CROSS Safety Report
Swimming pool ceiling collapses
This report is over 2 years old
A reporter writes about the collapses of two swimming pool ceilings due to failure of the fixings.
The fixings and support system had experienced significant corrosion due to the moist environment that they were exposed to.
Key Learning Outcomes
For owners and construction professionals:
Selecting the correct fixings and corrosion protection for the given environment is important to ensure they perform as expected
Consider carrying out a condensation risk analysis to ensure the correct form of construction is detailed to prevent condensation occurring
The anticipated life span of the fixings should be noted in the operation and maintenance manual
The fixings should be inspected as part of the ongoing maintenance programme by a person who is competent to inspect them
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A reporter writes about the collapses of two swimming pool ceilings. The first was due to a gradual build-up of condensation. This caused the 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 weight of the ceiling exceeded the strength of at least one of the tie bars. This caused progressive failure of the remainder of the bars resulting in the collapse of the ceiling. The swimming pool is located at basement level, beneath a multi-storey commercial premise.
Proper ventilation systems and moisture build up
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 of trading hours), thereby 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 ‘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. The supply of moisture from the pool and a jacuzzi also added to this.
Should a condensation risk analysis be carried out?
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 that was used was ineffective
A condensation risk analysis, using the computer software of the Fraunhofer Institute for Building Physics, was carried out. This found that condensation and then water was always expected to occur with the type of construction that was used.
Prior to the collapse of a part of the ceiling, there had been an issue with the appearance of brown-coloured 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.
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I note with interest yet more reports of failure in suspended ceiling systems. It's not mentioned in the second example, but in the first it seems like it might have been a post-installed anchoring system into the concrete slab over, although the initiating failure is reported in a tie bar itself rather than its supporting connection.
Of more interest to me though is that a progressive collapse mechanism followed. I recall a much larger example of this in the Sasago Tunnel ceiling collapse in Dec 2012, although this initiated from a creep failure of chemical anchorage into the tunnel roof. That progressive collapse can occur seems to me a much greater design failure which dominates the consequences of the event (fatalities in the case of Sasago) and about which I have not yet seen any attention given either as to responsibility borne by that designer nor in guidance / warnings to designers in future.
After all, given that a localised failure is initiated then is not a progressive collapse mechanism entirely foreseeable? One needs simply to design in continuity breaks (or a "structural fuse") to contain these potential results. I raised this directly with the Japanese Ministry responsible for Sasago Tunnel.
I also note in Comments for the report that "Any tension member will tend to fail suddenly and catastrophically" and then a series of questions directed at the designer and other presumably competent people concerning construction, inspection & structure performance. The actual failures in Sasago Tunnel, and in the Boston Big Dig tunnel 6 years earlier, reveal that the failure initiation itself was NOT sudden, being creep related, and provide ample forewarning to which all those who should have realised were apparently blind.
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CROSS has received a large number of ceiling collapse reports. You can search for safety information on ceilings on the CROSS website. 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.
Previous ceiling failures and alerts
The Standing Committee on Structural Safety (SCOSS) produced an Alert about this in 2012 (SCOSS Alert - Tension cable and rod connectors) and a fuller technical explanation can be found in Safety Hanging Systems: Lessons from CROSS Reports, was published in Volume 97 (2002) of the Structural Engineer.
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 the Uster swimming pool in Switzerland in 1985 killed 13.
Selecting the correct fixing or anchor is important
All designers should know that swimming pool environments can have a detrimental effect on 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 (e.g. 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.
All designers should know that swimming pool environments can have a detrimental effect on materials in the medium to long term
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 importantly 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.