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The Iron Bridge at Ironbridge

Sunday, 05 January, 2014 - Modified on Sunday, 05 January, 2014 at 9:30 pm

 

The Iron Bridge crosses the River Severn at the heart of the Ironbridge Gorge in Shropshire. 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.
 
Construction
In the early eighteenth century, the only way to cross the Severn Gorge was by ferry. However, the industries that were growing in the area of Coalbrookdale and Broseley needed a more reliable crossing.
 
In 1773, Thomas Farnolls Pritchard wrote to a local ironmaster, John Wilkinson of Broseley, to suggest building a bridge out of cast iron. By 1775, Pritchard had finalised the plans, but he died in December 1777, only a month after work had begun. 
 
Abraham Darby III, who was the grandson of the first foundry owner and an ironmaster working at Coalbrookdale in the gorge, was commissioned to cast and build the bridge. The iron for the new bridge was cast at his foundry.
 
Shares were issued to raise the £3,200 required, and Darby agreed to fund any excess. Although it had been predicted that 300 tons of iron would be needed (costing £7 a ton), in the end 379 tons were used, costing Darby and his company nearly £3,000. There would be many other costs to bear (masonry abutments, assembly, etc.), so that the project was far more expensive than first envisaged. Darby bore most of the cost overrun, and was in debt for the rest of his life.
 
Being the first of its sort, the construction had no precedent; the method chosen to create the structure was therefore based on carpentry. Each member of the frame was cast separately, and fastenings followed those used in woodworking, such as the mortise and tenon and blind dovetail joints, adapted as necessary to the different properties of cast iron. Bolts were used to fasten the half-ribs together at the crown of the arch. Very large parts were needed to create a structure to span 100 feet rising to 60 feet above the river. The largest parts were the half-ribs, each about 70 ft long and weighing 5.25 tons. The bridge comprises more than 800 castings of 12 basic types.
 
The bridge was raised in the summer of 1779, and it was opened on New Year's Day 1781.
 
Repairs
Just a few years after the construction of the bridge, cracks appeared in the masonry abutments, partly caused by ground movement. Some of the present-day cracks in the cast iron may date from this time, although others are probably casting cracks from defects such as blow holes. Some cracks were pinned with wrought iron straps, but others have been left free. By 1802, the southern stone abutment had to be demolished and replaced with temporary wooden arches before eventually being replaced by iron arches. 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. 
 
In 1972, a programme of major repairs took place on the foundations of the bridge. It involved creating a ferro-concrete inverted arch under the river. Inward movement of the bridge abutments had compressed the bridge and caused the centre of the arch to rise by a few feet. This counter-arch resists this compressive force from the 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. 
 
Please note:- All information relating to this article has been taken directly from “Wikipedia, the free encyclopedia”
 
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