The Iron Bridge (1779) Shropshire, England

​The Iron Bridge which crosses the River Severn in Shropshire, England was the first arch bridge in the world to be constructed of cast iron, a material previously too expensive to use in large scale structures. The founding of a new blast furnace nearby brought the cost of cast iron down and local engineers and architects were able to solve the long-standing problem of a fixed crossing to replace an inadequate ferry service. Plans were drawn up in 1775 and work began two years later. Abraham Darby III of the Coalbrookdale Foundry was commissioned to cast and build the bridge. 379 tons of iron for the bridge were cast at Coalbrookdale comprising more than 800 individual parts. The construction method was based on carpentry as there was no precedent for the use of iron. No plans or detailed accounts of the construction survived and many of the accepted assumptions were challenged with the discovery in 1997 of a previously unknown watercolour sketch which showed the bridge being built. A team of experts constructed a model of the bridge based on the sketch and determined that the method shown was possible. An archaeological survey followed and a detailed 3D model was built resulting in the finding that the method shown in the sketch was probably how the bridge was built.

Construction of the bridge was completed in 1779 but related roadworks delayed the official opening until 1st January 1881. The bridge is 60m (200 ft) long with an arch span of half that and it rises 18m (59 ft) above the Severn. The bridge is susceptible to cracks and has been extensively repaired many times the first as early as 1802. Many of the original cracks are visible today. Later cast iron bridges used much less iron and it is felt that the Iron Bridge was over-designed. The bridge was closed to vehicular traffic in 1934 and operated as a tolled pedestrian bridge until 1950. The bridge is a Grade I listed building and with the adjacent town of Ironbridge and the Ironbridge Gorge forms the UNESCO Ironbridge Gorge World Heritage Site. Links: Wikipedia | The Iron Bridge - How was it Built? (BBC)