Case Studies - Investigation of the Avonmouth Bridge Platform Collapse

This incident occurred when the bridge taking the M5 motorway over the River Avon was undergoing major repair and improvements. One end of a maintenance platform suspended on wheeled trolleys from a twin runway beneath the bridge became detached, causing four workmen to fall 30 metres to their deaths.

The method by which the runway beams were being replaced resulted in the platform straddling a gap in the runway as old beams were removed and new beams installed. The platform was intended to be restrained in both directions at all times, firstly by a system of wire ropes fastened between the platform and cross girders on the bridge, and secondly by the use of clamps on the runway beams. To move the platform along, in order to work on a new section of the runway, it was necessary to release the wire rope at one end whilst pulling in the rope at the other end. The beam clamps should have been released to allow movement and then refitted when the platform was stationary again.

The HSL investigation team examined all the components and looked in detail at the construction and method of restraint of the platform. A three-dimensional computer simulation was used to model the movements of the platform. It was concluded that at the time of the accident only one end of the platform had a wire rope attached and that the beam clamps were ineffective in providing horizontal restraint to movement of the platform. As a result the platform was relatively free to move in a northerly direction. It was demonstrated that a southerly wind gusting at approximately 30 mph could provide the necessary force to move the platform to the point where the trolleys at one end came off the runway beams. The weather conditions recorded on the day of the accident showed that there were winds of sufficient speed.

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