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Man with a will of iron

Structures Brunel's cast iron bridges

Almost 200 years after his birth, extensive archive research is providing new insights into the early career of Isambard Kingdom Brunel, discovers Robin Partridge.

Of all the famous 19th century engineering pioneers, Brunel is perhaps the most famed, recognised for outstanding achievements such as the Great Western Railway (GWR), his iron steamships and Bristol's Clifton Suspension Bridge. Yet little is known about Brunel as a man, the development of his ideas or about the projects of his early career.

'He was a very complicated man and not very knowable, ' remarks English Heritage inspector of ancient monuments Steven Brindle.

Brindle has just published a new biography, Brunel - the man who built the world (see box). The book has grown out of research on Paddington Station - the GWR's London terminus - which led in spring 2004 to the discovery of an early Brunel cast iron bridge. This had been long forgotten and lay buried within the brick spans of a later road bridge outside the station - Bishop's Bridge.

Brindle's discovery spawned further research into Brunel's work with cast iron: 'At the time, we thought Bishop's Bridge was his first and possibly only cast iron bridge, ' says Brindle, 'but we now know it was his fourth of at least 44 bridges, using a material that Brunel actually distrusted.' In the 1830s and 40s, Brunel's contemporaries were enthusiastically building larger and larger bridges using cast iron girders. But Brunel was wary.

In the 1870 biography of Brunel written by his son he is quoted as saying: 'Cast iron girder bridges are always giving trouble.' He referred to his bridge at Hanwell, west London, where the GWR crosses over the Uxbridge Road: 'Since 1838 [it] has always been under repair.' Hanwell was the first bridge on which Brunel used cast-iron girders. One beam broke during construction in June 1837 and another in March 1839, a year after the railway opened.

Design of the girders had proved difficult. 'Brunel used large I-section beams with equal top and bottom 'anges, some as thick as 4½ inches (115mm), ' explains engineering historian Malcolm Tucker, who is working with Brindle. Results of tests on cast iron had been published in London by Thomas Tredgold in 1822, purporting to show that tensile and compressive strengths were similar. But, Tucker notes, 'these tests used small specimens and did not pick up the scale factor'.

By 1830 more tests had been performed by Eaton Hodgkinson and it was calculated that a cast iron beam was about six times stronger in compression than in tension. In practical terms a ratio of about three to one was widely adopted. Robert Stephenson seems to have incorporated Hodgkinson's research into most of his cast iron bridges.

But, says Tucker, 'Brunel's mentors were in London, so this may be why he remained with equal top and bottom 'anges for Hanwell in 1836'. In his Book of Facts - a sort of office diary - Brunel recorded: 'Only three of the girders were actually tested by weight. The rest of the girders were tested by blows of a sledge hammer and listening to the sound produced by the impact.' Brindle adds, 'Brunel's design was more elaborate than any known cast-iron bridge on Stephenson's lines. The drawings show some interesting geometry and some curious tusk-tenoned joints.' These joints are reminiscent of timber construction, which Brunel loved.

Design was not Brunel's only problem, though. After the second failure in 1839, Brunel wrote to complain to the foundry:

'I have had a piece drilled off and it consists of a mass of half sand and half iron.' The beam was replaced, and Brunel lightened the bridge by removing its brick jack arches and replacing them with timber, Brindle says.

'Ironically, in May 1847, the timber deck caught fire from a locomotive 'rebox and the bridge was irreparably damaged.' Brunel rebuilt the bridge using the newly available wrought iron. His main girders were multi cellular and rectangular in form, anticipating the box girders to be used by Stephenson on the Conway Bridge in 1848 and Britannia Bridge in 1850.

It was with his problems at Hanwell fresh in his mind that Brunel approached the Bishop's Bridge crossing of the Grand Union Canal at Paddington. 'Its highly idiosyncratic design, of both beams and sof't plates, appears to represent Brunel thinking out a design from first principles in the wake of the Hanwell fiasco, ' Brindle says.

Though he did not increase the tension flange size, the beams form a shallow arch, so he may have been relying on arching action to help the tension flange perform, Brindle suggests.

Brindle's and Tucker's research is showing that Brunel was continually varying cast iron bridge design while the GWR was being built. Trough girders were used in the crowns of several small masonry arch bridges to meet clearance requirements.

Individual cast iron troughs for each rail and their longitudinal timber bearers performed as surrogate masonry to carry the compressive arching forces.

Investigation of Brunel's cast iron structures still has some way to go, but there is already talk of further research into his use of wrought iron. Brindle and Tucker will be asking whether Brunel invented the box girder.

Whatever discoveries lie ahead, research so far has reinforced in Brindle's mind that Brunel 'was a creative genius'.

'But as an individual he remains mysterious, ' Brindle adds. 'He was not very trusting, driven to personal possessive control of his work. He seems to have been restless and intellectually curious and at the same time a relentless perfectionist.'

Unreasonable route to success

Having immersed himself in Brunel's notebooks, drawings and correspondence, 'I feel I have actually met him', remarks Steven Brindle, following publication of his biography of Brunel.

The book places Brunel's early struggle with cast iron into the context of his whole career. Heavily influenced by his brilliantly inventive father Marc, Isambard received a sound education in maths and physics before joining his father's business, which familiarised him with cutting edge technology if not great economic success.

With this foundation, Brunel set forth on a solo career that would dazzle the early Victorian era. He had clearly learnt from his father's failures as well as his successes - Marc Brunel spent a spell in debtor's prison. Isambard was generally more focused on financial viability.

His irresistible urge to invent and his belligerent perseverance were tempered with caution and realism.

Appearing to advocate elasto-plastic analysis, albeit with a chunky safety factor, Brunel is quoted in 1854: 'Consider all structures . . . to be made of very elastic Indiarubber, and proportion them so that they will stand and keep their shape: you will by these means diminish greatly the required thickness. Then add 50%.' Brunel showed workaholic tendencies and set exacting standards for himself and those that worked with him.

He wrote to one assistant: 'You are a cursed, lazy, inattentive, apathetic vagabond. You have wasted more of my time than your life is worth.' His obsessive, abrasive personality is likely to have been one of the keys to Brunel's success, Brindle remarks. 'As George Bernard Shaw observed, since the reasonable man adapts himself to his circumstances, all progress must depend on the unreasonable man'.'

NCEI readers can buy copies of Brunel - the man who built the world, for the special price of £20.00 (rrp £25.00) plus £1.50 for postage by calling (+44) 1903 828503 and quoting ref JA23.

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