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'Disconcertingly original'

Special feature - Brunel Bicentenary

English Heritage's Steven Brindle believes that of all Brunel's lost structures, his bridge across the River Wye at Chepstow was the most important.

Brunel's career was dened by a refusal to accept proven solutions, says English Heritage inspector of ancient monuments Steven Brindle. 'Because Brunel was Brunel, he was never going to copy a design by [his friend and rival Robert] Stevenson, or even copy one of his own designs.

Each time he was faced with a problem he came up with something completely new.'

In his biography of the industrial demi-god, Brunel, the man who built the world, published last autumn, Brindle examined the evolution of Brunel's career and the way ideas cross-fertilised between the engineering disciplines he straddled. His research has shed light on hitherto forgotten or overlooked aspects of Brunel's work, such as his experimentation with cast iron (NCE 12 January).

Brindle says that selecting a single most important structure from the hundreds that have been lost and forgotten is a tough challenge. But bridges exemplify Brunel's constant invention. And despite being 'one of the most consistently innovative builders in timber there ever was', it is Brunel's iron bridges that stand out.

Chepstow Bridge, carrying the South Wales Railway across the River Wye, was 'disconcertingly original'.

The 182m bridge was divided into two distinct halves. Three unremarkable plate girder spans, each of 30m, cleared the river's foreshore. To leap the next 91m, however, Brunel developed an entirely new structural system, on rst impressions a truss of immense depth, but described by Brindle as an enclosed suspension bridge.

'Chepstow was the direct ancestor of the Royal Albert Bridge at Saltash, ' Brindle claims. 'At either end of the main span you had a tower. These supported a pair of huge, tubular wrought iron beams.

'The deck was carried on straight suspension chains hung from the tubular beams. It looked like an extremely large girder with diagonal stiffeners, but actually it was acting as closed suspension system. The top tubes took some load from the suspension chains, but their primary role was to brace the towers apart.'

In 1849, when Brunel started designing Chepstow, suspension bridge design and construction was becoming well understood: Brunel knew that a suspension bridge could have spanned the Wye, but that it would have been dangerously dynamic under train loading. The only precedent for carrying a railway across such a large span was Stevenson's 122m tubular girder at Conwy.

Brunel had watched the structure being oated into position and admired Stevenson's solution, Brindle notes.

Always ondent that a better solution could be found, Brunel evolved an entirely new structural concept. Chepstow's twin trusses were fabricated on the river foreshore and oated into position on barges. They were then jacked 31m up into position using a hydraulic system developed by Sir William Fairburn and Stevenson for erection of the Conwy span. Chepstow cost £77,000. By contrast, Stevenson's Conwy Bridge cost three times that sum.

Royal Albert Bridge at Saltash rened the structural concept developed at Chepstow and used the same construction methods. It came in at half the price of Stevenson's Britannia Bridge. Chepstow and Saltash showed Brunel it was possible to build long span rail bridges for less than half the cost of those designed and built by any contemporary engineer, Brindle says. 'The idea that Brunel was an extravagant engineer doesn't stand up. These were very economical bridges.'

Brunel was inuenced by Fairbairn's work on tubular wrought iron beams in the run-up to construction of the Conwy Bridge. When he started designing in wrought iron himself, in 1847, 'it is obvious who Brunel was getting his ideas from', Brindle notes.

'But Brunel was consistently creative and didn't necessarily use an idea again because it worked well once. He constantly re-invented.'

When in 1849 he came to design the Chepstow crossing, Brunel drew on developments in ship building and boiler making, Brindle says. 'The top tubes used plated construction. Their scale mirrors the contemporary upping of scale in the fabrication of iron ship hulls and in boiler making.'

Greater size was accompanied by greater strength and improved seal between individual wrought iron plates, Brindle adds. 'The late 1840s were critical years.

Hull, boiler and bridge building were all marching forward together. Brunel harnessed that to create a huge single span compression tube. That's why Chepstow was so remarkable. It was a completely new idea.'

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