6.1 New Tay Bridge
So the collapse of the bridge was probably caused by premature fracture of the lugs, perhaps aided by fatigue (Input 10). Once the wind braces had been lost, the stability of the piers was drastically reduced because each trio of columns became separated (Figure 47). It only needed a further small sway to cause toppling, because of the shift in centre of gravity of the piers.
The collapse of the Tay Bridge sent shock waves through the engineering community. The immediate reaction was to cancel Bouch's next major project for a bridge across the Forth, and to build a replacement bridge across the Tay.
The new bridge carried a double railway track and needed wider girders and piers to support the greater width of permanent way (Figure 49). The main piers were fabricated from iron plates in the form of a massive tubular arch supported by a wrought iron internal space frame and had an ample resistance to lateral wind forces. The new bridge runs parallel to the course of the old one and the stumps of Bouch's piers can still be seen from a passing train. Many of the girders from the old bridge were incorporated into the structure of its successor.
Given all the problems with estimating the wind speed – and hence the pressure – acting on the bridge at the time of the disaster, a royal commission was set up shortly after the enquiry to study the problem. Stokes was a principal member of the commission, and he helped establish a number of experimental stations across the country to measure wind speeds over a period of time. He developed an anemometer design for this purpose, one of which still survives and is presently in the Science Museum.
Despite the probability the disaster was not caused by the storm, wind pressure now became an important design criterion for bridge builders, and is recognised as such today in the relevant British Standard (BS 5400).
There were several other consequences of the disaster. Bouch's other bridges were all carefully examined for their design, construction and safety. Many were reinforced or otherwise modified to increase their stability, although his similar ‘water pipe’ bridge at Belah survived intact till 1966. On the other hand, it only carried light traffic on a minor branch line.
Bouch himself died on 30 October 1880 at the young age of 58, a demise probably hastened by the Rothery report (see Paper 3, linked below), which placed the blame for the disaster on him personally.
Click 'View document' below to open Paper 3 (35 pages, 39 MB).
Major-general Hutchinson, the inspector who had approved opening the Tay bridge for traffic after testing it, escaped criticism because he had added a comment about wishing ‘… to observe the effect of wind on the structure …’. He never did meet this obligation.
Many years later, Hutchinson produced his own report on the failure of the Portland Road bridge near Norwood Junction station in 1891. It fractured suddenly while a train was passing over it, but fortunately no-one was killed because the train managed to reach the other side before the track gave way under its weight – the track did not fracture however, remaining in festoons over the gap. A cast-iron beam laid across the gap fractured at two places on the tension side, the primary crack growing vertically from a large casting defect. The defect was completely hidden from view, so had remained undetected since manufacture. The report does not mention the possibility of fatigue, but the bridge had lasted 31 years before failure. The immediate cause was apparently an increase in locomotive weight using the track.
As a direct result of this disaster, all cast-iron bridges where the material was put into tension were replaced by steel or wrought iron constructions. The company involved, the London, Brighton and South Coast Railway, were asked by their consulting engineer to rebuild 20 bridges, followed by another 60 within three years. Other companies followed their example.
Why such cast-iron bridges had not been replaced years before following the Dee bridge accident is unexplained by the official report, although it does mention that no new bridges could be built with cast-iron beams after August 1883. Presumably this regulation stemmed directly from the Tay Bridge disaster.
The materials of construction for bridges naturally received great attention, given the problems with cast-iron found in the Tay bridge disaster. Bessemer steel had been available for some time before the disaster, and it is surprising it was not used in the Tay Bridge. Its greater toughness should have made it a prime material for structural application, and it was soon approved for the new Forth Bridge built shortly after the disaster on the Tay.