Many people driving under Ironbridge, where the Great Western Railway crosses Uxbridge Road in West London, will be unaware that it consists of three bridges side by side. And since two of them are composed of two spans of a different character, there are actually five bridges there. However Ironbridge is not to be confused with Brunel’s famous Three Bridges (which are actually only two), only a few hundred yards away where a road crosses a canal, which itself crosses over a railway line at the same point.
The illusion (that there is just one bridge there) at Ironbridge can be understood from the attached pictures, courtesy of Google Maps, which show views from either side of the ‘bridge’.
But, I hear you say, if there are five bridges there, where are the other two? To answer that, we must go back 189 years to 1835 when Brunel was planning his Great Western Railway. Immediately after the Wharncliffe Viaduct, that magnificent series of brick arches crossing the valley of the River Brent just after Hanwell Station, Brunel had to bridge what was then the Uxbridge Turnpike. And being Brunel and, in a perversity that he was to repeat more than twenty years later with his Three Bridges, he elected to design a structure to cross the turnpike at the exact point where it was crossed by Windmill Lane (and Windmill Lane is the road at the Three Bridges).
The difficulty of building a bridge across a crossroads was compounded by the fact that the railway crossing was highly skewed; the angle between the main (Uxbridge) road and the line of rails was only 23°—equivalent to a ‘skew angle’ of 67°—meaning that from simple geometry, the length of a single span would have to have been more than twice the combined width of the road and walkways on either side. The Act of Parliament authorising the building of the GWR required a forty foot width of road with two walkways each of ten feet either side. To cross a 60 foot gap at a skew angle of 67° requires a bridge span of more than 150 feet. Because of the highly skewed angle, an arch bridge either in brick, stone, or cast iron would have been impossible. A girder bridge was the only viable option, but in 1835 the only girders available were of cast iron, and they were limited in length to around forty feet. Ever resourceful, Brunel first convinced the Commissioners of Roads to reduce the road width to thirty feet, the walkways to five feet each, and also to allow him to ‘straighten’ the Uxbridge Turnpike so that the skew angle was reduced to 60°. In addition he was permitted to reduce the height of the road somewhat. He then devised an ingenious structure using a number of intermediate columns to support a grid of cast iron girders, none of which was more than 34 feet in length. Two lines of eight columns each were erected on either side of the Uxbridge Turnpike between it and the walkways, and girders were laid longitudinally along their tops, supporting transverse girders over the roads. The gaps were filled in with brick arches. But Isambard was not content just to produce an elegant solution to an awkward engineering problem, he wanted to do so with a flourish and make a statement. It may be that the prospect of his proposed flamboyant design helped to persuade the Road Commissioners to agree to his reduction in the road width and the other concessions, since the bridge would undoubtedly be a notable and interesting landmark on the road. He designed it such that the approaches, when viewed from either direction along the Uxbridge Road, appeared to be symmetric. On the right hand side was the end of the embankment with its brick abutment; on the left was a brick ‘folly’ closely mirroring the masonry on the opposite side. There were also some leading overhead girders between the two, quite superfluous to the bridge structure, which created a sort of ‘proscenium arch’ portico, framing the lines of pillars and girders in a most dramatic way. The effect is strikingly apparent in J C Bourne’s beautiful lithograph of the bridge dating from shortly after its completion in 1839. The skew bridge at Hanwell was quite unique, and demonstrated Brunel’s determination to find original solutions to difficult engineering problems.
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AuthorWelcome to the Mirli Books blog written by Peter Maggs Archives
March 2024
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