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Bridges and viaducts

Brunel is perhaps best remembered for designs for the Clifton Suspension Bridge in Bristol, begun in 1831. The bridge was built to designs based on Brunel's, but with significant changes. Spanning over 702 ft (214 m), and nominally 249 ft (76 m) above the River Avon, it had the longest span of any bridge in the world at the time of construction. Brunel submitted four designs to a committee headed by Thomas Telford, but Telford rejected all entries, proposing his own design instead. Vociferous opposition from the public forced the organising committee to hold a new competition, which was won by Brunel.

Afterwards, Brunel wrote to his brother-in-law, the politician Benjamin Hawes: "Of all the wonderful feats I have performed, since I have been in this part of the world, I think yesterday I performed the most wonderful. I produced unanimity among 15 men who were all quarrelling about that most ticklish subject—taste".

Work on the Clifton bridge started in 1831, but was suspended due to the Queen Square riots caused by the arrival of Sir Charles Wetherell in Clifton. The riots drove away investors, leaving no money for the project, and construction ceased.

Brunel did not live to see the bridge finished, although his colleagues and admirers at the Institution of Civil Engineers felt it would be a fitting memorial, and started to raise new funds and to amend the design. Work recommenced in 1862, three years after Brunel's death, and was completed in 1864. In 2011, it was suggested, by historian and biographer Adrian Vaughan, that Brunel did not design the bridge, as eventually built, as the later changes to its design were substantial. His views reflected a sentiment stated fifty-two years earlier by Tom Rolt in his 1959 book Brunel. Re-engineering of suspension chains recovered from an earlier suspension bridge was one of many reasons given why Brunel's design could not be followed exactly.

Hungerford Bridge, a suspension footbridge across the Thames near Charing Cross Station in London, was opened in May 1845. Its central span was 676.5 feet (206.2 m), and its cost was £106,000. It was replaced by a new railway bridge in 1859, and the suspension chains were used to complete the Clifton Suspension Bridge.

The Clifton Suspension Bridge still stands, and over 4 million vehicles traverse it every year.

Brunel designed many bridges for his railway projects, including the Royal Albert Bridge spanning the River Tamar at Saltash near Plymouth, Somerset Bridge (an unusual laminated timber-framed bridge near Bridgwater), the Windsor Railway Bridge, and the Maidenhead Railway Bridge over the Thames in Berkshire. This last was the flattest, widest brick arch bridge in the world and is still carrying main line trains to the west, even though today's trains are about ten times heavier than in Brunel's time.

Throughout his railway building career, but particularly on the South Devon and Cornwall Railways where economy was needed and there were many valleys to cross, Brunel made extensive use of wood for the construction of substantial viaducts; these have had to be replaced over the years as their primary material, Kyanised Baltic Pine, became uneconomical to obtain.

Brunel designed the Royal Albert Bridge in 1855 for the Cornwall Railway, after Parliament rejected his original plan for a train ferry across the Hamoaze —the estuary of the tidal Tamar, Tavy and Lynher. The bridge (of bowstring girder or tied arch construction) consists of two main spans of 455 ft (139 m), 100 ft (30 m) above mean high spring tide, plus 17 much shorter approach spans. Opened by Prince Albert on 2 May 1859, it was completed in the year of Brunel's death.

Several of Brunel's bridges over the Great Western Railway might be demolished because the line is to be electrified, and there is inadequate clearance for overhead wires. Buckinghamshire County Council is negotiating to have further options pursued, in order that all nine of the remaining historic bridges on the line can be saved.

When the Cornwall Railway company constructed a railway line between Plymouth and Truro, opening in 1859, and extended it to Falmouth in 1863, on the advice of Brunel, they constructed the river crossings in the form of wooden viaducts, 42 in total, consisting of timber deck spans supported by fans of timber bracing built on masonry piers. This unusual method of construction substantially reduced the first cost of construction compared to an all-masonry structure, but at the cost of more expensive maintenance. In 1934 the last of Brunel's timber viaducts was dismantled and replaced by a masonry structure.

Brunel's last major undertaking was the unique Three Bridges, London. Work began in 1856, and was completed in 1859. The three bridges in question are arranged to allow the routes of the Grand Junction Canal, Great Western and Brentford Railway, and Windmill Lane to cross each other.