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

Whatever the intention behind the bridge’s foundation rituals, as it turned out, they brought no good to the Jews of Prague. During Easter 1389, as the bridge was nearing completion, the clergy in the city inflamed the latent anti-Semitic feelings of the population by announcing that Jews – long held responsible by Christians for the death of Christ – had desecrated the host, the Eucharistic wafer that becomes the body of Christ during the mystery of the Roman Catholic Mass. Murderous chaos followed which resulted in the Jewish ghetto being pillaged and burnt, and much of the Jewish population of Prague – estimated at around 3,000 people – being murdered.

Four years later the bridge – still incomplete – became itself the focus of a grim event that, in later centuries, did much to define the character and spiritual aspirations of the city. On the night of 20 March 1393, John Nepomuk was killed by being thrown from the bridge on the orders of Wenceslaus, King of Bohemia, who in 1378 succeeded his father, Charles IV, as ruler of Prague. Nepomuk was a principled cleric who displeased Wenceslaus by refusing to divulge to him the secrets of his Queen’s confession. In consequence Nepomuk was tortured (apparently his tongue was removed in no uncertain manner) before being tossed from the Charles Bridge. His sufferings and manner of martyrdom led Nepomuk to be canonized, made the patron saint of Prague, the official holy protector against floods, and inspired within Bohemian architects for years to come a morbid interest in tongues. As motifs, as plan forms or vault patterns, stylized tongues, small or vast in scale, enliven the sacred architecture of the region.

The Charles Bridge stood firm and largely unaltered for nearly 300 years, maintained by a toll collected by the crusading military order of the Knights of the Red Cross and Star whose mother-house was located next to the bridge. Then, in the 1680s, the bridge’s lurid past caught up with it. This was a time of Roman Catholic resurgence in Bohemia, following the dramatic defeat in 1620 of Protestant forces at the Battle of the White Mountain, and if the bridge’s foundation had anything to do with the ancient arts of magic, alchemy or the Kabbalah, then the Catholics felt something had to be done about it. And it was. From 1683 until about 1714, the bridge’s parapets were loaded with statues carved of stone, mostly of saints and clerics – including, of course, an image of St John Nepomuk. The bridge was turned into a Roman Catholic shrine – walking along it became a mini-pilgrimage – with the flamboyantly posturing parade of saints, carved in ostentatious baroque manner, being a tremendous late flowering – in theology and in art – of the Counter Reformation. Virtually every one of these statues has now gone, their weathered and battered hulks carted off to the Lapidarium museum and replaced by replicas. But the fourteenth century bridge endures, a tribute to its robust construction, to the skill of the master mason Peter Parler who supervised building works and – perhaps – to the strange ritual of its foundation.

STRUCTURAL PRINCIPLES

During the late fourteenth century in Europe, when the Charles Bridge was being built, the technical approach to bridge building was starting to change. Masons like Peter Parler tended to have an almost intuitive understanding – honed by years of experience and exposure to the trade ‘mysteries’ of their craft – of the structural forces engaged in bridge construction. Their responses to these forces of nature, and to the manner in which loads are contained or transmitted by structures of different forms or materials, were usually pragmatic and the result of empirical observation, practical experiment and trial and error. This resulted in safe and conservative designs with few great and dramatic leaps forward – which is what makes the unusually wide-span elliptical arches of the mid fourteenth century Ponte Vecchio in Florence so novel and interesting (see page 150 (#litres_trial_promo)). Throughout the fifteenth century things started to change, gradually at first, as bridge building became more theoretical and finely calculated. But it was not until the late sixteenth century that scientific understanding of the theory of bridge construction started to dominate the business of bridge building.

Crucial to this new understanding was the ground-breaking research and analysis undertaken in the late sixteenth and early seventeenth centuries by mathematician, astronomer and philosopher Galileo Galilei. This allowed late Renaissance engineers to calculate the ways in which the shape and size of structural members – for example beams and trusses – and the materials from which they were made would affect their ability to carry and transmit loads. Significantly Galileo identified the ‘scaling problem’. He established the principle that as a beam increases in length it decreases in strength, unless its thickness and breadth increase disproportionately. He also demonstrated that this escalation of scale has very definite limits dictated by nature. Quite simply, if a beam is increased in scale beyond a natural limit it will be capable of supporting no loads at all and break under its own weight.

