The Iron Bridge
The Iron Bridge
|Locale||Ironbridge Gorge near Coalbrookdale|
|Design||cast iron arch bridge|
|Longest span||30.6 metres (100 ft)|
|Clearance below||16.75 metres (55.0 ft)|
|Construction begin||November 1777|
|Construction end||January 1781|
|Opened||1 January 1781|
The Iron Bridge crosses the River Severn in Shropshire, England. It was the first arch bridge in the world to be made of cast iron, a material which was previously too expensive to use for large structures. However, a new blast furnace nearby lowered the cost and encouraged local engineers and architects to solve a long-standing problem of a crossing over the river.
In 1934 it was designated a Scheduled Ancient Monument and closed to vehicular traffic. Tolls for pedestrians were collected until 1950, when ownership of the bridge was transferred to Shropshire County Council. It now belongs to Telford and Wrekin Borough Council. The bridge, the adjacent settlement of Ironbridge and the Ironbridge Gorge form the UNESCO Ironbridge Gorge World Heritage Site. The bridge is a Grade I listed building, and a waypoint on the South Telford Heritage Trail.
Abraham Darby I first smelted local iron ore with Coalbrookdale in 1709, but the expansion of industry was limited without a bridge over the Ironbridge Gorge, the nearest being at Buildwas 3 kilometres (1.9 mi) away. The use of the river by barge traffic and the steep sides of the gorge meant that any bridge should ideally be of a single span, and sufficiently tall to allow ships to pass.
The Darby family had made the valley famous for its iron, and a lack of timber made iron a logical choice for bridge construction. However, the design suggests caution, with iron used in the same fashion as timber and assembly details in common with contemporary wooden bridges.
In 1773, Thomas Farnolls Pritchard wrote to a local ironmaster, John Wilkinson of Broseley, to suggest building a bridge out of cast iron. In March 1776, the Act to build a bridge remedying the situation received Royal Assent, and the grandson of Abraham Darby I, Abraham Darby III, an ironmaster working at Coalbrookdale in the gorge, was commissioned to cast and build the bridge. The site, where a ferry had run between Madely and Benthall, was chosen for its high approaches on each side and the solidity of the ground. Pritchard died in December 1777, only a month after work had begun.
Decorative rings and ogees between the structural ribs of the bridge suggest that the final design was of Pritchard, as the same elements appear in a gazebo he rebuilt. The final design was Pritchard's third attempt, after design for bridges of 36.5 metres (120 ft) and 27.4 metres (90 ft) were rejected. A foreman at the foundry, Thomas Gregory, drew the detailed designs for the members, resulting in the use of carpentry jointing details.
The bridge was closed to vehicular traffic in 1931, and was designated a Scheduled Ancient Monument in 1934. Tolls for pedestrians were collected until 1950, when ownership of the bridge was transferred to Shropshire County Council.
The masonry and abutments were constructed between 1777-8, and the ribs were lifted into place in the summer of 1779. The nascent bridge first spanned the river on 2 July 1779, and it was opened to traffic on 1 January 1781. It was the only bridge on the River Severn to survive the flood of 1795, due to its strength and small profile against the floodwaters.
The bridge is built from five cast iron ribs that give a span of 30.6 metres (100 ft). Exactly 378 tons 10 cwt (847,800 lb or 384.6 t) of iron was used in the construction of the bridge, and there are almost 1700 individual components, the heaviest weighing 5.5 long tons (5.6 t). Components were cast individually to fit with each other, rather than being of standard sizes.
In December 1784, less than four years after the completion of the bridge, cracks were found in the south side of the arch, and the neighbouring abutment showed signs of movement. The Gorge is very prone to landslides, and over 20 have been 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. In 1800, the stone-faced embankment behind the south abutment was replaced with two small timber arches to relieve pressure on the main span. The timber arches were replaced with cast iron ones in 1821, and in May 1862, the bridge was the subject of further repairs.
