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The famous Iron Bridge, which spans the River Severn at Coalbrookdale in Shropshire, England has again come under the spotlight regarding its construction. Built by the Shropshire ironmaster Abraham Darby III in 17/9, it is generally accepted to be the first bridge to be built of iron in the world. But the mystery remains - what kind of equipment did Darby and his men have at their disposal to assemble the 378 tons of cast iron from which the bridge is made? The heaviest castings are the main ribs of the arches, these are in two halves and fit into an iron key to form the arch. Each half-rib is 70 feet (21m) from end to end and weighs 6t. There are five identical arches placed alongside each other over a distance of about 26 feet (8m) to give the bridge its width. A further series of ribs and pillars make up the superstructure. The inner pillars resist the thrust of the main arches and are the second heaviest castings at about 4 tons each. These pillars are connected by the cross-stays to the back outer pillars, which are supported by the stone abutments. For those who like numbers, there are 1400 separate castings and 900 rivets, pins and wedges. Castings are 'jointed' into one another forming an assembly that any carpenter would be familiar with.
Harry Hudson has been studying the bridge since 1957, and is confident of the sequence of erection. He has produced a scale model from castings of each component, a task that took 6 years. Using this, the most likely positions of the lifting tackle used to assemble the bridge have been calculated. Most arched bridges are constructed on a wooden...