Forensic engineering: a reappraisal of the Tay Bridge disaster

Abstract

The Tay Bridge disaster of 1879 shocked the world and led to important changes in bridge design, construction, and inspection. The Court of Inquiry produced its final report in six months, and condemned the structure for its design and materials defects. However, the court did not specify exactly how the final collapse of the 'high girders' section occurred on the night of the accident. By reexamining the wealth of evidence surviving from the time, in particular the photographic archive and the court proceedings, we have looked again at the causes of the disaster. Our reappraisal confirms the conclusions of the original inquiry, but it also extends them by suggesting that lateral oscillations were induced in the high girders section of the bridge by trains passing over a slight misalignment in the track. The amplitude of these oscillations grew with time, because joints holding the bridge together were defective, and this in turn resulted in fatigue cracks being induced in the cast iron lugs, which reached criticality on the night of the disaster. Numerous east–west lugs fractured when a local train passed over the bridge in a westerly gale on the evening of 28 December 1879. The express train which followed was much heavier, and the towers in the high girders collapsed progressively as the train was part way over the section. Although wind loads contributed to the disaster, the bridge was already severely defective owing to failure of its most important stabilising elements.