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

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СКАЧАТЬ And when timber is used the beam-type design, incorporating piers, is the obvious choice if reasonably short unsupported spans are acceptable. Two early timber bridges survive and are available for scrutiny – not in physical reality but in informative contemporary descriptions. During the Gallic War, in 55 to 53 BC, Julius Caesar built two military bridges over the River Rhine. The purpose of the bridges was both strategic and political. They were to demonstrate to aggressive tribes east of the Rhine that Roman power was ubiquitous, that the legions could roam at will – even the mighty Rhine was no obstacle – and that they didn’t need to scrabble around in boats, but could move with majesty and dignity over the water.¹²

      The first bridge was located between Andernach and Neuwied, and took 40,000 legionaries ten days to build. It was 140–400 metres long and 7–9 metres wide, across 9 metres of water. Caesar’s description of the bridge’s construction is obscure and has led to disputes about its exact design. But it is clear that the carriageway rested on piles, each formed by a pair of pointed logs 0.46 metres (1.5 feet) thick and fastened together 0.61 metres (2 feet) apart. The two-pronged piles were driven into the riverbed using pile-drivers, ‘not vertically…but obliquely’, set alternately so as to be ‘inclined in the direction of the current’ and ‘in the opposite direction to the current’. Such was the ingenuity of the design that ‘the pairs of piles…each…individually strengthened by a diagonal tie between the two piles’ formed a ‘structure…so rigid that in accordance with the laws of physics, the greater the force of the current, the more tightly were the piles held in position’.¹³ This description suggests that trusses were created to stiffen the bridge. After the bridge was used for punitive raids and to demonstrate the power of Rome, it was completely destroyed so that it could not be used by attacking forces. Clearly Caesar was not convinced that his threats had entirely worked on the warlike tribes of Germania.

      The second bridge was constructed in a similar manner and for similar reasons at Umitz and then partly dismantled. At about the same time, Vitruvius was writing his Ten Books of Architecture. Although his text touches on most aspects of building he says virtually nothing about bridges. That such an important civic and military work should be omitted is very strange, and suggests that Vitruvius’s work was either incomplete or that parts are now missing. In contrast, Leonardo da Vinci tried his hand at the design of timber bridges, perhaps inspired by Roman precedent, with one surviving sketch of 1490 showing a complex trussed construction incorporating a two-tier passageway.¹⁴

      In 1513 Fra Giovanni Giocondo published a scholarly rendering of Caesar’s bridge in his edition of Caesar’s Commentaries. Andrea Palladio, always keen to try his hand at the analysis and reconstruction of ancient buildings, included in his Quattro Libri of 1570 a very accurate interpretation of Caesar’s account (Chapter IV, Book Three).

      Palladio also, in his Quattro Libri, included an illustration (Plate VI, Book Three) of a bridge just completed to his own designs – at Bassano del Grappa in north Italy – that must in part have been influenced by Caesar’s description of the Rhine bridges. Palladio’s bridge is a most curious affair that crosses the River Brenta by means of timber beams supported on timber piles formed by logs set obliquely – somewhat as described by Caesar – to counter the current of the river. The bridge, roofed to protect both its passengers and its timbers from the weather, survived until 1748. It was then rebuilt but again destroyed – this time during the Second World War – and has since been faithfully rebuilt to Palladio’s original 1569 design.

      A perhaps more remarkable design in the Quattro Libri – remarkable, that is, because of its pure utility – shows a series of variations for the construction of a bridge over the 35-metre-wide Cismone River (Plates III to V, Book Three). All the variations show trussed or triangulated timber structures – some slightly arched, others with flat carriageways – with individual timber members joined with wrought-iron straps and pins.¹⁵

      Timber-built bridges became something of a speciality in those regions where wide rivers or gorges abounded and where timber, rather than stone or brick-clay, was in ready supply as a building material. Timber was the dominant construction material in much of the Himalaya region – for example, the roofed cantilever bridges of Bhutan and Tibet and in Japan, where a sensational and very beautiful example is the Kintaikyo Bridge at Iwakuni. Here five steeply rising timber-built arches – with timbers wedged and dove-tailed together – leap from stone piers, like a great serpent. The bridge was built in 1673 but the timber arches have been regularly rebuilt (originally without nails) in the traditional manner.

      The Kintaikyo Bridge, Iwakuni, Japan, was first built in 1673. The centre three timber spans – each a 35 metre width – has been rebuilt every 20 years and narrower outer spans every 40 years – initially without nails. All fully rebuilt in the early 1950s after wartime neglect.

      The Blenheim Covered Bridge, New York State, USA. Built between 1855 and 1857, it incorporates a clear span of 64 metres.

      Eighteenth-century Switzerland was also a place in which timber bridges became something of a speciality. Here the brothers Johannes and Ulrich Grubenmann, both skilled carpenters, constructed a series of timber bridges of pioneering form and large scale. Most have been long destroyed, such as the one of 1757 at Reichenau that had a span of 67 metres, but their Rümlang Bridge at Oberglatt survives. Built in 1766, it spans 27.5 metres by means of struts, trusses and arches. To help protect the structural timbers from the weather, the carriageway is roofed.

      Carpentry was also the solution when a bridge was required in many regions of North America during the first half of the nineteenth century and before the ready availability of cheap iron. Some of these American structures were of huge size and, through engineering ingenuity, achieved surprisingly long spans. For example, the bridge of 1812 across the Schuylkill River, just outside Philadelphia, had a clear span of 103.56 metres and so earned itself the name Colossus. The designer, Louis Wernwag, was in part able to achieve such a wide span by incorporating iron rods with the timber beams. In 1838 the bridge suffered the fate feared by all builders of wooden bridges: it caught fire and was utterly destroyed. The other thing greatly feared by the builders of timber bridges – especially those constructed with softwood – was rapid decay. Softwood can only survive the elements if regularly painted, a hard or very expensive thing to do with bridges, so American bridge builders tended to take up the other option – a shingle-clad roof over, and wooden walls around, the structural timbers and trusses.

      The Colossus wasn’t roofed, but a timber bridge built across the Schuylkill River a few years before had been. The Permanent Bridge in Philadelphia, designed by Timothy Palmer and opened in 1805, is regarded as the first covered bridge built in North America.¹⁶ Covered bridges built of softwood, if properly detailed, with roof cladding maintained, and if fortunate enough to escape fire, can protect themselves from the weather and prove incredibly long lasting. The Permanent Bridge lasted until 1850. The oldest covered timber bridge surviving in the USA today is located at Hyde Hall, East Springfield, New York State, and dates from 1825.

      As early as the 1840s, large-scale, covered bridges had become internationally recognised as something of a North American peculiarity. Charles Dickens, when travelling through the land in 1842, went to explore one crossing the Susquehanna River in Pennsylvania, and described it as ‘nearly a mile in length…profoundly dark…interminable…with great beams crossing and recrossing it at every possible angle’. He admitted to being ‘perplexed’ and, due to the gloom and the echo of the ‘hollow noises’, like being ‘in a painful dream’.СКАЧАТЬ