Several theories have been put forward as to why the Tay Bridge collapsed, three of which are summarised below. Read the conclusions that the experts have drawn and then decide whether the evidence supports these theories in the weigh up the evidence section.

Bill Dow
"The rear carriages of the train derailed and ran into one of the coverplates. The force of this impact would shatter the cast iron lugs - leaving the bridge in a high wind without its proper structural support. The girder closest to Wormit, in the high girder section of the bridge, had bent during the lifetime of the bridge. This girder had been dropped during construction and was bent. It was straightened out and reused, but over time it started to return to the bent shape. As a result, the rail tracks which ran over it developed a kink. This kink, combined with the high wind, may have been responsible for the derailment." Theory in full

Tom Martin
"The disaster was investigated using modern computer analysis techniques in conjunction with a modern approach to wind loading. The bridge was examined with and without the train on the bridge to see what effect it had on the performance of the pier structure. And a pier was analysed under various load conditions with a view to proposing a mechanism for collapse of the navigation piers. The bridge was simply not strong enough to withstand the strength of the wind on that night. Although the presence of the train does affect the structure, even without the train it would still have fallen." Theory in full

Peter Lewis
"It was almost certainly dynamic effects on the bridge itself well before the disaster that brought the bridge down. These led to gradual deterioration of the ironwork supporting the high girders. On the night of the storm, the bridge piers were no longer capable of supporting the applied load. Within that, fatigue, crack propagation, probably played an important role." Theory in full