The bridge was designed by Thomas Farnolls Pritchard with ironmaster Abraham Derby III and is, in many wonderful ways, a bridge between worlds. It marks the coming of the new industrial age of mass-production, yet it still possesses a Georgian elegance and regard for ornament. It is made of iron, yet its iron components are mortised, wedged and screwed together as if made of timber. It is a work very much of transition: Derby knew that it was essential for this new material to be used in compression – which is cast-iron’s strength – not in tension, which is its weakness (see page 182). So the bridge is designed to ensure that, as far as possible, loads are transmitted vertically down and individual components are in compression.
The possibilities offered by this newly available construction material were soon appreciated by avant-garde architects and engineers such as Thomas Telford, who in 1795 specified cast-iron to form the troughs of the vast Pontcysyllte canal aqueduct across the Dee Valley outside Llangollen, North Wales. The troughs, made with cast-iron slabs dovetailed together – like keystones in an arch of lintel – for greater compressive strength, sits on 35-metre-high, stone-built piers, each 16 metres apart. These are of slender, tapering form and of ingenious honeycomb construction (with stone laid in mortar containing ox blood as a hardener) to reduce weight and materials used. Within the cast-iron troughs water rises almost to their brims, and a railing and towpath are placed only on one side so that barges float high above the valley, enjoying unrestricted views, as if navigating through the clouds. The aqueduct was opened in November 1805 and is one of the most beautiful, robust and continuingly useful creations of Britain’s early Industrial Revolution.
Key characteristics of cast-iron were that components made in the material were not only strong – which meant they could be slender in section and still possess greater strength than equivalent timber components – but also quick and cheap to produce. These characteristics ensured that the model offered by the Iron Bridge at Coalbrookdale was soon emulated. They also made it possible to develop a new commerce in export architecture, transporting British-forged buildings and bridges – reduced to flat-packs for ease of travel – to all corners of the growing empire.
One of the first and most significant developments of the Iron Bridge was the stupendous Sunderland Bridge across the River Wear in County Durham. It was started in 1793, completed in 1796, rose to a height of nearly 30 metres above the surface of the water (which had involved the design and erection of a superbly designed system of timber scaffolding), and had a span of 72 metres. Its promoters and designers were Rowland Burdon and Thomas Wilson. The huge arched bridge, made in an elegant manner out of pioneering material, was regarded as one of the technological glories of the age. But, sadly, glory proved only transitory, and due to gradual deterioration all was demolished in 1929. But the great bridge left more than a trace behind. Such was its fame, that copies were eagerly desired around the world and – thanks to the character of cast-iron that favoured pre-fabrication – this desire could be fairly easily fulfilled with components cast to the required scale and then shipped abroad for assembly on site. And so, in 1800, crates of cast-iron arrived in Spanish Town, Jamaica which by 1810 – after the construction of substantial stone abutments – had been bolted together to form a reduced-scale version of the Sunderland Bridge. Known as the Rio Cobre Bridge, it is now the oldest iron bridge in the western hemisphere.
If the Rio Cobre Bridge is one of the most curious of iron bridges, then Isambard Kingdom Brunel’s bridge masterpiece – the Royal Albert Bridge, Saltash, Cornwall – is one of the most stunning. The last great work of Brunel’s career was completed in 1859, and all ornament and all reference to history has been dispensed with: it is a masterpiece in pure, raw, and beautiful function.
Vast trusses of ‘lenticular’ form span 139 metres between piers, the centre founded in a small rocky island, and rise 24 metres above high-water mark. The lenticular trusses incorporate massive tubular arches made from wrought-iron that rise above the track, with their ends tied with chains forming inverted arches. The object of this design was to prevent the trusses transmitting any outward horizontal thrust. It is fascinating to compare the construction of the Saltash Bridge with the near contemporary works of Robert Stephenson, such as the High Level Bridge in Newcastle-upon-Tyne, with which Brunel was very familiar (see page 213). The trusses on Stephenson’s slightly earlier High Level Bridge in Newcastle-upon-Tyne (see page 190), perform in much the same manner as Brunel’s ‘lenticular’ trusses and must, to a degree, have been an inspiration.
One of the particular problems Brunel encountered at Saltash was the digging of the foundations for the piers that rise from the riverbed. The solution he chose – ‘pneumatic caissons’ – was a very modern one that had not been tried and tested. The system had been used first by John Wright in 1851 for setting the foundations for piers for the bridge across the River Medway at Rochester. Pneumatic caissons were an intriguing and seemingly very sensible idea that utilized the potential of modern technology. Airtight caissons – essentially vast drums open at top and bottom – are wrought, towed into position, sunk so that their bottom edges sink into, and are sealed by, soft mud or sand. Then the tops, standing well above high water mark, are sealed, and compressed air pumped in to keep water and mud from entering. To retain air pressure inside the caisson, workmen were to enter and exit the sealed chamber by means of an airlock.
A lenticular truss of the type used by Isambard Kingdom Brunel for his Royal Albert Bridge, Saltash, England, completed in 1859. The large wrought-iron tube forming the top arch of the truss is tied, or restrained, by the lower, inverted, arch made of a chain fabricated of wrought-iron bars. This design means that the arched truss exerts no outward thrust.
There was even a system devised for removing material – ‘muck’ as it was called – to the surface by means of a pit, filled with water to act as an airlock, via a ‘muck tube’ that was cleared by a crane fitted with a clamshell bucket. The water-filled ‘muck tube’ also helped to regulate pressure in the working chamber. Excess air pressure could escape through it and the tube could be used to prevent water pressure dropping – which was essential if water and mud were to be prevented from flowing into the caisson.
It was all very ingenious – the only problem was that the system killed or crippled people. The engineers didn’t know anything about what happens to the human body when it toils within a compressed atmosphere, and they had no concept that bubbles of gas can build up in the blood, and expand when a normal – decompressed – atmosphere is re-entered too quickly, causing not only great pain but serious, possibly terminal, physical damage. This distressing condition is also suffered by divers who ascend rapidly after spending too long a time at a great depth and has become known as ‘the bends’.
Although this mysterious ailment was quickly associated with pneumatic caissons, indeed it became known as ‘caisson disease’, ambitious engineers did not abandon the use of this useful time-saving invention and in 1872 it even caused the disablement of the engineer Washington Roebling when supervising the construction of New York’s Brooklyn Bridge (see page 287). When James Eads started construction of his epic railway bridge across the Mississippi at St Louis in 1867, he was under intense time pressure with loans only becoming available when specific construction targets had been achieved. So, naturally pneumatic caissons were the preferred option which, with men working up to 28 metres below water level, resulted in the deaths of fifteen men, the crippling of two, and serious injury to a further seventy-seven.
Due in part to these woeful sacrifices, Eads completed construction on time, and as it happens created one of the most significant bridges ever built. It crossed the river in three mighty arches, the longest with a span of 158 metres, and all constructed out of steel – which was the first time this material had been