Bridges

Many of the important innovations in the technology of iron and steel occurred in civil engineering, especially bridge building.

Though not, strictly speaking, a part of the present story, the importance of bridges in iron construction cannot be overlooked. They were, and have remained, the main advertisement of what could be done with iron, and they provided a testing-ground for developments which later found application in other spheres.

The first major iron structure in  Britain was a bridge at Coalbrookdale that was constructed between 1777 and 1781.

The first and most effective symbol for the industry was the  Iron  Bridge  at Coalbrookdale 1777-81. Designed by Thomas Farnolls Pritchard in conjunction with the ironmaster Abraham Darby III, the 100ft. 6in. (31 metre) span arches, plus their bracing and subsidiary ribs, were put together like timber construction. The castings are threaded through each other, or are mortised and fixed by wedges: there are few bolts and no rivets.

Thomas Telford, who was responsible for extensive canal and road building projects, was an early exponent of iron bridges.

As a pioneer work, the Iron bridge at Coalbrookdale has fully deserved all the attention has received, but in the development of cast iron bridges it was less influential than its successors, particularly those designed by Thomas Telford.  Telford's 170ft. (52 metre) span My the Bridge at Tewkesbury  1823-6 is both more logical and elegant.Telford  was an enthusiastic exponent of single arch iron bridges and used them extensively in his canal and roadbuilding projects. His most ambitious projects, the 1800 proposal for a 600 ft. (183 metre) span London Bridge and the 1811 scheme for a 500ft. (152 metre) arched bridge over the Menai Strait on the London to Holyhead road, were not executed but their publication drew attention to the potential of the emerging iron technology. The Menai scheme was superseded by the first version of a suspension bridge design which, after many modifications, was finally built in 1819-26. As seen today, the suspension chains are not original, and the decks have been widened, but essentially the bridge is as he knew it. At Clifton, near Bristol his suspension design for crossing the Avon Gorge was set aside in favour of a scheme put forward by I.K. Brunel. Begun in 1836, that project foundered, to be finally completed in 1864 as an act of piety by Brunel's fellow engineers.

The tubular wrought iron Britannia Railway Bridge over the Menai Straits set an important precedent not only for the widespread use of this material for major structures but also for the design and construction techniques that were employed.

The problem of establishing a railway connection across the Menai Strait, near to Telford's suspension bridge, occasioned the most dramatic advances in the understanding of the behaviour of iron. Cast iron arches were not feasible for navigational reasons, so Robert Stephenson first of all conceived of a series of suspended tubes, of circular or elliptical section, through which trains would run. As built in 1845-50, the masonry towers of the  Brittania Bridge had openings for suspension chains, but the tubes were self-supporting box girders with cellular flanges, 460ft. (140 metres) long on the principal spans. The omission of the chains, whether Stephenson's idea or, more likely, attributable to his collaborators William Fairbairn and Eaton Hodgkinson, symbolised a major advance in the use of wrought iron. It proved not only the superiority of the rectangular tube made up of riveted wrought iron plates but, what in the long run was more important, the necessity of scientific calculation testing as the basis of engineering design.

From around 1850, the lattice truss in wrought iron became the standard configuration for long-span bridge.

The Brittainia Bridge did not produce a large progeny of similar designs. Instead the lessons from it emerged in the more logically thought-out bridge trusses which became the standard types after 1850. Some of the most popular lattice and triangular types started life as combinations of timber and wrought iron, for instance the Howe truss (patented in 1840-1) and the Pratt truss (patented in 1844), but eventually all-iron versions were preferred. The simple equilateral triangles of the Warren Truss, so named in 1848 though in use before, became the most familiar of all work in railway bridges and the spanning of wide architectural spaces.