Cross Reports - Report 946

Highlights the potential for condensation to undermine the structural integrity of swimming pool ceilings  

Condensation is a well recognised issue in the design and operation of buildings that include swimming pools. While the impact on a building’s main structural elements is well understood, this report emphasises the effect of condensation on secondary structural support to ceilings and the subsequent risk of failure. The first [collapse] resulted from a gradual build-up of condensation, causing corrosion of the pressed metal angle tie bars and fixings that supported the suspended ceiling above a swimming pool to the overhead concrete floor slabs. Eventually, after about 12 years, the ceiling weight exceeded the strength of at least one of the tie bars, which caused progressive failure of the remainder and then collapse of the ceiling. The swimming pool was located at basement level, beneath a multi-storey commercial premise.

The second case concerns the effects of a gradual build-up of condensation that caused the collapse of a rigid insulation board and suspended ceiling grid in another swimming pool. This was a commercially operated venue, located in the basement of a sports centre. The pool’s ventilation system was switched off each night (outside trading hours), creating the conditions needed to cause condensation. For example, the guidance of the Canadian Building Digest is that the ventilation system is an essential part of a swimming pool’s design and that “it must be properly operated if serious building problems are to be avoided”. Condensation was forming on the colder surface of the concrete roof, which was at ground level. This caused wetting of the insulation boards and corrosion of the support system for the suspended ceiling tile system. The condensation was generated by temperature differences between the 30-degree Celsius environment of the pool hall, and the outdoor temperature of the concrete slab, combined with the supply of moisture from the pool and a jacuzzi. The ceiling tile system was provided for aesthetics and acoustics; it was not meant to be airtight or waterproof. The warm, moist air came into contact with the concrete roof where the silver-coloured foil on the insulation was missing or ineffective. This was occurring at holes or gaps in the insulation; where joints between individual boards were not sealed with tape; where the tape was not properly installed along joints; or if the type of tape used was ineffective.

A condensation risk analysis, using the computer software of the Fraunhofer Institute for Building Physics, found that condensation and then water would always be expected to occur with the type of construction used. Prior to the collapse of a part of the ceiling, there had been an issue with the appearance of brown stains on the ceiling tiles. This was managed on an ongoing basis by replacing the affected tiles. It is clear from images of the pool hall ceiling from one year prior to the incident that this was a widespread issue.

Comments

CROSS has received a large number of ceiling collapse reports, and they usually follow the same pattern; there is failure of a single hanger, which precipitates a cascade failure across the whole roof, just as reported here. SCOSS produced an Alert about this in 2012 (SCOSS Alert – Tension cable and rod connectors), and a fuller technical explanation can be found in The Structural Engineer, Vol 97 (2002) Issue 9, A P Mann: ‘Safety of Hanging Systems: Lessons from CROSS Reports’.

Ceilings can be very heavy, and their failures have the potential to cause injury and death. A ceiling collapse (precipitated by a hanger failure) over a swimming pool in Uster, Switzerland, in 1985 killed 12 people. All designers should know that swimming pool environments are highly deleterious to materials in the medium to long term.

Designers need, universally, to ask themselves the question: ‘How do I know it is safe?’ Any tension member (eg, a ceiling hanger) will tend to fail suddenly and catastrophically; this is in contrast to bending or compression design, which tends to fail with forewarning. In the cases reported, how did the designers know that the hangers would remain intact? During the operational phase was there an inspection and replacement regime? Most important, were competent persons engaged in these roles? The safety-critical aspects of fixings and anchors are too often ignored.

Owners and managers of swimming pools should be made aware of the sensitivity of fixings to corrosion. See SCOSS Alert – Stainless Steel: A Reminder of the Risk of Failure due to Stress Corrosion Cracking in Swimming Pool Buildings, from 2005.

To search for ceiling failures on the CROSS database, visit structural-safety.org and enter ‘ceiling’ in the ‘quick search’ box.      

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