materialsb2

Brick

People sometimes say "cast in stone" to imply permanence, yet casting, the pouring of a material into a mould, is not possible with stone. The advantages of casting are obvious. By making a mould, you can in principle make any shape you like, and if you can remove the mould from the workpiece unharmed, you can repeat the process. You are in fact in mass production. Sculptors can use a pointing device, rather like a three-dimensional pantograph, to make copies, even at different scales, but that is a slow method of reproduction. It is in fact not strictly true that stone cannot be poured, as some kinds pour out as lava from volcanoes, which are, however, definitely uncontrollable, and the resulting stone is not necessarily of structural quality.

Bricks, though they may be formed in moulds, are not generally poured; rather, the material is forced into the rectangular containers by hand or by machine. And unlike the casting process, the brick making process requires heating the material after it has been put into the moulds, rather than before, producing irreversible chemical changes that do not take place in casting. In ancient times they might simply have been left to bake in the sun. A wide range of materials, including even mud, has been used to make bricks, though clays are probably the commonest. The best bricks are very durable. Straw was often put into ancient bricks, creating an early form of composite material. Such materials can offer the benefits of each individual component in a way which overcomes the deficiencies of the other component. A common example is the use of fibres within a matrix, for example resin bonded glass fibre and reinforced concrete. Many years ago, a strong insulating material called Tufnol was invented. In most of these fibrous composites, the matrix withstands compression and the fibres take tension. Pre-stressed concrete is a special case in which the fibres (steel wires) are put into tension even before any loads have been applied to the structure.

An essential feature of most brick making is the firing of the bricks to a high temperature, resulting in chemical changes that create a strong, hard material. Brick, like many rocks, is porous, and care is needed in order to prevent the ingress of water. A building may be provided with an impervious damp course just above ground level, to prevent water creeping up from the ground by capillary action. Water in brick renders the material subject to frost damage, when the water expands on freezing. Flakes may split off the surface, or major cracks may occur. Cotswold limestone suffers in the same way.

Should the ground settle, large scale cracking may propagate through a brick wall, often on a diagonal. In many instances the cracks avoid the bricks and go through the mortar, but it others, they may go straight through the bricks. Note how the crack in the second example has started at a corner.

As with stone blocks the mason has to choose the type of mortar with care, to suit the bricks.

Compression is the stress that bricks resist best, and so they are used to greatest effect in arches and walls.

Exercise – Find out which ancient civilizations used bricks, and find out how they made them.

Here are some links to web-sites about bricks –

Brick Development Association

Roman bricks

Adobe bricks

A bricklayer’s story

Here are some links about bridge construction –

Masonry arches

Skew arches

Here are some pictures of structures built in brick.

Cast Iron

People may speculate about the first bridges in wood and stone: about cast iron there is no doubt. The bridge is well documented and still exists. It was designed by Abram Darby and built in Coalbrookdale between 1777 and 1779. Cast iron bridges rapidly became popular and were only superseded when wrought iron became cheap enough to supplant cast iron. During the 19th century many cast iron bridges failed. These failures were due almost entirely to the great weakness of cast iron – its inability to withstand much tension. This did not matter when only arches were built in it, but once people started to use it for beams and trusses the trouble began. The failure rate in the 19th century, especially in railroad bridges was very serious, even well-known engineers being by no means immune.

There are advantages in casting. You can make the mould in a non-structural material that may be much cheaper than the actual part being made. In the case of a molten metal the mould must withstand high temperatures. For a bridge there is no need for the mould or the metal to create fine detail. A disadvantage is that making large parts requires making large moulds. For the Coalbrookdale bridge the builders got round this by designing ingenious ways of joining the parts – ways which no subsequent builder would use. Such is life when you are the first. Builders such as Telford produced elegant and confident designs in cast iron.

What is cast iron? It is a material comprising around 2% to 4% carbon and the remainder iron. It is very strong and rigid, but it is not ductile and it cannot absorb much energy. In this it resembles glass. Moreover, it is weak in tension. This means that when it experiences bending moments it will break readily. The lack of ductility means that little or no warning is given before failure – a most undesirable property. Why is a warning important? Especially during construction, when loads are applied relatively gradually as a structure grows, unexpected deflections indicate problems. During the construction of the first Quebec bridge, buckling was observed in the lower chords near the supports. Nobody on site seems to have realised the serious implications, and before any attempt had been made to recall any workers, the structure collapsed, with great loss of life. During the life of a structure or equipment, inspections can detect early signs of trouble if the materials are capable of showing them. Often the main problem is to devise reliable and cheap means of detection which will not provide too many false positives. The field of non-destructive testing is devoted to these procedures.

