To date there are four theories proposed to explain why the navigation spans collapsed:
Can Philosophy help when when faced with deciding between competing theories? William of Occam's abiding contribution to philosophy is a principle simplicity and economy, known as Occam’s razor. It states that ‘entities are not to be multiplied beyond necessity.’ In modern language Occam’s razor would translate something like: Given we have a potential set of solutions to a problem, which one is the most likely to be true: The one with the least amount of pre-suppositions and the theory which contains the simplest ideas.
The train could have come off the rails assisting the bridge to collapse. Dynamic effects and fatigue could have been a contributory factor although they are weakly supported by evidence. Bad workmanship may have exacerbated the situation. But these are secondary issues. The stongest evidence points to the bridge being underdesigned for wind. It blew over due to inadequate bracing.
In terms of loss of life, the Tay Rail Bridge disaster is one of the worst structural collapses in the UK in recorded history. The loss of the Stranraer to Larne Ferry Princess Victoria in 1953 is the worst maritime disaster on the UK coast outside wartime. To mark the 50th anniversary of this tragedy a new account of the event has been published (16). In this book the author attributes the fundamental source of the disaster to bad design and suggests that for those who died it was 'death by design'.
In the case of the Tay Bridge the wind loading was seriously underestimated; in the case of the Princess Victoria the stern doors (see picture above) were inadequate to withstand heavy seas and the scuppers were not large enough to efficiently drain water from the car deck. Could it be said that, as for the Princess Victoria disaster, for those who perished in the cold waters of the firth of Tay that night, it was death by design?