cast concrete floor. The ceiling was constructed as part of a new office building, and was situated externally above steps forming part of the public highway. Early in the morning, the ceiling unexpectedly collapsed, and the event was captured on CCTV. The only warning was some falling debris moments earlier. No one was injured. The construction of the ceiling was as follows: aluminium channels wired together to form a grid, upon which expanded metal lath (EML) was fixed. This was then rendered. The grid was wired to hangars which were fixed into the precast units above by means of an expanding hammer-in bolt. The ceiling was not inspected as part of the Building Control process as the BCO was not aware of its design, and had thought that the render was directly onto the precast units. The failure seems to have a number of causes. Firstly the render was applied very thickly, up to 100mm in parts which greatly increased the weight. Secondly inappropriate fixings were used, which over time seem to have become loose due to vibration from the adjacent highway structure.

Comments

The key learning point from these two reports (148 and 149) is that there seems to have been no appreciation of the effects of vibration on the fixings. Dynamic action can cause fixings to work loose in substrates and/or nuts to loosen. Whenever there is cyclic action from traffic, wind effects, or machinery then fixings must be chosen that are suitable for both static and dynamic loads. SCOSS issued an Alert in 2009 on problems associated with the incorrect choice of secondary fixings and this can be found on: www.scoss.org.uk/publications/rtf/SCO8048A-Alert%20-Fixings-Final.pdf. There have been a number of CROSS reports about fixing failures which has resulted in the Construction Fixings Association promoting a new BS Code of Practice for the use of anchors in safety critical applications. The drafting is due to commence shortly. In addition, the fixings industry, in conjunction with SCOSS, is drafting a guide to ceiling fixings.

 There have been recent press reports of other ceiling collapses, including one from the Middle East about an incident which saw decorative panels fall from the ceiling of a large new mall. There were shoppers present at the time but fortunately nobody was injured. Another ceiling collapse, this time in the UK, resulted in the closure, for an extended period, of a Grade ll listed art gallery. CROSS has been informed of a number of ceiling failures and they have in common that fact that the loads are uncertain, that the failure of one hanger increases loads on adjacent hangers and that rapid cascade type failure then occurs. It is also the case that the engineering of such ceilings often seems to be absent and no one takes responsibility. The large number of failures (some accompanied by injury) suggests that the design teams are not identifying ceiling design as an important issue. Fixing details get very little attention by the designer yet they pose significant safety hazards unless spotted by a ‘passing site engineer/design engineer’. We would encourage further examples to illustrate this point. The Introduction provides useful contemporary information on fixings.

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