The span of history
The world's first iron bridge was across the Severn at Coalbrookdale, and represents the cutting edge of 18th century materials science. From small-scale iron components relying on tie bars, dowels and straps, larger building elements had became possible, largely via the skill of the blacksmith. And with the adoption of casting, previously used mainly for the production of cannon, items such as columns and railings could also be made in the same way.
Still, the acceleration of progress would not have happened without Abraham Darby who invented the method of smelting iron with coke. Small-scale iron production, first in a Bloomery and later the blast furnace to be wrought or beaten into shape, had already turned to casting to produce intricate, highly accurate or decorative work. But use of charcoal limited capacity for casting, which was still effectively a cottage industry.
Conversion to coke meant production could be industrialised, and when steam engines were harnessed to blow air into the furnace, the essential parts of what still remains the method of iron production were in place. In the early 18th century no more than 5 per cent of iron was cast, by 1750 this figure was 20 per cent. This, along with geography, dictated the material and form of the pioneering iron bridge.
First effort The germ of all that was to follow in bridges of iron and steel, as de Mare describes it in The Bridges of Britain, was an entirely iron bridge designed by Shrewsbury architect Thomas Farnolls Pritchard. Following on from an earlier design in 1774 for a masonry bridge where the structural scheme was supplemented with iron ribs, 'it contained a deep iron beam, hollowed out in the form of an arch and pierced at the spandrels'.
On the strength of this, when the ferry across the Severn at Coalbrookdale was to be replaced by a bridge, Pritchard was the obvious choice as designer, and, also due to the proximity of the foundries of Wilkinson and Darby, so was the choice of iron as a material.
Ironbridge might have paved the way for the use of iron and steel, but was in de Mare's view still imperfect. 'The designer was still thinking largely in terms of traditional materials? the curious combination of techniques apposite to masonry and timber rather than to iron. The pure arch form and the solid iron blocks, like keystones? suggest masonry, while the solid rectangular sections of the ribs and the plain iron slabs of the roadway suggest timber. Ironbridge possesses that unusual charm which goes with naivety.'
None the less, the seed was planted. Over the following years, the Severn in particular saw several bridges by the engineer Thomas Telford (1757-1834), of which the Mythe bridge is regarded as the best. On the site of his first at Buildwas, built in 1796, only a monument remains.
The single arch Mythe bridge stands just outside Tewkesbury. Its arch is lower than that of the Ironbridge, but spans further.
The spandrels are stiffened not with radial ribs, but with cross members describing a series of diamonds and triangles, increasing in size towards the ends of the span - an elegant but entirely engineering-led solution.
Chain gang At the same time (1819-26) Telford's bridge over the Menai Straits at Anglesey was also being constructed. This started life as a simple arch but, as Robert Stephenson was to find later when commissioned to design a sister rail crossing, the arch would not allow tall ships to pass underneath, the requirement being for a clear 100 feet (30m) below the bridge's structure. Another solution had to be found. Chain bridges had been in existence since at least the 8th century in China, and Telford adopted the principle of suspending the bridge deck on wrought-iron chains, of which there were 16 holding up the 579ft deck. The central section of chain weighed 23.5 tonnes and was manoeuvred into place from a raft by block and tackle.
The bridge, although modified and strengthened since, is much as Telford left it.
The wooden deck was replaced by steel in 1893, and the chains were replaced with steel copies between 1938 and 1940. Telford would probably have approved.
His design for a suspension bridge at Clifton, which would have rounded off his domination of Severn crossings, was usurped in competition by that of Isambard Kingdom Brunel, who produced another suspension structure.
Echoes of Egypt Twenty years after the opening of Telford's Menai bridge, in 1846, Stephenson also adopted a typically innovative design for his Britannia Bridge. In its role of bringing the railway to Holyhead, Stephenson's pedigree was impressive, as the son of locomotive pioneer George. He devised rectangular iron tubes through which the trains would pass totally enclosed, a vast improvement on the original proposal to uncouple the carriage and bring them over the road bridge by horse to a waiting locomotive on the other side.
Stephenson, like Darby before, collaborated with an architect - in this case one Francis Thompson. Thompson's Egyptian style admirably accommodates the holes through which suspension cables were to pass, but after tests it was found that the tubes were capable of supporting rail traffic without extra support, leaving the pylons looking like refugees from Memphis.
Raising the tubes proved harder than expected. The treacherous waters of the Menai caused one section to be swept out to sea (it was only just recovered), and the hydraulic lifting equipment failed. Disaster was averted by Stephenson's precaution that stonework be built up under the ends of the tubes as they were raised. The tube fell only nine inches (220mm) before stopping.
Unfortunately the bridge was so badly damaged by fire in 1970 that it now bears little resemblance to the original, with the exception of Thompson's pylons.
In some ways Brunel's Clifton suspension bridge forms a missing link between Thomas Telford and Brunel's contemporary and rival, Stephenson, in that it contains features recognisable in both the Menai Straits road and rail bridges, namely suspension elements and the use of 'Egyptian' pylons.
The Clifton suspension bridge was eventually completed in 1864, 33 years after its start, and five years after Brunel's death, largely at the behest of 'fellow engineers'. Its span was, at the time, one of the longest for a suspension structure at 705 feet (212m).
Refinements Refinements in calculation and form from the engineering practice of Sir Marc Brunel (1769-1849) and Isambard Kingdom Brunel (1806-59) allowed the performance ofa material (in this case iron) to be predicted empirically, without resorting to over-design or destructive testing. The Brunels in particular were responsible for the development of shaped sections of iron for particular tasks; rails or bulbous 'I' bars for ships (still used, although now steel, and known as bulb flats). These evolved to today's beam and column sections. In Brunel's Saltash rail bridge, the 'Royal Albert' (1852-59) another link can be seen between Stephenson's Menai bridge and much later Benjamin Baker's Forth Bridge - the first all-steel structure - in the use of tubular trusses.
The use of iron, wrought and cast, in bridges allowed engineers to experiment with a 'pure' form of structure, leading to a gradual but complete understanding of the material's properties which, in turn, enabled the use of long spanning structures in buildings.
Most architectural historians identify the Ironbridge as the precursor to a long line of industrial and transport structures ranging from Paxton's Crystal Palace to Barlow and Ordish's 1868 St Pancras station roof.
After the widespread adoption of the converter process invented by Sir Henry Bessemer in 1856, which reduced the carbon content of iron from about 4 per cent to (typically) 0.2 per cent to form steel, the similar but better material began to replace iron as the choice for long-spans and bridges. The techniques developed for forming and fixing were, for the larger part, the same, but it was possible to do more with less. Further developments of the converter process, by such as SG Thomas in 1879 and later the Linz-Donawitz method, allowed greater control of the carbon and alloy content of steel (see Dutch Buildings, MetalWorks Spring 2004) further refining the ability to put strength where it was needed (higher carbon steels), and increasing corrosion resistance (weathering and stainless steels).
All these developments stem from a modest bridge across the Severn in the Midlands.
As Siegfried Giedion comments in Space, Time and Architecture, with breathtaking understatement, the bridge at Coalbrookdale 'opens a path for developments of great importance'. As it says in Judges (Chapter 6, verse 34): 'And the spirit of the Lord came upon Gideon - and he blew a trumpet.'