The world’s first cast iron bridge still stands in Shropshire, England, across River Severn. It’s more than two hundred years old.
Although cast iron has been used since ancient times to make pots and pans, cannon balls, and decorative pieces such as window grills and chimneypieces, it was never used for structural purposes until architect Thomas Farnolls Pritchard suggested that a cast iron bridge be constructed to span the Severn Gorge in Shropshire. The Severn Gorge, later renamed to the Ironbridge Gorge after the bridge, is rich in coal, iron ore, and limestone, and a significant industry developed in the area to exploit these resources during the later part of the 18th century.
As the industry around the gorge grew, there arose the need for a strong and durable bridge to transport goods across the river. Because the gorge was deep and the banks instable, the bridge had to be single span and sufficiently high to allow tall ships to pass underneath, the river being a key trading route. The only material that was acceptable was cast iron, but nobody had built a cast iron bridge at this scale before. An iron bridge was begun at Lyons in 1755, but was abandoned because of cost, and a 22-meter-span wrought iron footbridge exist over an ornamental waterway in Kirklees, Yorkshire, since 1769.
Undaunted, architect Thomas Farnolls Pritchard proposed an iron bridge which would link the parishes of Madley and Benthall over one of the busiest rivers in the country. Pritchard’s designs were approved by Act of Parliament and in 1777 construction began.
Abraham Darby III, an ironmaster at Coalbrookdale, was commissioned to cast and build the bridge. When Pritchard died just a month after work had begun, the responsibility of the project fell upon Darby.
Darby cast all the parts necessary for the bridge—over 1,700 individual components, with the heaviest weighing 5 long tons— in his own foundry, and each component was cast individually to fit with each other. He borrowed techniques from carpentry, such as mortises and tenons, dovetails and wedges, and adapted them to the different properties of cast iron.
When the bridge was completed in 1779, it measured 100 feet across and weighed just short of 400 tons. Curiously, there is no reliable record and no witness account that describe exactly how Darby managed to hoist a mass of iron and suspend it over a river. But in 1997 a small watercolour sketch by Elias Martin came to light in a museum in Stockholm. The painting shows a moveable wooden scaffold consisting of derrick poles standing in the river bed being used as a crane to position the half-ribs of the bridge, which had been taken to the site by boat from Darby’s foundry 500 meters downstream. To rest the credibility of the engineering solution as depicted in the painting, a half-size replica of the main section of the bridge was built in 2001 as part of the research for a BBC.
Elias Martin’s painting of the Iron Bridge under construction, July 1779.
The success of the Iron Bridge inspired the widespread use of cast iron as a structural material across Europe and America, which was unfortunate because cast iron is brittle and has poor tensile strength. Throughout the 19th century, many cast iron bridges failed spectacularly. The most infamous was the Tay Bridge disaster of 1879, where 75 people lost their lives.
In 1943, the bridge was closed to vehicular traffic to avoid subjecting it to unnecessary stress and risk collapse. That same year, it was designated a Scheduled Ancient Monument. In the subsequent decades, the bridge was strengthened by building a reinforced concrete strut across the bed of the river to brace the two abutments.
Today, the bridge is celebrated as a symbol of the Industrial Revolution.
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