These three pictures show firstly a cast iron drain cover, secondly a steel tube forming part of a gate, and thirdly a steel barrier. All have failed. The cast iron drain cover was broken by a heavy vehicle: there is no sign of deformation – it has failed by brittle fracture. The steel parts have deformed extensively, beyond their elastic limits, but have not broken.

At the opposite extreme from glass and cast iron are substances such as kevlar and rubber, and a great many organic substances such as leather, spider web, tendon and chitin. In fact, it is only in recent years that artificial materials have been found to substitute satisfactorily for organics in energy absorbing applications.

In spite of its limitations, cast iron still finds plenty of applications in applications such as garden furniture and in the well-known Aga and Rayburn stoves. For progress in structures, however, builders had to turn to wrought iron.

Some links about cast iron –

Telford’s bridges

Some examples of cast iron bridges –

Wrought Iron

What is wrought iron? It is almost pure iron, containing very little carbon, less than 0.3% in fact. In contrast to cast iron, it is ductile, and it is more resistant to corrosion than many steels, and most important, it can withstand tension. Like many other technologies, it was expensive when first introduced, but became very popular.

What is more, where masonry had awaited the right idea, the arch, to come of age, its inverse, the suspension bridge, hitherto a matter of ropes and wood, was ushered into a new era by a new material, wrought iron. Not as we know it, with wires, but with iron eye bars. An outstanding example was the Menai Bridge of Thomas Telford, begun in 1819 and opened in 1826. The main span of 580 feet is held up by sixteen chains of eight foot wrought iron eyebars, which were treated with linseed oil to retard rusting. It was the longest span in the world, and a significant step forward in bridge building. It is still in use, after some refurbishment.

Not far from the Telford bridge, the Britannia bridge of Robert Stephenson (1850) is based on wrought iron rectangular tubes, with main spans weighing 1500 tons. The original idea was to support the tubes using chains, and the towers provided for that, but extensive and careful tests showed that the tubes could be self-supporting.

The Clifton suspension bridge of I K Brunel, with a span of 702 feet, 250 feet above the river Avon, was completed in 1864, after the death if the designer. It, too, had wrought iron chains, but the method was nearing the end of its rather short life, because a new material, steel, was soon to take over, in the form of wires. Wrought iron was relegated to ornamental garden gates and the like. But before that came to pass, Brunel completed the Royal Albert bridge, a work of great originality and economy, in 1859. It was his last creation: after its completion, he was taken across it on a truck which had been modified so that he could recline on it in order to view his work. Not long after, he died.

The bridges of Gustave Eiffel, and his famous tower in Paris, were built of wrought iron. The truss could now be built strongly and reliably. Pont de Garabit (1880 – 1884). Tour Eiffel (1887 – 1889).

From the Menai bridge (1826) to the Tour Eiffel (1889) was a span of 63 years, the glory years of wrought iron. It was steel that released designers from their chains and enabled them to contemplate spanning even such gaps as the Straits of Messina.

The relatively short time spans of cast iron and wrought iron in bridge building, compared with the longevity of wood and masonry, probably tell us something about the accelerating pace of technical development over the last few hundred years.

Here are some links about wrought iron –

History and technology – one

History and technology – two

Steel

Steel is not a material, but many materials, all of them alloys of iron with one or more of the other chemical elements, including some carbon. Steels have been created for high tensile strength, corrosion resistance, good performance at high temperature, and elasticity, for example. Many of the specialist steels are expensive, and so the choice of materials for a large structure such as a bridge has to balance performance in terms of quantities like strength-to-weight ratio against cost.

The cost of any material is related to its abundance in the earth’s crust, its distribution, its location, and the cost of extracting it from the ore or other raw material. The oceans, for example, are full of water, but if we want drinking water, we have to use energy to remove the other substances. The water is full of hydrogen, a useful fuel, but the result of burning hydrogen is water, so to extract hydrogen from water, even with perfect efficiency, requires as much energy as we will get from burning it.

