Cast iron became a very popular construction material in the late 1700s. It had previously been too expensive to produce in a large enough quantity for construction, but the advent of many new blast furnaces made cast iron more widely available. Through the Industrial Revolution cast iron bridges became very popular and many still remain.
Cast iron has a very high resistance to compression which makes it ideal for use in arch bridges where the entire cast iron piece is put under compression. For this reason the first large cast iron bridge, the Iron Bridge, used cast iron for its supportive arch. The bridge was constructed in 1779 across the River Severn in Shropshire, England.
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Cost of Cast Iron
During the Industrial Revolution, manufacturing cast iron became easier as there was a widespread rise in factories and furnaces. This led to cast iron becoming a very cheap building material. Bridge designers saw the opportunity in its use and set about building dozens of new bridges with the material. The advent of the railway made bridge construction an even bigger demand. The cheap cost and availability of the material made it ideal for these new projects.
Although cast iron has a very high resistance to compression it is also very brittle and has poor tensile strength. This is not typical of most metals or even other types of iron. When the bridges were being designed, many architects mistakenly put the cast iron beams into tension rather than compression in their designs because no one yet knew about this property of cast iron.
The combination of widespread use and weak strength led to many disastrous collapses of cast iron bridges. The Tay Bridge in Scotland in 1879 was one of the most serious examples of such a collapse. The center portion of the bridge collapsed taking a train with it during a violent storm. All 75 people on board the train were killed. As a result, many cast iron bridges were rebuilt or demolished and most surviving bridges have been refitted with steel.