The Iron Bridge

The Iron Bridge is a cast iron arch bridge that crosses the River Severn in Shropshire, England. Opened in 1781, it was the first major bridge in the world to be made of cast iron. Its success inspired the widespread use of cast iron as a structural material, and today the bridge is celebrated as a symbol of the Industrial Revolution.

The geography of the deep Ironbridge Gorge, formed by glacial action during the last ice age, meant that there are industrially useful deposits of coal, iron ore, limestone and fire clay present near the surface where they are readily mined, but also that it was difficult to build a bridge across the river at this location. To cope with the instability of the banks and the need to maintain a navigable channel in the river, a single span iron bridge was proposed by Thomas Farnolls Pritchard. After initial uncertainty about the use of iron, construction took place over 2 years, with Abraham Darby III responsible for the ironworks. The bridge crosses the Ironbridge Gorge with a main span of , allowing sufficient clearance for boats to pass underneath.

In 1934 it was designated a scheduled monument and closed to vehicular traffic. Tolls for pedestrians were collected until 1950, when the bridge was transferred into public ownership. After being in a poor state of repair for much of its life, extensive restoration works in the latter half of the 20th century have protected the bridge. The bridge, the adjacent settlement of Ironbridge and the Ironbridge Gorge form the UNESCO Ironbridge Gorge World Heritage Site.

History

Background

The Ironbridge Gorge was formed at the end of the last ice age by the overflowing of Lake Lapworth, which resulted in the exposure of useful deposits of resources such as coal, iron ore, fire clay and limestone near the surface where they were readily mined. With the river providing a means of transport, the local area was an important centre of the emerging Industrial Revolution.

Abraham Darby I first smelted local iron ore with coke made from Coalbrookdale coal in 1709, and in the coming decades Shropshire became a centre for industry due to the low price of fuel from local mines. The River Severn was used as a key trading route, but it was also a barrier to travel around the deep Ironbridge Gorge, especially between the then important industrial parishes of Broseley and Madeley, the nearest bridge being at Buildwas away. The Iron Bridge was therefore proposed to link the industrial town of Broseley with the smaller mining town of Madeley and the industrial centre of Coalbrookdale. The use of the river by boat traffic and the steep sides of the gorge meant that any bridge should ideally be of a single span, and sufficiently high to allow tall ships to pass underneath. The steepness and instability of the banks was problematic for building a bridge, and there was no point where roads on opposite sides of the river converged.

The Iron Bridge was the first of its kind to be constructed, although not the first to be considered nor the first iron bridge of any kind. An iron bridge was partly constructed at Lyons in 1755, but was abandoned for reasons of cost, and a span wrought iron footbridge over an ornamental waterway was erected in Kirklees, Yorkshire, in 1769.

Proposal

In 1773, architect Thomas Farnolls Pritchard wrote to his 'iron mad' friend and local ironmaster, John Wilkinson of Broseley, to suggest building a bridge out of cast iron. Although he specialised in the design of chimneypieces and other items of interior decoration, and in funerary monuments, he had also previously designed both wooden and stone bridges.

During the winter of 1773–74, local newspapers advertised a proposal to petition Parliament for leave to construct an iron bridge with a single span. In 1775, a subscription raised funds of between £3000 to £4000 (equivalent to £ to £ in 2016), and Abraham Darby III, the grandson of Abraham Darby I and an ironmaster working at Coalbrookdale, was appointed treasurer to the project.

In March 1776, the Act to build a bridge received Royal assent. It had been drafted by Thomas Addenbrooke, secretary of the trustees, and John Harries, a London barrister, then presented to the House of Commons by Charles Baldwyn, MP for Shropshire. Abraham Darby III was commissioned to cast and build the bridge. In May 1776, the trustees withdrew Darby's commission, and instead advertised for plans for a single arch bridge to be built in "stone, brick or timber". No satisfactory proposal was made, and the trustees agreed to proceed with Pritchard's design, but there was continued uncertainty about the use of iron, and conditions were set on the cost and duration of the construction. In July 1777 the span of the bridge was decreased to , and then increased again to , possibly in order to accommodate a towpath.

Construction

The site, adjacent to where a ferry had run between Madeley and Benthall, was chosen for its high approaches on each side and the relative solidity of the ground. The Act of Parliament described how the bridge was to be built from a point in Benthall parish near the house of Samuel Barnett to a point on the opposite shore near the house of Thomas Crumpton. Pritchard died on 21 December 1777 in his towerhouse at Eyton on Severn, only a month after work had begun, having been ill for over a year.

The bridge is built from five sectional cast-iron ribs that give a span of . The construction of the bridge used of iron, and there are almost 1,700 individual components, the heaviest weighing . Components were cast individually to fit with each other, rather than being of standard sizes, with discrepancies of up to several centimetres between 'identical' components in different locations.

The masonry and abutments were constructed between 1777 and 1778, and the ribs were lifted into place in the summer of 1779. The bridge first spanned the river on 2 July 1779, and it was opened to traffic on 1 January 1781.

In 1997, a watercolour by Elias Martin was discovered in a Stockholm museum, which showed the bridge under construction in 1779. 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 downstream. Using the approach 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 the BBC
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