3.3.2 Planning the investigation
A plan was needed to determine the chain of events leading up to and during the collapse. That sequence would necessarily depend on which parts had broken first, and a fault tree would enable a plan of action in isolating the cause (or causes) of the disaster. Such a systematic approach is known as fault-tree analysis or FTA, and is part of the armoury of methods used by accident investigators. With large-scale and devastating accidents, all possibilities, however remote, need evaluation in the light of all the available evidence. In this way, the list can be whittled down to the vital one or small handful of most probable causes. Such a systematic approach is vital where both the material and the witness evidence is extensive, not just for the analysis of bridge or building failures where destruction is almost complete, but also for marine and aerospace disasters.
One action that was taken almost immediately was to close a bridge of very similar design some miles upstream, at St Mary's. That bridge would not just be subject to rigorous inspection but would also become the basis for experimental work on its dynamic behaviour when loaded under controlled conditions.
The possibility of wind action could also be ruled out, because the wind at the time of the accident was parallel to the long axis of the bridge, and was only about 6 mph. Likewise, there was no evidence that the masonry piers were involved. Indeed, they survived almost unscathed (Figure 35).