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Iron Bridge
River Severn, Ironbridge, Shropshire
Iron Bridge
associated engineer
Abraham Darby III
date  November 1777 - December 1780, opened 1 January 1781
UK era  Georgian  |  category  Bridge  |  reference  SJ671034
ICE reference number  HEW 136
photo  PHEW
The first major bridge in the world to be constructed wholly of cast iron. The iconic Iron Bridge spans the River Severn between Broseley and Madeley Wood near Coalbrookdale and gives its name to the town that sprang up around it. It is Britain's best-known industrial monument and remains in use by pedestrians.
Rural east Shropshire may seem an unlikely location for a structure that has come to symbolise the Industrial Revolution in Britain. At the start of the 18th century, a shortage of timber — used for making charcoal to fuel blast furnaces — affected iron production so dramatically that pig iron had to be imported from Europe. In 1709, Abraham Darby (1678-1717) developed a method for using coke instead of charcoal to fuel the smelting furnaces at his Coalbrookdale foundry (SJ665050).
Fortunately, the area lived up to its name — there were plentiful coal seams near the surface. Other important raw materials such as iron ore, water, sand, clay and limestone were also readily available in the Severn gorge. And with Abraham Darby’s new technique, high quality iron could be produced in much larger quantities than ever before.
Better infrastructure was needed for the iron trade to continue to grow. In September 1775, a bridge was proposed between Broseley and Madeley Wood, using a single span to avoid constructing piers in a river prone to flooding. In March 1776, the Parliamentary Act for a cast iron bridge received royal assent.
Thomas Farnolls Pritchard (1723-1777) was the bridge’s architect. The Iron Bridge we see today was his third design, which was approved in June 1776. Two previous designs, for 36.5m and 27.4m single spans, had been rejected. Abraham Darby's grandson, Abraham Darby III (1750-91), was in overall charge of the construction and casting.
Drawings for the detailed design of bridge members were made at the Coalbrookdale foundry by Thomas Gregory, a foreman pattern maker who usually worked with wood. This is probably why the bridge uses carpentry jointing details such as mortises and tenons, and dovetails and wedges, despite the change of materials from timber to iron.
Work started in November 1777, but Pritchard died the following month. During 1777-8, the masonry abutments were built and the castings prepared. The bridge has a single semicircular arch of 30.6m span, made up of 10 half ribs, each cast in one piece. It contains 385 tonnes of ironwork and almost 1,700 components, the heaviest weighing 5.5 tonnes.
The ironwork was completed in 1779 — the date is marked on the outer rib — and the whole structure was finished in December 1780, with its official opening as a toll bridge on New Year’s Day 1781. In 1788, the Society of Arts presented Abraham Darby III with their gold medal, in recognition of his achievements in building the bridge.
However, in December 1784, cracks were found in the south side of the arch, and the neighbouring abutment showed signs of movement. It was feared that the sides of the gorge were moving towards the river, forcing the feet of the arch towards each other. Repairs were carried out in 1784, 1791 and 1792.
In 1800, the stone-faced embankment behind the south abutment was replaced with two small timber land arches to relieve pressure on the river span. The timber arches were replaced with cast iron ones in 1821. In May 1862, the bridge was the subject of further repairs.
Iron Bridge continued to carry vehicular traffic until 1931, when it was closed to all but pedestrians. The bridge was designated a Scheduled Ancient Monument in 1934 and remained a toll bridge until 1950. The toll house (SJ672033) is on the south side.
By the time the Ironbridge Gorge Museum Trust was founded in 1967, the bridge needed major work as a result of ongoing movement of the gorge sides. In 1972-5, a reinforced concrete inverted arch was constructed over the river bed between the abutments. The northern abutment’s rubble fill was replaced with concrete.
In 1980, the bridge was blast cleaned and painted with a five coat system. It was Grade I listed in 1983. In 1999, the structure was inspected, the paint retouched a new top coat applied to all the ironwork.
Also in 1999, a watercolour sketch of Iron Bridge being built was discovered. By Swedish artist Elias Martin (1739-1818), and thought to be the only surviving contemporary record of the construction, it sparked an in-depth investigation of how the bridge was designed and made.
The bridge was surveyed in 1999-2000, using 3D EDM and CAD techniques, enabling accurate computer modelling of the structure. The data helped English Heritage to assemble a half-size physical model in 2002, reproducing the original construction methods and validating its design. They found that most of the components had been cast individually to ensure a good fit.
In 2001-2, cracks in the original cast iron parapet railings were repaired with 3.8mm thick carbon fibre reinforced epoxy plates. The repairs are almost invisible and circumvented the difficulty of welding cast iron in situ.
Architect: Thomas F. Pritchard
Contractor: Abraham Darby III, Coalbrookdale
Patternmaker: Thomas Gregory, Coalbrookdale
Research: ECPK
"Bedlam comes alive again" by Richard Fifield, in New Scientist, Vol.57, No.839, pp.722-725, 29th March 1973
"Industrial Biography: Iron Workers and Tool Makers" by Samuel Smiles, 1863, reprinted by Europaeischer Hochschulverlag GmbH & Co. KG, Bremen, 2010
reference sources   CEH W&W

Iron Bridge