In the late sixteenth century the scientific and mathematical approach to construction was in fact being explored by many and evolved at a rapid rate. For example, the architect Andrea Palladio’s Quattro Libri dell’Architettura of 1570 included the first published illustration of a triangulated truss – a robust structure for transferring loads through a rigid system of triangular forms. Other important publications pioneering, promoting or explaining theories of bridge construction included Machinae Novae of 1595 by Fausto Veranzio, which includes information on tied-arch bridge construction, the oval lenticular or lentil-shaped truss, and the iron chain-link suspension bridge. A key later work containing much technical information is Traitre des Ponts of 1716 by Hubert (Henri) Gautier.

BRIDGE DESIGN AND CONSTRUCTION

The permutations of materials and structural principles employed in bridge construction are seemingly many, varied and complex – timber, brick, stone, cast and wrought iron, steel, hydraulic cement, mass concrete and steel-reinforced concrete, arches of diverse form, beams, cantilevers, pylons, cables and masts. But in its aim bridge construction is straightforward and construction simple. The object is to link two points of land as safely and efficiently as possible. If the obstacle being bridged is running water, then the ideal is to achieve wide spans with minimal support rising from the water to make the bridge easier to build and maintain, to avoid disturbing navigation, and to reduce the risk of the bridge being swept away.

In essence bridge construction is of two basic types. The carriageway – be it for vehicles or pedestrians – is either supported from below or suspended from above. If the ‘dead’ load of the carriageway (its weight) and the ‘live’ load of the carriageway (the weight of the use it carries plus the ‘environmental load’ comprising the weight and pressure of rain, snow and wind) are supported from below it must be carried on arches or vaults of varied types; on beams either cantilevered from, or supported by, abutments and piers; or set within a lattice-like engineered truss wrought of timber or metal. There are two models of nature for support from below: rock formations that arch over; and timber logs or beams laid across, chasms or rivers.

If the ‘dead’ and ‘live’ loads are supported from above, the carriageway must be suspended from well-anchored cables or chains stretching over masts to form inverted arches of strong catenary shape, or from natural features – a system known in China from the second century BC. Cables can also be stayed or anchored firmly to a single support to create a cable-stayed bridge. The prototype in nature for these types of suspension bridges is a walkway formed by, or supported by, hanging vines and vegetation.

These different approaches are determined by a variety of circumstances but all are responses – in various and appropriate ways – to the four basic types of forces that act on bridges, either singly or in combination: tension, or a tendency to stretch or pull apart; compression which pushes together and compacts; shear, which is a sliding force; and torsion which is a twisting force.

(#litres_trial_promo) The form of the bridge, and the materials used in its construction, also create different – and utilize different – structural forces.

A bow-string truss or tied-arch bridge: the horizontal thrust of the arch, from which the carriageway or deck is supported from above, is restrained by the horizontal tie on which the carriageway sits.

For example, the beam in a beam bridge is under both compressive and tensile forces, itself exerting a downward, compressive force on its piers. The weight of arched bridges is carried downwards from the crown to the ends of the arch and then not only vertically but also laterally because an arch, by its nature, thrusts outwards. The force of an arch’s lateral thrust depends largely on its form, but also to a degree on its materials of construction and sheer mass and weight. Clearly, a shallow, elliptical arch of masonry will have more lateral thrust than a semi-circular arch formed with timber members. For the arch to function structurally this outward thrust must be contained – possibly by a horizontal tie linking both ends of the arch but usually, in bridges, by abutments that exert a compressive force to prevent the arch from spreading apart.

In addition, the material from which a masonry arched bridge is constructed is under compression, being forced together by the loads carried and by gravity. This is why an arch made of brick or stone voussoirs is such a perfect form for a load-carrying structure – the more weight that is placed upon it (within reason) the more rigid it becomes because the load ensures that the components are locked more firmly together.

