Bull bridge accident
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As a goods train was passing over a cast-iron bridge at Bullbridge, near Ambergate in Derbyshire on 26 September 1860, it failed suddenly and derailed most of the train of wagons. There were fortunately no casualties, but it was a warning of the fundamental weakness of most such bridges on the British rail network.
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[edit] The Accident
It happened on the Midland Railway track between Derby and Chesterfield, on a dark and foggy night of 26 September 1860. With visibility only about 10 yards, the train was proceeding north at only 14 mph. It was a long train, with 27 wagons loaded with salt, two loaded goods vans, engine, tender and brake van. The heavy load was causing some slippage on the rail. Half a mile beyond Ambergate station, the driver suddenly noticed that his rear wheels were no longer on the rails. He shut off steam, stopped the engine and went to investigate. His tender was attached to only two wagons, and they were all off the rails too. There were two more wagons about 10 yards behind, close to Bull bridge, a small viaduct over a local road. The next nine wagons behind were piled in a heap about 25 feet high from the bottom of the road, reaching up to the telegraph wires by the side of the track. The guard in the brake van had been thrown head first against the front panel when the accident occurred, but was not seriously hurt. All the wagons behind the bridge were still on the line.
[edit] Investigation
When examined by the driver and fireman, they found that one of the cast iron girders had fractured, unusually near to one of the abutments, rather than at the centre of the beam. The track was supported by a pair of identical girders with Barlow rails cut to length laid across the inner flange, with asphalted ballast on top, just below the sleepers. When Captain Tyler of the Railway Inspectorate examined the fracture surface, he found it rusted over, but the evidence of those who had seen it first, showed just what had happened. The cast iron girder had broken vertically from an enormous defect in the web and flanges, where there was a complete gap between the sides. The girder was completely defective here, but why no one had spotted it before the accident remains a mystery. Tyler suggests that it may have grown from an internal void with time, simply because such a defect would surely have been discovered either by the foundry or the builders. The girder had been installed 23 years before, in 1837, and should have supported a central load of 90 tons if in good condition. The pair of girders should have supported a total load of 360 tons distributed over all the structure, and Tyler stated that the girder was much bigger than was needed for the job. The engine weighed 31 tons and the tender 18 tons. The source of the defect remains unknown, but may have been caused initially by a cold shut, where the molten metal does not fuse together during casting. It often occurs when two waves of fast cooling cast iron meet, but why it should have occurred only about 10 feet from one end remains unknown. It could have grown with repeated loading from passing traffic, and Tyler thought it lucky that it had failed when a goods train was passing, rather than a passenger train, when casualties would have been inevitable. So it is possible that fatigue had caused the failure.
[edit] Implications
Together with the Wooton bridge collapse in the same year, there should have been some concern within the railway companies about the number of cast iron girders used as under-bridges on railway lines. Engineers would also have been aware of the problems of cast-iron in railway bridges after the failure of the Dee bridge in 1847. Whether or not such structures were inspected is unknown. Later events seem to negate the possibility, because several more collapses occurred some years later, at the Inverythan crash and the Norwood Junction rail accident. Thereafter, all cast-iron underbridges on the rail network were replaced by steel structures.
[edit] See also
[edit] References
Peter R Lewis, Disaster on the Dee: Robert Stephenson's Nemesis of 1847, Tempus (2007).
