|Cast iron is made by pouring molten iron into a mould. This
enables large single pieces to be made to any desired shape. For early bridge
builders, this seemed the ideal material. Beams and other components could be
made economically to suit individual locations, enabling rapid assembly on
site. This was known technology when the early railways were built, because
cast iron was already in use in canal and road bridges.
Cast iron is strong in compression, but not in tension. This had not been a problem with road bridges, because loadings were light. The weight and live loading of even the earliest trains imposed stresses that could cause fracturing and failure of cast iron components. This was demonstrated by the collapse of the Dee bridge at Chester in 1847. Railway companies strengthened bridges, usually with additional wrought iron beams, but continued to use cast iron arches.
In May 1891 a cast iron bridge at Portland Road, Norwood Junction collapsed under a train. The Board of Trade Inspector who investigated the incident, Major General Hutchinson, found that there had been a flaw in a cast iron girder. However, even if the flaw had not been present, he considered that the bridge would have been under-strength for use by the heavier locomotives then working the line. He recommended the replacement of cast iron bridge girders by wrought iron or steel. The London, Brighton and South Coast Railway alone had to rebuild eighty bridges, and other companies replaced many others.
Most cast iron structures that were dismantled went for scrap, but Castle Street bridge, Salisbury escaped the blast furnace. This was a single span, with four parallel cast-iron arches. The cast-ironwork was reused in a new bridge nearby, carrying Nelson Street over the River Avon.
Cast iron can still be seen quite widely in railway bridges, but normally in columns (where it is in compression, quite safely), lightly-loaded overbridges, or in components that are not load-bearing.
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This page was last updated 19 March 2011