5.15 Further investigation is possible
There are still many mysteries that surround the Tay Bridge disaster, largely because so little was recorded at the time of construction. For instance, questions remain about the details of reject rates for the castings, and modifications made to the first designs of the piers and their component parts.
Although enlargement of the BoT set of pictures has helped clarify the various failure modes described by Henry Law and others at the enquiry, it has also revealed yet more mysteries. Why were the jacking columns left in (Figures 25 and 26)? Perhaps there may have been problems in stabilising the structure as it was being built, the attached wings on the two columns shown in the pictures allowing extra struts to be inserted. There is no mention of these designs in the enquiry, and other useful information from the enquiry itself is now lost, such as drawings made by witnesses and extra pictures taken at the time of failed components.
Virtually all the material evidence Law collected, and the samples Kirkaldy tested have also disappeared, making the task of the modern investigator more difficult. Most of the fallen piers probably still lie buried in the estuary. The brick piers were deliberately tipped into the estuary during the construction of the replacement bridge.
The most intriguing suggestion that has emerged from this enquiry is the possibility of fatigue at the critical lower lugs. If corroborated by further research, perhaps it would encourage more detailed examination of the history of use of the bridge from construction to fall. The story of the increased awareness of fatigue during the Victorian era is chequered.
Various engineers examined and specified the failure mode, especially Wöhler in Germany. It was he who developed apparatus to test axles in a realistic way by applying both repeated bending and rotation to the shaft. He recognised the importance of stress concentrators for starting cracks, and showed how two distinct zones occur: a slow crack growth region and the fast growth part where the crack accelerated suddenly to break the component into two halves.
Rankine and Fairbairn also developed some understanding of the problem at about the same time as Wöhler, in the 1850s and 1860s. But it appears their published works were not read by practising engineers, because many later railway accidents were probably caused by fatigue, especially causing the failure of engine and carriage axles.
One of the first large scale disasters happened at Versailles outside Paris when an axle on a locomotive fractured on 11 May 1842. A principal cause of the large number of deaths – over 40 – was the subsequent fire from which passengers could not escape because the carriage doors were locked. However, the accident was almost certainly initiated by fatigue crack growth at a sharp fillet radius on the shaft, and such accidents became common for the lack of appreciation of the stress concentration.
Wrought iron tyres also caused many accidents, culminating in the Shipton-on-Cherwell disaster on Christmas eve 1874. A tyre on one of the carriages fractured suddenly, and caused progressive derailment of the train. There were many casualties – 34 dead, 69 injured – and Colonel Yolland, the investigator, pointed towards the problem of tyre fractures from bolt and rivet holes. That the bolt holes in the Tay bridge lugs were a problem could have come as no surprise to him. However, fatigue is not mentioned at all in his report of the court of enquiry, and is the most likely source of the many sudden fractures that had been experienced on the railways up to and including the Shipton accident.
The problem of fatigue remains with us to this day, and is not restricted to rail, but has caused some of the worst aerospace disasters in living memory. The Comet crashes of the 1950s were caused by fatigue cracks growing from corners in portholes. The Japanese Airlines JAL8119 disaster of 1985 occurred when a fatigue crack grew to criticality from a rivet hole in the tail. The subsequent crash produced the highest number of casualties in a single airplane crash ever – 520 deaths. The extra rivet hole had been made during an incorrect repair to the tail. Appreciating the importance of fatigue as a basic failure mode in all structural materials is clearly a lesson still to be learnt by many engineers.