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 two years, with Abraham Darby III responsible for the ironwork. 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 Lyon 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 £3,000 and £4,000, 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 giving authorisation 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 Benthall Bridge, Severn Act 1776 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's Timewatch programme, which was shown the following year.
Design
The bridge is to a carpenters' design typically used for wood structures, built from five sectional cast-iron ribs that give a span of. The interlocking rings between the arches make this a truss arch. Exactly of iron was used in the construction of the bridge, 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.Decorative rings and ogees between the structural ribs of the bridge suggest that the final design was Pritchard's, as the same elements appear in a gazebo he rebuilt. A foreman at the foundry, Thomas Gregory, drew the detailed designs for the members, resulting in the use of carpentry jointing details such as mortise and tenon joints and dovetails.
The two outer ribs are engraved with the words: "This bridge was cast at Coalbrook-Dale and erected in the year MDCCLXXIX".
Two supplemental arches, of similar cast iron construction, carry a towpath on the southern bank and also act as flood arches. A stone arch with a brick vault carries a small path on the northern bank.
Material
The Iron Bridge is made of cast iron. This is immensely strong in compression, but performs less well than steel and wrought iron when subjected to tension or bending moments, because of its brittleness and lower tensile strength. Analysis of an arch and a strut from the Iron Bridge revealed the following elemental compositions:The presence of 0.1% sulphur in cast iron is at the upper limit of what is acceptable, but the presence of sufficient manganese leads to the formation of harmless manganese sulphide.
Puddled wrought iron was a much better structural material than cast iron but not widely available until after 1800, eventually becoming the preferred material for bridges, rails, ships and buildings until new steel making processes such as the Bessemer process were developed in the late 19th century.
Cost
Darby had agreed to construct the bridge with a budget of £3,250 and this was raised by subscribers to the project, mostly from Broseley. While the actual cost of the bridge is unknown, contemporary records suggest it was as high as £6,000, and Darby, who was already indebted from other ventures, agreed to cover the excess. However, by the mid-1790s the bridge was highly profitable, and tolls were giving the shareholders an annual dividend of 8 per cent.Later history
The opening of the bridge resulted in changes in the pattern of settlement in the gorge, and roads around the bridge were improved in the years after its construction. The town of Ironbridge, taking its name from the bridge, developed at the northern end. The trustees, as well as local hotel keepers and coach operators, promoted interest in the bridge among members of high society.Repairs
In July 1783, a wall was built in order to prevent the north bank from slipping into the river. Cracks were found in the stone land arch on the south side in December 1784, and the neighbouring abutment showed signs of movement. The gorge is very prone to landslides, and over 20 are recorded in the British Geological Survey's National Landslide Database in the area. It was suspected that the sides of the gorge were moving towards the river, forcing the feet of the arch towards each other, and consequently repairs were carried out in 1784, 1791 and 1792.It was the only bridge on the River Severn to survive the flood of February 1795 undamaged, due to its strength and small profile against the floodwaters. The medieval bridge at Buildwas was replaced with a cast iron bridge by Thomas Telford, which, by virtue of superior design, required half the quantity of iron despite a longer span of. The Buildwas Bridge survived until 1906. Contrary to a common misconception, Telford was not responsible for the construction of the bridge itself.
In 1800 the trustees commissioned repairs which lasted for several years, which involved the replacement of the stone land arches with wooden ones to relieve pressure on the main span. A proposal to build a rigid support between the abutments to keep them apart was found to be impossible with the available technology, but was achieved during the later restoration of the bridge in the 1970s. In 1812, its construction was described as "very bad" by Charles Hutton, and he predicted that it would not last for long, "though not from any deficiency in the iron-work", but due to cracks that had appeared in the stonework. The timber arches were replaced with cast iron ones in December 1820, and further repairs were necessary throughout the remainder of the 19th century.
Around 1870, “Sir B. Baker stated that it had required patching for ninety years, because the arch and the high side arches would not work together. Expansion and contraction broke the high arch and the connexions between the arches. When it broke they fished it. Then the bolts sheared or the ironwork broke in a new place. He advised that there was nothing unsafe; it was perfectly strong and the stress in vital parts moderate. All that needed to be done was to fish the fractured ribs of the high arches, put oval holes in the fishes, and not screw up the bolts too tight."
On 24 August 1902, a length of parapet collapsed into the river, and a section of deck plate weighing around fell from the bridge in July 1903. The opening of a toll-free concrete bridge in 1909 caused concern among the trustees, but it continued to be used by vehicles and pedestrians.