Partial roof collapse at shopping centre

There was a recent partial roof collapse at a Shopping Centre which was, says the reporter, built in the early 1970s in a form which may have been used extensively throughout the UK during this period and possibly beyond. Luckily, due to the timing in the early hours of the morning, this roof collapse did not claim any loss of life but has raised considerable concerns.

The failure mechanism was ascertained from site visits, but the underlying cause is not known, and the report was submitted in the hope that it will be useful in the understanding of the mode of failure and in the identification of similar connections which may occur in other buildings. The building is of reinforced concrete construction, with columns supporting a grid of beams at roof level, which support the flat roof construction over shop units and a mall. The flat roof had a mineral felt weathering, on insulation board, on profiled metal decking. The metal decking spanned between steel lattice trusses, which themselves spanned between the reinforced concrete beams at roof level.

The lattice girders were connected to the high level reinforced concrete (RC) beams via a top flange bearing plate which was bolted to an angle cleat (bolts ‘B’) which was in turn bolted to the RC beams by bolts ‘A’ . The arrangement is shown in the figures below and the diagrammatic section. It was clear that some of the bolts used to secure the angle cleats to the RC beams (bolts ‘A’) had failed in tension. They appeared to have been pulled out of the concrete. A closer inspection of some bolts which were found in the debris showed that the bolts which had failed (‘Drop-In Wedge Anchors’ — manufacturer unknown) had done so within the shield of the anchor, within the embedded depth of the bolts. The threaded steel shields were later found to be no more than 1.6mm in thickness.

The reporter did not see any evidence of corrosion but was surprised at how thin the walls of the shells were. It appeared that it is most likely that the failure occurred as a chain reaction when a single bolt, or group of bolts at a connection failed (4 No bolts ‘A’ per connection). It was clear to see that the failure started in one corner, where the bolt shells had failed in tension. But further along, where the support cleats were still in position on the RC beam, the fixing bolts ‘B’ between the lattice girders and the cleats had failed in shear as the roof collapsed. The collapse followed a significant period of heavy rainfall.

The fall of the roof towards rainwater outlets (which would dictate the ‘deep end’ for any ponding water) should have accumulated more loading and possibly cause failure at that end if the outlets were blocked or flow restricted. However, this was not the case and failure occurred at the ‘shallow end’, the opposite end to the outlets. Remedial works to enhance the loading capacity of all of these connections were necessary.