The mystery of building the Iron Bridge at Coalbrookdale

摘要

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 1779, 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 Hodson has been studying the bridge since 1957, and is confident of the sequence of erection. Re 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 centre, especially stone bridges, erected to support the structure during building. But this practice could not be used on the Iron Bridge that spanned a navigable section of the river Severn that had to be kept open while building the bridge. Contemporary reports tell us that the bridge parts were cast nearby and brought to the site by boat. It is generally agreed that a scaffold was used, this is borne out by the 'Phillips Text' of 1782, that reads, 'A high Scaffold being previously erected, and the work carried out without a single accident, or the least obstruction to the navigation of the river.' A remarkable achievement indeed, the problem is; what did this Scaffold look like?