The original Tay Bridge was made of cast iron which is prone to failure at the best of times but as the enquiry after the fact stated the bridge had been "badly designed, badly built, and badly maintained".RJDG14 wrote: ↑Sun Jul 31, 2022 16:18 The Forth Rail Bridge looks pretty well designed for a railway bridge from the 19th Century. The original Tay Bridge looks pretty flimsy by modern standards with little to no high wind support. The original Tacoma Narrows Bridge looked fairly well designed for its day in terms of its suspension and towers but from the pictures and opening booklet that I can find, the main deck looked very basic with virtually no regard made for airflow underneath.
Badly designed as Bouch had not made an adequate allowance for wind loading.
Badly built as the cast iron columns were not made by the iron works he has used previously and proved to be of poor quality. So bad were they that holes in the side of the column were filled with a mixture of beeswax, iron filings and rosin.
Badly maintained as the horizontal tie bars had not been properly tightened and this meant the trains would sway as they crossed the bridge.
The final blow was that on the night in question the cross wind was very high making the train sway as it crossed, its believed that the locomotive struck one of the cross members which caused a massive structural failure, it also seems to have been exceeding the bridge speed limit.
The result so damaged public faith that the Forth Bridge was massively overdesigned. As part of the final testing Two trains, each consisting of three heavy locomotives and 50 wagons loaded with coal, totalling 1,880 tons in weight, were driven slowly from South Queensferry to the middle of the north cantilever, stopping frequently to measure the deflection of the bridge. This represented more than twice the design load of the bridge: the deflection under load was as expected.
As for the Tacoma Narrows bridge when it was designed it was not normal practise for bridge designers to take the aerodynamics of the bridge into account. In the case of a massive steel structure such as the Forth Rail bridge this simply was not an issue. Do remember that both the original Severn and Forth road bridges have been found to have problems of their own, mainly to do with corrosion inside the suspension cables and attachment points.