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The Dee bridge disaster was a rail accident that occurred in 1847. A new bridge across the river Dee in Chester was needed for the Chester-Holyhead railway, a project planned in the 1840s for the expanding British railway system. It was built using cast iron girders, each of which was made of three very large castings dovetailed together. Each girder was strengthened by wrought iron bars along the length. It was finished in September 1846, and opened for local traffic after approval by the first Railway Inspector, General Pasley. However, six months later, a local train fell through the bridge and five died. The bridge had been designed by Robert Stephenson, and he was accused of negligence by a local inquest. Although strong in compression, cast iron was known to be brittle in tension or bending, yet that same day, May 24th 1847, the track was covered with track ballast to prevent the oak beams supporting the track from catching fire. Ironically, Stephenson took this precaution because of a recent fire on the Great Western Railway at Uxbridge, London, where Isambard Kingdom Brunel's bridge caught fire and collapsed.
The accident occurred a few hours later when the locomotive reached the final girder. It cracked in the middle, allowing all the carriages to fall into the river Dee fifty feet below. The extra load of ballast undoubtedly helped cause the accident. The design of the bridge was seriously flawed, although different authors have emphasised different causes. Lewis and Gagg state that failure occurred in tension at the bottom of the girders, exacerbated by stress concentrations. However, Henry Petroski notes that the wrought iron bars would tend to exacerbate compression in the beams, and as they are eccentric they increased the tendency towards failure by lateral torsional buckling. William Fairbairn had warned Stephenson of the problem of cast iron girders only a few months before construction of the bridge at a meeting at the Institution of Civil Engineers in London, but his advice was ignored. A subsequent Royal Commission condemned the design and the use of cast iron in railway bridges. It had been used very successfully in the The Crystal Palace of 1851 and the Crumlin viaduct in south Wales (built in 1857), but the first Tay Rail Bridge of 1878 failed catastrophically due to its poor use of the material, putting the cast iron lugs on the columns into tension. The Tay disaster stimulated engineers to use steel, as exhibited by the Forth Railway Bridge of 1890.
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