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Brunel Cast iron excuse

Heritage Brunel's use of cast iron has faded from engineering history.English Heritage inspector Steven Brindle aims to change all that, he tells Andrew Mylius.

Last year's discovery of a Brunel-designed cast iron bridge in west London created a buzz of excitement. The modest two-span canal crossing at Paddington was nestled within the lumpen brickwork of a later structure and only revealed during a major bridge replacement project (NCE 18 March 2004).

Brunel's notebooks revealed that the banana-shaped beams making up the 1838 canal crossing were his fi rst experiment in the use of cast iron.

The saving of a hitherto forgotten Brunel work came hard on the heels of the engineer's election in a BBC poll as one of the greatest ever Britons - second only to Churchill - and briefly refuelled his fame.

But after the Paddington bridge had been excavated and removed by English Heritage for safe storage and eventual re-erection, attention waned.

Cast iron is a material few associate with Brunel. Wrought iron was Brunel's material of choice. Paddington was seen by most as a curious anomaly.

However, to English Heritage inspector of ancient monuments Steven Brindle, whose research revealed the Paddington bridge's existence, it was just the first of what he hopes will be a series of discoveries.

'I've done a lot more investigating since then with the help of [engineers] Malcolm Tucker and James Sutherland, ' Brindle says. 'Far from Brunel never or rarely using cast iron bridges he actually did at least 30 and probably more like 50.'

Brindle is still in the early stages of trawling through the 1,200 largely unexplored pages of Brunel's Facts - the six volumes of notebooks in which he recorded his technical research.

His findings so far suggest that a significant rewriting of Brunelian history may be needed.

'Facts escribes runel's programme of work in experimental cast iron, ' Brindle says. They record that in the 1830s and 40s Brunel conducted a series of destructive tests to determine the deadload strength of cast iron beams. He also set about finding the neutral axis of beams he designed by chasing six to 10 longitudinal grooves into them and fitting these with wrought iron bars.

Tension and compression in the beam resulted in various degrees of elongation or buckling in the bars, depending on whether they were below the neutral axis and in tension, or above it and in compression.

These tests paved the way for construction of a number of bridges on the Great Western railway line between London and Bristol, five of which are recorded. As well as Paddington, they are a railway overbridge in Sidney Gardens, Bath, at Windmill Lane, Southall, and at Ealing in West London. A bridge carrying the railway over London's Uxbridge Road had to be demolished following failures during construction and finally cracking in use.

Facts indicates he built numerous other railway bridges elsewhere in the country as well, says Brindle.

Brunel's engineering instinct was to distrust cast iron: he was worried by its performance in compression as well as in tension, and by the difficulty involved in achieving consistent quality. But he was spurred into using it by his contemporaries, who adopted it enthusiastically, notably his friend and great rival Robert Stevenson, Brindle says.

'Brunel was using out of date beam designs with equal top and bottom flanges where his contemporaries were using bigger bottom flanges to resist tension.' Paddington demonstrated Brunel's resolution to draw his own conclusions, even when they went against received wisdom, with its large bulb-shaped top flanges designed to resist what, presumably, he considered the greater danger of compression, Brindle observes.

'It was a unique design.' Cross referencing his findings in Facts with railway records and local searches, Brindle expects to have identified other possible cast iron bridge survivors by the end of this year.

'Nobody's ever studied the evolution of his work in timber or cast iron - nor properly in wrought iron, come to that.' Brindle is looking forward to confounding what he describes as 'the classic account' of Brunel's evolution as an engineer.

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