Dan Cruickshank’s Bridges: Heroic Designs that Changed the World. Dan Cruickshank

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      The Malabadi Bridge, Diyarbakir, Turkey, was constructed around 1146 and its masonry-built and slightly pointed arch has a span of 38.8 metres - at the time one of the longest in the world. Rest rooms for travellers, a caravanserai, - are located within the bridge’s haunches.

      In western culture, immensity of scale – particularly of span – grew increasingly important as the practical demands of bridge construction increased. This culminated with the brick-built Maidenhead Bridge in Berkshire, England, designed by Isambard Kingdom Brunel for the Great Western Railway and completed in 1839. It incorporates a pair of shallow segmental aches, each with a span of 39 metres, making them the longest and flattest brick-built arches ever formed. In 1902 the Rockville Bridge over the Susquehanna River was completed in Harrisburg, Pennsylvania and which – with a length of 1,150 metres, became – and remains – the longest stone-arched bridge in the world. This bridge – offering a vast, open and noble promenade across the river – would surely have delighted Charles Dickens as much as its now long-lost and ‘profoundly dark’ precursor across the river ‘perplexed’ him.

      Metal

      The age of all-metal bridge construction starts, to all intents and purposes, in England with the epoch-making Iron Bridge, in Coalbrookdale, Shropshire. Completed in 1779, it was the boisterous and precocious child of the Industrial Revolution. It was not the first bridge to use iron, nor was its design revolutionary – indeed its basic form is that of a masonry bridge and its cast-iron members are disposed much as in contemporary timber bridges. But it was the world’s first all-iron bridge, it was the product of new technology – steam power and the efficient manufacture of strong cast-iron components – and it would usher in a new world of bridge construction.

      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 СКАЧАТЬ