The cast iron bridge stood firm for more than 200 years

The Iron Bridge, the world's first large cast iron bridge, was completed in 1779 and still stands today over the River Severn, Shropshire.

The Iron Bridge over the River Severn. Photo: Bs0u10e0/Flickr

Cast iron is an alloy of iron and carbon, used since ancient times to make pots, pans, cannonballs, and decorative items such as window grilles and fireplace mantels. However, the material had never been used for structural purposes until architect Thomas Farnolls Pritchard proposed building the Iron Bridge of cast iron at the Severn Gorge in Shropshire, England.

The Severn Gorge, later renamed Ironbridge Gorge after the bridge, was rich in coal, iron ore and limestone. Mining industries for these resources also flourished in the area during the late 18th century.

As industry developed, the need for a strong bridge to transport goods across the river arose. Because of the deep gorge and unstable riverbanks, the bridge had to be single-span and high enough to allow ships to pass underneath. The river below was also an important trade route.

The only acceptable material was cast iron, but no one had ever built a cast iron bridge on such a large scale. The Iron Bridge was the first of its kind, although it was not the first bridge made of iron. In 1755, an iron bridge was built at Lyons, but it was abandoned because of cost concerns. In 1769, a 22-metre wrought iron bridge was built over a waterway at Kirklees, Yorkshire.

Architect Thomas Farnolls Pritchard proposed a cast iron bridge, connecting Madley and Benthall, across the River Severn. Pritchard's designs were subsequently approved and construction began in 1777.

Abraham Darby III, an ironworker from Coalbrookdale, was tasked with casting and building the bridge. Pritchard died just a month after construction began, and responsibility for the project fell to Darby.

Darby cast all the parts needed for the bridge—more than 1,700, the heaviest of which weighed five tons—in his workshop. Each part was cast individually to be assembled. He borrowed techniques from carpentry, then adapted them to the properties of cast iron.

Elias Martin's painting Iron Bridge under construction in July 1779. Photo: Amusing Planet

When completed in 1779, the Iron Bridge was over 30 metres long and weighed nearly 400 tonnes. However, there are no authentic documents or eyewitness accounts of exactly how Darby lifted the iron blocks and spanned the river. In 1997, a small watercolor sketch by artist Elias Martin was discovered in a museum in Stockholm.

The painting depicts a wooden, movable scaffolding. The scaffolding consists of piles placed in the riverbed and is used as a crane to move the bridge components into place. These components are brought to the construction site by boat from Darby's workshop 500 meters away. To confirm the reliability of the technical solution in the painting, a half-size replica of the bridge was built in 2001, according to a BBC study.

The success of the Iron Bridge inspired the widespread use of cast iron as a structural material in Europe and America, despite its brittleness and poor tensile strength. During the 19th century, several cast iron bridges suffered serious failures, the most famous being the Tay Bridge disaster in Scotland in 1879, which killed 75 people.

In 1943, the Iron Bridge was closed to traffic to avoid unnecessary stress on the bridge and the risk of collapse. That same year, the structure was listed as a British monument. Over the following decades, the Iron Bridge was reinforced with reinforced concrete struts. Today, the bridge is one of the symbols of the Industrial Revolution.

Thu Thao (According to Amusing Planet )

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