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Brunel's Clifton Suspension Bridge - West of England Branch

[fa icon="calendar"] Mar 11, 2016 10:30:00 AM / by IQ News Update

IQ News Update

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The West of England branch welcomed David Anderson, Bridge Master for Brunel’s Clifton Suspension Bridge over the River Avon in Bristol to give an outline history of both the bridge and the work going on today to maintain it as an important link for traffic across the Avon Gorge.

The idea for a crossing the Avon was first put forward in 1754 and at the time of construction there was only one other crossing in Bristol. Today, around 4 million vehicles each year use the bridge, which raises a toll of around £2 million each year, helping to support maintenance, staffing and education.

In 1829 a competition was launched for a design. Initially bridge builder Thomas Telford, famous for his Menai Straights Bridge, put forward a proposal that was based on two towers built on each bank of the River Avon. He felt it would be impossible to cross the gorge in one span. The draw back with this design was that it would have blocked the access down the riverbanks and in the longer term would have prevented the construction of the railway on one side and the Portway road on the other.

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Brunel only saw the two towers built before he died in 1859 aged 53, as work was stopped due to lack of funds. The construction of these alone was quite a feat requiring on one side a structure built out from the rock to form the base of the tower. The speaker went on to read from an article written at the time describing how there had been a near miss during blasting operations to cut into the rock face showering rock onto the other side of the river. As a result it was decided that blasting would only take place at certain times such that people could take themselves to a place of safety. Prior to this people had to just run for cover when the blasting occurred.

The towers are largely constructed from Pennant sandstone quarried locally. A few courses of Lias Limestone have been built into the lower part of one tower. The top of the towers under the cast coping is from Bath Stone from Box Ground Quarry chosen for it’s durability.

Work resumed on the bridge in 1862 with financial support from the Institute of Civil Engineers (ICE) and was finally completed in 1864, some 5 years after Brunel’s death.

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The wrought iron suspension chains were an early example of recycling, having initially been installed on the Hungerford Suspension Bridge in London, which was demolished to make way for a new railway bridge, before being brought over and used on the Clifton Bridge.

The timber platform of the bridge is carried on the original iron cross members supported from the hangers. It is probably the only bridge of this design with the original ironworks largely in place. This is a credit to foresight of the designers to have an access for a maintenance cradle slung underneath, allowing the mandatory inspections required by the Act of Parliament granting permission for the bridge and also enabled the routine treatment with zinc paint, which has protected the structure. The deck of the bridge is also original having been protected over the years. In modern times the tar and sand treatment has been replaced with asphalt, which is laid by hand when being replaced.

Considerable work has taken place over recent years on the stonework of the towers. Past treatment using cement based pointing had a detrimental effect on the stonework and has been replaced with a more traditional lime sand mortar along with some badly weathered stones. This work has restored them to the bridge to its former glory, being a Grade 1 listed building.

David closed the session informing people of the interesting Visitor Centre at the bridge. After questions the new branch chairman, Peter Mole, gave a vote of thanks and presented a £250 cheque to the speaker towards their funds.

Topics: West of England Branch, Technical Evenings

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