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    Monday, 14 July 2014

    World's 1st cast iron Bridge

    The Iron Bridge is a bridge that 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 be used 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.
    Industry started in the area when Abraham Darby I first smelted local iron ore with coke made Coalbrookdale coal 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 high to allow tall ships to pass underneath.

    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 was conservative in its details, 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 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.

    A 1979 exhibition by the Royal Academy celebrated the bicentenary of 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 on 21 December 1777 in his tower-house at Eyton on Severn, 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.

    Crack and repairs in bridge                    
                                                                  Cracked supports

    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 . Components were cast individually to fit with each other, rather than being of standard sizes, with discrepancies of up to several centimeters between identical components in different locations.

    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 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. 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.

    Between 1972 and 1975, a programme of repairs took place on the foundations of the bridge at a cost of GB£147,000.[19] 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.

    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.

    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.

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