CROSS Safety Report
Roof collapse at a social club
This is a report about a roof collapse that occurred, in about 1985, because only half as many parallel faced timber roof trusses were installed as there should have been.
Key Learning Outcomes
For designers and the construction team:
- Be careful to ensure that the construction drawings match the manufacturers' drawings
- If trusses have nearly parallel top and bottom faces ensure that they are marked accordingly to ensure they are installed correctly
- Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
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This is a report about a collapse that occurred in about 1985.
The roof of a social club designed by the practice had collapsed at about 3.00 p.m. during a heavy rain storm. Half an hour earlier and the roof would have collapsed onto a busy room full of patrons, thankfully, as it was, the room was unoccupied at the time of the collapse.
The collapsed building formed the main assembly hall for the newly refurbished club. It was 12.0m wide and about the same length with brick side walls and a flat roof covered with a high-performance membrane. The architect had drawn seven lines on the roof plan indicating roof trusses at 1.2m centres. Channel-reinforced wood-wool slabs spanned between the trusses and there was a lay-in grid ceiling.
A flat roof was required and trussed rafter manufacturer provided a drawing of a parallel faced truss with a note to say at "600mm centres".
The truss internal members were arranged in 'W' formation with an extra vertical in every other bay reducing the in-plane buckling length of the top boom by a half. The top and bottom booms were of the same sectional area so that there was no obvious 'right-way-up' for a parallel boom girder except by reference to the pattern of the internals.
The builder solved this problem by looking at the architect's drawing and, noting that the vertical members were ceiling hangers supporting the bottom boom between the nodes. The builder then erected the 7 trusses at 1.2m. centres and the wrong way up. The quantity surveyor had counted the 7 lines on the plan and measured 7 roof trusses not noticing that the truss supplier's drawing specified 600mm centres.
This absolutely flat roof was therefore built with half the required number of trusses which were installed upside-down; The inadequate strength roof would have deflected noticeably under only self-weight allowing rain to pond in the centre and it collapsed under a heavy rain shower.
The initial cause of failure was the error in not obtaining the services of a structural engineer for the project. The importance of the vertical members in the trusses, to restrain the top booms from buckling, was not obvious to the non-engineering members of the building team.
The second error was to do with the number of trusses required. All members of the team had the opportunity to note that the manufacture’s drawing of the truss said these were to be used at 600mm centres but the working drawings, from the team, indicated trusses at 1.2m. centres.
The absence of a structural engineer's 'seeing eye' meant that nobody noticed. It is to be hoped that current CDM regulations would make it impossible for anything like this to happen now.
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