Some have suggested that the "High Girders" section (about half a mile in length) was blown over by the storm, but the original inquiry in 1880 found that the bridge had been badly designed, constructed and maintained. The tall piers which supported the main girders was a space frame made from cast iron columns braced together with wrought iron tie bars (figure 6).

Figure 6: Section of the Old Tay bridge showing the High and Low girders.

Although the columns were mainly in compression, the bracing bars were tensioned to lugs cast integral with the columns. The cast iron here was in tension, and much weaker than had been realised at the time it was designed. The lugs broke very easily (Figure 7), and it is likely that the whole structure was oscillating laterally when trains passed over. High winds would have added extra loads onto a structure which had been vibrating for many months before the final crash. Defects showing the weakness of the structure were found during maintenance, but their significance not appreciated. The tie bars had loosened, and bolts came apart, falling onto the pier foundations.

Figure 7: Close-up of fractured lug, with intact lug and wedged joint at right

The demands of local industry assured that the bridge would be replaced, and the new bridge was finished in 1887. It was much more stable laterally since the girders were put on arches, and they carried a double rather than single track (Figure 8). It remains in use safely to this day, together with a new road bridge downstream.

Figure 8: The New Tay Bridge with the piers of the older strcuture still visible to the right

The Forth estuary presented quite different problems. Although the estuary was much narrower, the land either side was much higher, so a tall but shorter bridge was needed. A suspension bridge had been suggested earlier by the same engineer (Bouch) who built the first Tay bridge. When he was blamed for the disaster, the contract was removed and eventually given to Baker and Foster. They proposed a quite different way of crossing the Forth: a cantilever design with three massive piers to support the double rail track (top right).

The entire bridge was built from mild steel, a relatively new material for engineering, but much tougher than cast iron (especially in tension). The structure was held together with rivets, preventing any loosening with vibrations from passing trains. Although the structure is very safe, 57 workers were killed during construction, a sad reminder of the risks taken by Victorian engineers. Steel is also susceptible to rusting in the salty conditions of the estuary, so must be repainted regularly (a fact remembered by popular simile!). Together with the new Tay bridge, journey times decreased substantially, and freight for expanding industry was available at lower cost. The completion of the bridge in 1890 may have signalled the end of the great railway era, but enabled industry in Scotland to thrive and prosper.

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Content last updated: 18/08/2005

About our expert

Dr Peter Lewis (CEng FIM MAE MFSS) is a senior lecturer in the Department of Materials Engineering at the Open University, with extensive experience of failure investigation of polymer and composite products. He is course chair of T838 (Design and Manufacture of Polymer products) and T839 (Forensic Engineering).

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