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

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      A covered bridge constructed using Burr Arch trusses. The system, incorporating a wide-span timber arch to unite and bolster the trusses, was patented in 1817 by Theodore Burr.

      The essential principle underpinning the Burr arch truss design was that the arch should carry the entire load of the bridge while the trusses, of king-post form, should keep the bridge rigid. Good examples of Burr arch truss bridges are found mostly in India and Pennsylvania, such as the Baumgardener’s Covered Bridge of 1860 with a 32-metre span, in Lancaster County, Pennsylvania. But fine early examples survive also in West Virginia – notably those at Philippi of 1852 and at Barrackville, of 1853, both designed by Lemuel Chenoweth. The longest automobile-carrying covered bridge in the US is the splendid 140-metre-long Cornish-Windsor Bridge, of 1866 in Cornish, New Hampshire incorporating a clear span of 62m.

      Trestle viaducts

      Characteristic in parts of North America was the practice of carrying railway tracks on often highly-elevated and prodigiously long timber-built trestle viaducts. Such things had been built in Europe during the early years of the railway age when speed and economy were required – indeed Isambard Kingdom Brunel was quite a master of the art, constructing over forty in Cornwall alone during the 1850s – but there was nothing to compare with the huge scale of the American creations. Trestle viaducts must have been, in the early years in the United States, truly astonishing and unprecedented things to behold – they must have convinced the innocent and those uninitiated into the wonders of the railway age, that smoke-belching and bellowing steam engines and such monstrous creations as the viaducts were all just part of the Devil’s ride from hell. Studying early photographs of trestles – for many of the best are now long gone – it is still possible to capture some of the original drama of their first appearance.

      Unlike traditional buildings, their close-packed structure gives them no obvious grace, and they often look impossibly bizarre or outlandish as they cross ravines or gorges that had lain pristine for eons and impassable to man. They possess instead a sort of crude, utilitarian strength and power that is almost sinister, as if they are bridges built for brute creation. For these reasons, the mightiest trestles are mesmerising to behold, awe-inspiring and tremendous examples of the emotions Edmund Burke explored in the mid eighteenth century in his Philosophical Enquiry into the Origins of our Idea of the Sublime and Beautiful: ‘The emotion caused by the great and the sublime…[the] terrible with regard to sight… is Astonishment…that state of the soul, in which all motions are suspended, with some degree of horror…’.¹⁸ Good examples are the Portage Viaduct, one of the ‘wonders of the age’ when completed in 1852; the vastly long Lucin Cut-off Viaduct across the Great Salt Lake in Utah, built with Douglas fir from 1902 to 1904, disused since the 1950s and now being dismantled; and the long-demolished Bloedel Donovan’s Viaduct of 1920 over the Skykomish River, in King County, Washington.

      Of course, long before the end of the nineteenth century, metal started to replace wood for viaduct construction, which resulted in a great reduction in structural bulk and increase in elegance. The Garabit Viaduct in France of 1884, is a supreme example (see page 264), but also memorable are the 625-metre-long and 92-metre-high Kinzus Viaduct of 1882 in McKean County, Pennsylvania, which was grievously damaged by a tornado in 2003; the Meldon Viaduct, Okehampton, Devon, England, built between 1871 and 1874; and the Lethbridge Viaduct, in Alberta, Canada, of 1909, which is the longest metal-built viaduct in the world.

      Masonry

      Pioneering brick-built bridges with carriageways supported on vaults were constructed in Mesopotamia from at least the sixth century BC – for example, the famed bridge across the Euphrates in Babylon. Stone vaulted and arched bridges, as well as stone lintel bridges are recorded in the Anatolia and Aegean regions from the second century BC. Possibly far older masonry bridges survive in the Peloponnese, Greece, where there are a number of what appear to be Mycenaean bridges dating from around 1,300 to 1,190 BC. A good example is the 22-metre-long Arkadiko or Kazarma Bridge, near Tiryns, that is constructed with cyclopean masonry in typical Mycenaean manner and incorporates a corbel arch. However, the earliest surviving masonry bridges, with verified histories and that represent a coherent body of work, are those built by Rome (see pp 56). A good, typical and early example is the Ponte Fabricio, in Rome of 62 BC and restored AD 19 that crosses the Tiber to an island by means of two semi-circular arches, separated by a narrow higher arch to allow floodwater to pass without putting undue lateral pressure on the bridge. Other examples include the Ponte Milvio in Rome, of 115–109 BC, that carries the Via Flaminia into Rome on five wide semi-circular arches, and the Ponte d’Augusto, of AD 50, in Rimini. A fascinating Roman period bridge is the Romi or Sasani Bridge at Dezful, Khuzestan, Iran. It was constructed in AD 250 by Shapur I, ruler of the Second Persian Empire, who may have used Roman engineers captured when he defeated Emperor Valerian.

      Although Roman structural theory – and aesthetics – preferred the use of arches of semi-circular form, the advantages of segmental arches were recognized (with the possibility of greater span they could bridge a river with fewer piers) and occasionally applied. For example, the now ruined Limyra Bridge, at Lycia in Turkey, that incorporates wide-span segmental arches – some up to 15 metres in span – to create a bridge with a very low height; the Alconétar Bridge across the Tagus, in the Extremadura region in Spain, built perhaps during Emperor Trajan’s reign by Apollodorus of Damascus and now in ruins; and the also ruined Ponte San СКАЧАТЬ