CROSS Safety Report
Ceiling collapse in cinema
This report is over 2 years old
A reporter discusses a recent ceiling collapse (2008 report) at a multi-plex cinema in a major city.
Key Learning Outcomes
For civil and structural design engineers:
- Connections can often be the weak link in structures and attention to detail is required
- Careful consideration is required for fixings, particularly at interfaces between different materials. The role of tolerances should not be overlooked.
- An attribute of safety is to assure that the design is not disproportionately vulnerable to minor error
For construction professionals:
- Quality control and competent supervision on site can ensure ceiling systems are installed in accordance with the design
- Fixings should be installed in accordance with the manufacturer’s specifications
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A ceiling collapsed much more recently (2008 report) at a multi-plex cinema in a major city says a reporter. This too had a mass barrier acoustic ceiling of two layers of nominally 12.5mm plasterboard fixed to a two-way light gauge steel channel system suspended from an in-situ concrete roof slab composite with a steel deck.
Ceiling support specification
Ventilation ductwork was suspended below the mass barrier ceiling on threaded rods fixed to the roof slab soffit (i.e. through mass barrier ceiling) with single shot-fired fixings. There was a decorative lower ceiling suspended below the ductwork from the channel system with tie wires. The specification had however called for this ceiling to be suspended from the slab structure above.
One ductwork supporting rod (or its fixing) failed and it was found afterwards that a shot-fired fixing nail at this location was 16mm long and not 32mm as specified, and that the spacing of the threaded rods was excessive. The duct dropped onto the lower ceiling which in turn pulled down the end section of the mass barrier ceiling.
It was further found this ceiling was not properly fixed to the support system at the perimeter of auditorium and that the supporting channels were inadequately spliced. There was a progressive collapse of major part of the whole two layer ceiling system. Again, by good fortune, the auditorium empty so there were no casualties.
It was further found this ceiling was not properly fixed to the support system at the perimeter of auditorium and that the supporting channels were inadequately spliced.
The underlying causes, the reporter believes, were a lack of appreciation about the engineering risks inherent in the installation of a heavy ceiling system. There was reliance on a single shot-fired fixing (whereas the manufacturer recommended groups), coupled with an unworkable design resulting in unauthorised changes. There was no evidence of inspection or supervision of installation.
Expert Panel Comments
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This and other similar reports demonstrate a concerning trend of progressive collapse mechanisms in public buildings where there is a high probability of casualties in the event of failure. In each case there has been a fault, or a combination of faults, in the design, selection or installation of fixings, and a lack of appreciation of the magnitude of the dead loads from acoustic ceilings.
Inspection of fixings
These would have been compounded because it is not usually possible to inspect the fixings after installation. The danger of falling ceilings is not new; there is old cinematographic legislation that was brought in because of failures with lath and plaster ceiling fixings. The Home Office document ‘Recommendations on Safety in Cinemas 1955’, and which is still relevant says: ‘Ceilings shall be in such a condition as not to cause a danger to persons visiting the premises’.
Ceilings shall be in such a condition as not to cause a danger to persons visiting the premises.
There are other examples where the failure of a single component must not compromise the whole; for example, cable stayed bridges are designed so failure of one stay does not cause the bridge to come down. Similarly, what is needed for heavy ceilings is a robust design with a sensible appreciation by designers of the importance of what might appear to be trivial structural detail.
There must be a sound design tracing load paths back to a solid platform with a responsible person in charge. It may be that guidance there should be provided in Part A of the building regulations in a similar manner to the existing guidance on cladding systems.
CROSS has been concerned about fixings of various kinds for some time and these collapses illustrate very well the ‘3Ps’ promulgated by CROSS to illustrate the wide causes of failure
Those involved exhibiting a lack of structural engineering competence such that the safety critical implications of the work were not recognised.
Lack of attention given to the procurement of the work and in particular to ensure that one competent party is responsible for the overall design. A failure to appreciate that these support systems are just as important as primary structural members. Lack of supervision and checking of installations.
Specification (or choice) of the wrong product i.e. not fit for purpose.
Fixing failures such as the 1981 Hyatt Hotel walkway collapse in which 114 people died demonstrate the magnitude of tragedies that can unfold.
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