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Wood, brick, stone, cast iron, wrought iron, mild steel, high-tensile steels, alloy steels, aluminium, steel-reinforced concrete, pre-stressed concrete, glass-reinforced plastic. These are some of the materials that are available for bridge building.
Bending, compression, impact, oscillation, pressure, tension, torsion, vibration; contraction, corrosion, erosion, expansion, fatigue, friction, rain, river flow, sea-water, scouring, temperature changes, tidal flow, turbulence, waves, wind erosion, wind gusts, wind pressure. These are some of the stresses that bridge materials must withstand, in a variety of combinations.
What do we want from materialsStrength? But what is strength? It is a word with many meanings. Here are some attributes of a material that might be important, and which might come under the heading of strength.
One quantity is the amount of force in tension that it will withstand without breaking. In order to standardize on size, we use the concept of stress, which is force per unit area. So we refer to the ultimate tensile strength, though in reality one hopes that no bridge is designed in the expectation that any foreseeable loads will take it near to breaking point.
If the material stretches siginficantly before breaking, we might consider the stress that it can take while retaining the ability to regain its original condition. And we need to know how much it stretches for a given force, which is the modulus of elasticity, or Young's modulus, a ratio of stress to strain. After all, for any given piece of steel, we can find a big enough piece of rubber that will be as strong, but we wouldn't get much traffic on a rubber bridge. A related quantity is the amount of energy that a material can absorb. Organic materials such as tendon are superb in this respect, and only recently have engineers had at their disposal similar artificial materials such as Kevlar. As well as tension we must consider compression, torsion and bending. Resistance to tension, compression and bending stresses are all specified by Young's modulus.
We can combine materials to use their best properties. Fibreglass, or glass-reinforced plastic, is an example. So is reinforced concrete. We can take this further by employing pre-stressing, which enables concrete beams to withstand tension, which plain concrete, like cast iron, is very ill suited to take.
We have considered strength and elasticity. What else? Cost is extremely important. Some very strong and light materials are simply too expensive to use, except in specialist projects in which raw cost is over-ridden by some great necessity.
Assembly is more than a simple matter of connecting all the parts. Welding produces high temperatures, which produce expansion and distortion. Management of the cooling process is vital. Poor quality control of welding may allow detrimental changes to the properties of the metal. The weight distribution of a structure changes during assembly, requiring precautions such as adjustable jacking. Forcing two parts into alignment produces undesigned and undesirable stresses, which can start cracks.
We are still not done. Once we have bought our material and fashioned into the structure we want, we have to maintain it. Air contains mainly nitrogen and oxygen, but it contains much smaller quantities of gases which were generally not present 500 years ago. Sulphur dioxide and sulphur trioxide, and nitrogen oxides, are some of these. These gases are constantly available at at every surface, aided on occasion by rain, which wets the surface and increases the intimacy of contact. What we are talking about is corrosion. Riveted or bolted joints may be penetrated by thin films of liquid, which may do its insidious work unseen. There is an entire industry which provides solutions such as painting and anodizing.
Mechanical problems too, do not cease at the end of construction. Traffic may increase ten-fold or more within twenty years. Fatigue may change the internal structure of a metal or alloy. Creep may change dimensions.
The page about problems provides more detail about some of these topics.
Wood was one of the earliest materials used in structures, and it is still in use today. Unlike most structural materials it is anisotropic, meaning that its properties are not the same in all directions - it has a grain. The Young's modulus of a wood, for example, may vary by a factor of more than twenty between the longitudinal and radial directions. Cracks propagate radially and tangentially, but with difficulty in the longitudinal direction. Thermal expansion is also anisotropic. The pictures below show how wood behaves differently from most other structural materials.
Wood grows in many forms, from the very light balsa, beloved of modellers, to the hard and dense ironwood. Properly used and treated, wood can be strong and durable, with a life measured in hundreds of years. It is fairly easy to cut and shape, though it is limited to reasonably straight pieces. It does have some disadvantages. For example, it expands and contracts, not only with temperature change, but with change in water content. It is also liable to rotting and erosion from weather and fungi, and therefore requires careful preparation and maintenance. Wood is often attacked by insects, both in the living tree and after felling. Wood survives well in Antarctica because the climate is so dry.
Before use, wood must be seasoned, that is, allowed to dry out, because water is present inside the cells and in the cell walls. In practice, the drying may be accelerated by heating. Wood may also be impregnated with preservative, often using vacuum and high pressure.
We might think of wood as an inflammable material, and so it is. But in order to ignite it, a critical temperature must be reached. Wood is a fairly poor conductor of heat, and a large baulk of timber can remain quite cool inside while burning at the surface. Furthermore, the charcoal that remains on the surface is a very poor thermal conductor, and so a timber structure can survive surprisingly long times in a fire before collapsing. That is not to say that the structure is useful after the fire has been extinguished, but averting collapse is at least beneficial in avoiding injury to people and damage to property.
