This theory (10), attributed to the late Bill Dow, a retired lecturer in Physics, postulates that the train came off the track due to a kink in the rails with uplift of the train attributed to aerodynamic forces. In consequence, one of the carriages hits the bridge and the shock experienced by the pier causes the cast iron lugs connecting the wind bracing members to the columns to fracture leading to the subsequent collapse of the pier structure. To add credibility for the theory, the fact is highlighed that all the deck spans remained standing while the high girders, which the train was passing through, all collapsed. In other words, the train was necessary for the collapse.
This theory is similar to the one proposed by Sir Thomas Bouch to explain the collapse. It is possible that the train came off the rails to help trigger the collapse but further quantitative analysis is required to explain why the bridge failed as it did. The computer structural analysis indicates that the presence of the train on the bridge is not required for the bridge to collapse. Further computer analysis of the remaining piers left standing clearly show that, even with the weak bracing, the piers are very safe at Beaufort Force 10/11. There are two reasons for this:
- Wind load on the girders is greatly reduced as they are 145ft long as compared to the high girders which were 245ft long.
- The bending moment due to the wind load is lower due to the piers not being as high as the high girder piers.
This theory has been sited as the cause of the collapse by Charles McKean (Professor of Scottish Architectural History at Dundee University) in recently published book, Battle for the North (13). McKean conclusion that the wind was not the main agent in the collapse is based on:
- The wind was not particularly strong.
- Technical reports presented at the enquiry that confirmed the design of the bridge was adequate in relation to wind loading
Neither of these reasons stands up to scrutiny. McKean quotes Peter Burt (from his paper (14) about the strength of the wind at the time or the disaster) that the storm was not that severe. Burt was not inferring that the wind was less than storm force.
The computer analysis clearly shows that the bridge could have collapsed due to a Beaufort force 10/11 wind (storm/violent storm). Such a wind force is consistent with the experience of people who were in the area of the bridge at the time of the collapse(1,2,3). Data from the Meteorological office(15) in Edinburgh also support the Force 10/11 wind force estimate.
Tay Bridge disaster 