A suspension bridge, in which the carriageway is supported almost fully from above: the suspension cable, passing over the tops of the suspension towers and anchored firmly in the ground, is connected to the carriageway by vertical suspender cables.

The Forth Railway Bridge, Scotland, completed in 1890: balanced cantilever arms linked by suspended spans.

In contrast, bridges with their carriageways suspended from cables are structures operating under tension because the loads on the carriageway pull – or stretch – the cables.

These basic strategies can be combined to create composite structures in which different members are acting under both compression and tension. For example, the cantilever bridge is a more complex version of the beam bridge, utilizing additional structural principles. The Forth Railway Bridge of the 1880s in Scotland is a useful illustration (see page 294 (#litres_trial_promo)). It incorporates massive steel lattice-work pylon towers, forming projecting ‘arms’ that are, in effect, huge balanced cantilevers linked by suspended spans.

The loads at work within the lattice towers of the Forth Bridge are complex, with vertical members in compression and diagonal members in tension, but broadly speaking, the upper raking steel principle members of the cantilevers are under tension, being pulled down by the weight of the carriageway below them, while the lower steels of the cantilevers are under compression, being pushed down by the weight of the carriageway above them. The structure of the Forth Bridge confirms an ancient and elegant engineering ideal expressed in such great Gothic cathedrals as Notre Dame in Paris or Reims. In this ideal engineering model, conflicting forces are made to balance, to compensate for each other, with thrust met by an opposite and equal counter thrust, and weight balanced by weight, all calculated to create structural equilibrium and to achieve strength and solidity not through mass but through pure engineering know-how.

Another fascinating example of a bridge design embracing and utilizing different structural forces is the bow-string truss bridge, or tied-arch bridge, in which the outward, horizontal force of the arch is restrained by a horizontal tie rather than by abutments and the bridge’s foundations. Ideally the horizontal tie – when linked to the arch by vertical and diagonal members – also functions as the carriageways – as in the great examples of the type bridging Sydney Harbour and the River Tyne in Newcastle (see pages 219 (#litres_trial_promo) and 179 (#litres_trial_promo)).

The design of this type of truss – which by necessity must be made of metal or timber – was perfected in the second half of the nineteenth century by engineers who were fully able to calculate forces at work in bridge construction and in the natures of different materials. These engineers were obliged to do so because of the unprecedented methods of modern transport in which increasingly heavy and rattling railway engines and their carriages put particularly strong stresses on bridges. One of the pioneers of the precisely calculated and very strong metal railway bridge, was the American engineer Squire Whipple, who in 1841 patented his all-iron bow-string truss bridge design. In Whipple’s conception, a pair of these arched trusses, set side by side, carry a carriageway set on a platform built off the beams forming the strings. But the key point about Whipple’s bridges was not so much their form but the fact that all was calculated by scientific analysis and the size of all members dictated by the forces they carried. His book, A Work on Bridge Building of 1847, is one of the key nineteenth century publications on structural mechanics.

The choice of form chosen for the bridge usually depends on a number of factors: on the width, height and type of obstacle to be bridged; on the function of the bridge and estimated forces of ‘dead’ and ‘live’ loads; on time and materials available for use (masonry and cast-iron were really only appropriate for compression structures while more flexible or ‘elastic’ timber, wrought-iron or steel worked for tensile structures); and – of course – on the skill, knowledge, intentions and nerve of the bridge builder.

BRIDGE OVERVIEW

The principles of bridge construction, and the problems and potential of different forms and materials, are best explained in further detail by reference to a few specific examples of bridges. For reasons of clarity and instructive comparison, the examples are arranged and grouped according to primary construction materials.

Timber

Timber is, presumably, the earliest bridge-building material in large scale and continuous use. Only when grandeur or longevity was required, or in regions with no easily available timber – such as Mesopotamia – would stone or brick have been used in preference. 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.

(#litres_trial_promo)

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’.

(#litres_trial_promo) 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.

(#litres_trial_promo)

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.

(#litres_trial_promo)

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.

(#litres_trial_promo) 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’.

(#litres_trial_promo) 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.

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.