However, many of the cracks visible in the bridge today have been left untouched. The bridge was over-designed and subsequent bridges, such as those built by Thomas Telford, used much less cast iron. For example, his cast iron arch bridge at Buildwas, upstream from Ironbridge, used less than half the weight for a greater span (130-foot span, 170 tons of cast iron). However, it suffered similar problems of abutment movement and was replaced in 1902.
The cast iron bridge at Coalport downstream, built in 1818, is much more impressive because of its lean, streamlined design, and the higher quality of the cast iron arches. Thus it still carries vehicular traffic, albeit as a single carriageway. It has about half the weight of cast iron as the original Ironbridge, and is longer than the earlier iron structure. It was renovated in 2004 including replacement of the cast iron pavement by lighter equivalents.
More information about how the bridge was built came from the discovery in 1997 of a small watercolour by Elias Martin in a Stockholm museum. This showed the bridge under construction in 1779.
Between 1972 and 1975, a programme of repairs took place on the foundations of the bridge at a cost of GB£147,000. It involved creating a ferro-concrete inverted arch under the river to counter inward movement of the bridge abutments.
In 1999–2000, the bridge was renovated again, with replacement of the cast iron road plates with steel plates, and a lightweight top surface.
These renovations, together with recent research, revealed more about the building process and the manufacture of the cast iron parts. While the smaller parts were cast using wooden patterns, the large ribs were cast into excavated moulds in the casting sand. It is now known that 70 per cent of the components were made individually to fit, and as a result each is slightly different from the others. English Heritage, together with the Ironbridge Gorge Museums Trust, carried out a full archaeological survey, record and analysis of the bridge in 1999–2000. A half-size replica of the main section of the bridge was built in 2001 as part of the research for the BBC Timewatch programme which was shown in 2002.
Pedestrians crossing the Iron Bridge with Ironbridge in the background.
|Wikimedia Commons has media related to The Iron Bridge.|
- "Iron Bridge". European Route of Industrial Heritage. Retrieved 13 April 2014.
- Smith 1979, p. 4
- "History and Research: Iron Bridge". English Heritage. Retrieved 4 April 2014.
- "John Wilkinson and the Iron Bridge". Broseley Local History Society. Retrieved 4 April 2014.
- Petroski 1996, p. 160
- "Thomas Farnolls Pritchard". Retrieved 4 April 2014.
- "Why build an Iron Bridge in Coalbrookdale?". Ironbridge Gorge Museum Trust. Retrieved 4 April 2014.
- "Gazebo in garden of number 5 (not included)". English Heritage. Retrieved 13 April 2014.
- "Iron Bridge". engineering-timelines.com. Retrieved 13 April 2014.
- Briggs 1979, p. 8
- Briggs 1979, p. 9
- "The Iron Bridge - How was it Built?". BBC. Retrieved 14 April 2014.
- "Iron Bridge". English Heritage. Retrieved 4 April 2014.
- Petroski 1996, p. 161
- Briggs 1979, p. 7
- "Secrets of the past: How Ironbridge was built". sean.co.uk. Retrieved 13 April 2014.
- "Landslides in the Ironbridge Gorge, Shropshire". British Geological Survey. Retrieved 14 April 2014.
- "Ironbridge Gorge gets £12m government grant". BBC News. 4 October 2012. Retrieved 13 April 2014.
- Briggs, Asa (1979). Iron Bridge to Crystal Palace: Impact and Images of the Industrial Revolution. Thames and Hudson in collaboration with the Ironbridge Gorge Museum Trust. ISBN 9780500012222.
- Petroski, Henry (1996). Invention by Design: How Engineers Get from Thought to Thing. Harvard University Press. ISBN 9780674463684.
- Smith, Stuart (1979). A View from the Iron Bridge. Ironbridge Gorge Museum Trust. ISBN 0-903971-09-7.
- Iron Bridge & Tollhouse – Ironbridge Gorge Museums Trust
- Virtual tour, from the BBC (VRML plugin required, then use PgUp/PgDn to move between viewpoints)
- River Severn Bridges
- Iron Bridge at Structurae