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.11 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). 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,
