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). 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 and 179).
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.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