History and Research: Iron Bridge

The world’s first iron bridge was cast in Coalbrookdale by the local ironmaster Abraham Darby and erected across the River Severn in 1779.

Set in a spectacular wooded gorge, it is now part of a World Heritage Site, and is Britain’s best-known industrial monument.

Iron Bridge, the first bridge in the world to be constructed of iron

Iron Bridge, the first bridge in the world to be constructed of iron
© English Heritage

History

Although this symbol of the Industrial Revolution sits in what is today a rural area, in the late 18th century this part of Shropshire was an industrial powerhouse because of its rich coal deposits near the surface.

In 1709, Abraham Darby I, a former brass founder from Bristol, had begun to smelt local iron ore with coke made from Coalbrookdale coal.

The expansion of industrial activity here in the upper Severn gorge, however, was handicapped without a bridge, the nearest being at Buildwas 3km (2 miles) away. Intense barge traffic along the river also required a single-span bridge, as the steep sides of the gorge ruled out rising approaches to a stone central arch.  

Early concept

It was the Shrewsbury architect Thomas Pritchard who first suggested in 1773 to the ironmaster John Wilkinson that an iron bridge be built over the Severn.

The chosen crossing point, where a ferry had crossed from Benthall to Madeley Wood, had the advantage of high approaches on both sides and relative stability.

Pritchard drew up the designs, but he died in 1777, a month after work had begun on a single-span bridge of 30 metre (100 foot) with five main semicircular ribs.

Completing the Bridge

Abraham Darby III, grandson of the first foundry owner, agreed to continue the project, and all the iron was cast at his Coalbrookdale furnace.

Construction was completed in 1779, using in all 378 tons of iron, and the world’s first iron bridge was formally opened on New Year’s Day 1781, having cost over £6,000.

The construction methods are based on those used for woodworking

The construction methods are based on those used for woodworking
© English Heritage

Construction methods

Recent discoveries have shed much light on how the bridge was actually built.

In 1997, a small watercolour sketch of the bridge under construction was discovered in a Stockholm museum. Together with recent research this has revealed much about the building process and has overturned earlier assumptions.

It is now known that 70 per cent of the components – including all the large castings – were made individually to fit, and as a result each is slightly different from the others.

Darby’s workers employed woodworking joints – mortises and tenons, dovetails and wedges – and adapted them to the different properties of cast iron. A half-size replica of the main section of the bridge was built in 2001 as part of  the research.  

Commercial importance

The bridge had a far-reaching impact on the local society and economy.

It was always intended as a monument to the achievements of Shropshire ironmasters as well as a river crossing – it was an advertisement that gave their ironworks a competitive edge over their rivals.

The settlement that became known as Ironbridge quickly began to grow around the bridge after it had opened.  

The development of a single-span cast-iron bridge also represented a turning point in British bridge design and engineering, and cast iron became widely used in the construction of bridges, aqueducts and buildings.

The original builders, however, employing what was then new technology, had used much more iron than was necessary. The engineer Thomas Telford subsequently recognised that lighter cast-iron frames, such as that used at Cantlop Bridge, would allow the use of flatter profiles and less substantial foundations, while still enabling single spans and so avoiding the central piers that previously hindered river navigation.  

Later changes

The bridge’s great weight accounts for its massive strength over the years: it resisted the severe flooding of 1795 without damage. By then, however, there had already been some trouble with the enormous stone abutments, which caused cracks in the ironwork when the river banks shifted slowly over time.

The south abutment was modified several times and eventually replaced, initially by two wooden land arches and in 1821 by cast-iron arches.  

The bridge remained in full use for over 150 years, by ever-increasing traffic.

It was finally closed to vehicles in 1934, when it was designated an Ancient Monument.

Further massive works to strengthen the bridge have been undertaken since to counteract the constant tendency of the gorge sides to push inwards: in 1973 a reinforced concrete strut was built across the bed of the river to brace the two abutments.

English Heritage, together with the Ironbridge Gorge Museums Trust, carried out a full archaeological survey, record and analysis of the bridge in 1999–2000. The three-dimensional digital record now enables detailed understanding and management of the structure.

A World Heritage Site

The Iron Bridge today lies at the heart of a renowned complex of living history museums, and visitors can still cross it on foot.

There is an exhibition about its history in the brick tollhouse, which sits on the west side of the south abutment: the original tollhouse structure is contemporary with the bridge, and was extended in the mid-19th century.

The Museum of the Gorge, north of the river, provides an excellent introduction to the history of the gorge as a whole. 

Sources

Clark, C M 1993. 'The English Heritage Book of Ironbridge Gorge', London: Batsford/ English Heritage

Cossons, N and Trinder, B 2002. 'The Iron Bridge: Symbol of the Industrial Revolution', Chichester: Phillimore

Disclaimer

The text and pictures on this page are derived from the 'Heritage Unlocked' series of guidebooks published in 2004. We intend to review, update and enhance the content in the near future as part of the Portico project, whose objective is to provide information on the history, significance, research background and sources for all English Heritage properties.

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