Artefacts of great beauty have been made in wood, such as the Viking longships, and indeed many other vessels through the ages. A few Viking ships have been preserved under bog and mud, and are now in museums for all to see and admire. Boats and ships, exposed to wave and wind, exemplify the elegance that comes from design that is based on the need to withstand great forces. Wooden aircraft such as the Mosquito made by de Havilland could also be very graceful. The Mosquito was probably one of the fastest production wooden aircraft ever built.
In bridges, the use of wood has been strongly constrained by the need to build with straight pieces. The simplest wooden bridge is the beam, but over two thousand years ago, builders in Asia realised that by building cantilevers, or brackets, from either side of a gap, and connecting these with a beam, they could create longer spans.
An altogether more logical use of wood was made by Brunel in the 19th century, when he extended the Great Western Railway through Devon and Cornwall. The need to build many viaducts across wide and deep valleys, at a time when capital was limited, led Brunel to the use of wooden spans on tall masonry piers. His reasoning was that when the railway had made more money, the wooden spans could be replaced. Many of these spans were propped beams with fan-like arrays of struts, rather like inverted cable-stayed bridges. Click here to see pictures of a wooden propped beam that is still in use. The last of his wooden viaducts to survive was replaced in 1934, having been built in 1863.
With any material we face the problems of joining the parts. Wood can be obtained only in relatively short and narrow pieces. To make longer members, pieces of wood can be joined using scarf joints, in which ends of the wooden strips are cut at a shallow angle to provide a large area for joining. The angle must not be too shallow, or there will be long narrow noses which will be weak, nor too steep, or the bond area will be too small. Wood can be joined by boring holes and using bolts or screws. Many modern glues are very strong and weather resistant - they may even be as strong as the wood. Gluing enables numerous layers of wood to be laminated together to form very long and wide beams, which may readily be made in curved shapes by curving the laminae before gluing.
Numerous other types of composite material are made from wood, including plywood, blockboard, chipboard, fibreboard, as in "MDF", and cement bonded particleboard.
Here are some wooden spans -
Like wood, stone has a long history in construction, and like wood, stone is still in use. At school you may have heard terms like "stone age", "iron age", "bronze age", and so on. Whatever the relevance of these terms, we can be sure that at these times nobody built bridges of iron and bronze. The cost alone would have been prohibitive. The technical knowledge of structures would have been insufficient, even if enough material could have been amassed.
Here is a picture of an early construction in stone, in the place that is now called England. The first construction at the site probably began around 5000 years ago, and other parts were added very much later. Compared with what had already been built in other parts of the world, Stonehenge could not possibly have been called "state-of-the-art", but it serves to illustrate one vital discovery - that a gap could be spanned by a beam of stone. This is sometimes called trabeate architecture. The ancient Greeks built extensively in this way, and seemed uninterested in the arch.
Bridges, some of them of great antiquity, with stone beams are often called clapper bridges in England. The bridge illustrated here is at Eastleach. It is not especially old. The stone beam is extremely limited in span, firstly because very long pieces are not easy to obtain, and there is no means of joining small pieces to make a beam. The second reason is that in order to increase the span, the depth of a beam has to be increased more rapidly than the span increases. Clearly, when the depth is equal to the span, the point of absurdity has been passed. Using materials such as wood, iron and steel, builders could postpone the inevitable by using structures such as plate girders and trusses, which are impossible in stone. Masonry could only span large gaps after the arch had been invented, an event that began a reign of over two thousand years. The masonry arch was still going strong in the eras of canal and railway of the 18th and 19th centuries.
Construction in masonry looks, at first sight, relatively simple. You get the rock out of the ground, cut it to shape, and put it in place using mortar. In practice, life is far more complicated for the mason. Rock may vary greatly, even within a quarry. A sedimentary material, for example, may be subject to sorting by the speed of water currents while being deposited, resulting in a gradation of properties tens of millions of year later. Swirling or circulating water may result in very sudden variations in deposition. Even uniform rock may be porous, allowing water to penetrate deep inside. Limestones, especially the lighter types, such as Cotswold, are subject to this. Mortars must be matched to the stone so that neither can abstract water from the other.
Stone can be corroded by chemicals in air and water, especially in industrial areas. Wind can carry dust particles which erode the softer materials - the Egyptian sphinx and pyramids are notable cases. Some examples of corrosion and erosion are shown below.
In spite of all these possibilities, the use of the right materials in the right way can result in structures that last for thousands of years. All over the world, stone structures testify to the skills of builders of many different cultures. In any discussion of decay, we must not forget the depredations of man. The famous stone ring at Avebury was raided by local people for stone to make their dwellings. Many another monument is smaller than when built, for the same reason. Stone comes from quarries, and a stone structure, for some people, is a convenient quarry containing ready made blocks.
Here are some links about masonry arches -
A few examples are shown below. In two of these you can see the corbels on which the centring was set.