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’.17 The bridge was indeed nearly a mile long, its carriageway supported on piers, and it was burned and destroyed during the Civil War.
The timber- built truss structure of a mid-nineteenth century North American covered bridge. Its soft wood structural timbers are protected from the weather by timber boarding.
Blenheim Bridge, Schoharie Valley, New York State, built between 1855 and 1857, is a very fine surviving and early example of a timber-built covered bridge. It has a total length of 70.7 metres that incorporates a clear span of 64 metres – the longest single span of any wooden covered bridge in the world. The bridge is built of pine with its length formed by three trusses, with huge pine posts, braces and counter-braces, which are based on a prototype designed in 1830 by Stephen H Long, known as the ‘long truss’, and once much-emulated. The centre truss rises higher than the other two, and within it is enclosed a pair of arches – wrought of oak – rising from the lower ‘cord’ or carriageway up to the level of the roof ridge. The engineer-cum-carpenter of the Blenheim Bridge, a masterpiece of big-boned, heavy-duty timber construction, was Nicolas Powers from Vermont.
The Bridgeport Covered Bridge of 1862 over the South fork of the Yuba River near Grass Valley, California, incorporates a clear-span almost as long as that of the Blenheim Bridge – 63 metres. The Bridgeport Bridge includes two parallel trusses based on a design that was patented in 1840 by William Howe, a Massachusetts millwright. Howe trusses are designed so that – unusually – diagonal members are in compression and vertical members in tension. The designer of the bridge, David Ingefield Wood, was seemingly unsure about the ability of the Howe trusses to bridge the wide span or to carry expected loads, so he beefed them up with wide and shallow timber arches. These arches are based on the Burr arch truss – a type designed by Theodore Burr in 1804 and patented in 1817 – which consists of timbers bolted together, squeezing between them the members of the truss. The arches, essentially an auxiliary and independent structural system, rise from huge granite blocks placed slightly below each end of the bridge to just below the eaves of its roof. These arches are expressed externally and give the bridge a powerfully engineered appearance. Other examples of the Howe truss survive in the Jay Bridge of 1857, in Jay, Essex County, New York, and in the 22-metre-long Sandy Creek Covered Bridge of 1872, in Jefferson County, Missouri.