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Safety concerns over cinema ceiling fixings

The Standing Committee on Structural Safety (SCOSS) this week warned cinema operators in the UK about acoustic ceiling fixing failures in cinemas.

Three cinema ceiling collapses have been reported to Confidential Reporting on Structural Safety (CROSS), part of Standing Committee on Structural Safety (SCOSS) and as a result SCOSS will be writing to cinema operators in the UK. A suspended ceiling collapse in an educational building was also reported

In all four cases it seems that the suspended ceiling fell due to poor selection of fixings.

CROSS comments: "These are important reports that demonstrate a 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. These would have been compounded because it is not usually possible to inspect the fixings after installation.

"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."

The first failure occurred at a cinema complex in a major UK city some few years ago. The heavy ceiling was suspended by drop rods fixed to a U section channel system attached to the underside of a composite steel deck and insitu concrete slab. The design was for each rod to be connected to a nut with a washer over a pre-formed hole in bottom of the channel. In practice the washers were too small; one (or more) pulled through its hole, initiating a progressive collapse of the whole ceiling. At the time the cinema was operational but fortunately the auditorium was empty so no one killed or injured.

It was also reported a ceiling collapsed much more recently at a multi-plex cinema in another major city. This too had a mass barrier acoustic ceiling and ductwork suspended from an insitu concrete composite roof slab . A decorative ceiling was suspended from the ductwork, but this ceiling should have been suspended from the roof slab 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. The auditorium empty so there were no casualties.

A third cinema had several separate auditoria. Each had a ceiling consisting of a metal grillage supporting plaster board and on top of that was network of cables and ventilation ducts. Some of the fixings pulled out in one of the auditoria (both shot fired nails and drilled in fixings) setting up a cascade reaction such that the whole ceiling fell down. Inspections were carried out in the adjoining auditoria and it was found that the fixings there had partially failed. The collapse occurred at night so no one was injured

The suspended ceiling in a large teaching hall collapsed days before the official opening of a new educational building. The contractor had procured correctly rated brackets, but had procured compression brackets not tension brackets. All were overloaded but just working until one failed, which set off a chain reaction.

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