Iron is one of the commoner elements in the universe, and probably accounts for much of the core of the earth, where it cannot be reached. But it is also reasonably easy to extract from crustal materials. Iron has been known for thousands of years, but only towards the end of the 19th century did people learn how to make steel economically. Because of costs, steel will have a much longer life in bridge building than did cast iron and wrought iron, simply because any superior materials are very much more expensive. Only concrete can compete, and it of course includes steel within it.

Steel was still a relatively new material when the Forth railway bridge was completed in 1890 (a year after the wrought iron Tour Eiffel), so the bridge was innovative in both conception and material. The age of steel continues, and shows no sign of ending.

Steel can be forged and rolled into shapes ranging from I-beams to wires, and in fact the properties of steel, like those of many other alloys, depend strongly on the treatment imposed before use. The treatment may include mechanical processes such as forging, rolling or drawing, or thernal treatments such as quenching and annealing. An extreme example is the Samurai sword, which is made by a long process of multiple heating, folding, beating and quenching. If you fold 16 times, for example, you get 65536 layers.

The treatment of steels generally leads to benefits in performance, though remanent stresses may result from large scale forming, as in the preparation of rolled steel joists (RSJs). Large quantities of steel are hidden away in the form or reinforcing bars (rebars) in reinforced concrete, and in the form of stressing wires in pre-stressed concrete.

Steel may be joined by rivetting, bolting, and numerous varieties of welding. Joining, especially welding, where large spatial temperature differentials occur, may lead to remanent stresses in the structure.

The formulation of different types of steels may be optimised for making girders, suspension cables, hangers, bolts, rivets and so on, and for different processes such as forging and casting. Also to be considered are conditions of use such as corrosive conditions, variable forces, requiring fatigue resistance, etc. Corrosion is of particular concern in the relatively narrow cables of cable-stayed bridges as opposed to the thicker main cables of suspension bridges, because the ratio of surface area to mass is greater for the thinner wires.

Steel is used in the longest suspension bridges, the longest cable-stayed bridges, the longest arch bridges, the longest cantilever bridges, and the longest beams, and of course in thousands upon thousands of lesser structures.

Some links about steel –

About steel

Steel facts

Some examples –

Concrete

Concrete, perhaps more than most other structural materials, excites both negative and positive feelings of great strength. Many will point to the dark grey brutalist buildings of the 1960s as examples of drab and ugly buildings, especially on the duller kind of British day. Others will point to the achievements of the pioneers Maillart and Freysinnet, and their numerous distinguished followers.

Concrete is found in three main forms, simple concrete, reinforced concrete, and pre-stressed concrete. Concrete with no additions may be found in such simple constructions as private driveways and patios, but for anything of more than trivial size, reinforcing bars will be used. These are the modern equivalent of the straws used in early brick making.

The important property of concrete is that it is relatively weak in tension, and is therefore rather like brick or stone. It is intrinsically poor for making beams. Its great advantage is that it can be poured into moulds, usually called formwork or shuttering, to to take up any required shape, whether it be a gigantic dam or a small foundation for a shed.

Reinforced concrete contains numerous iron bars, often with serrated surfaces to grip the concrete. These bars are placed strategically inside the formwork before any concrete is poured. Placing and fixing of the reinforcement is a most important part of any design in concrete, because the reinforcement is going to take any tensile forces that arise. It should be noted that until a structure is loaded, there is little stress in the bars. The bars do not necessarily prevent the concrete from cracking, but they do prevent the structure collapsing if cracks do form. It is clear that if the tensile stress on a reinforced concrete member is enough to stretch the bars to the point where the concrete is stressed to its limit, the concrete will crack. The picture shows a pile of reinforcing bars prior to use. In use they would of course be spaced out, and often held in place by thinner bars placed across them.

Other materials, such as glass fibre and bamboo, have been used in reinforced concrete. The relative mechanical and thermal properties of the concrete and the reinforcement are very important for the integrity of the structure. The pouring and subsequent treatment are also important. Because of the heat developed during pouring, the quantity in a pour is limited. For structures such as large dams, refrigeration pipes may be included in the structure. After pouring, the concrete is subjected to a brief period of vibration to eliminate voids.

The principle and purpose of pre-stressed concrete are quite different from those of reinforced concrete. Here the wires, or tendons, are under tension all the time, to the extent that the surrounding concrete is always under compression and can therefore never crack.That is how the pioneers thought, but more recently, engineers have realised that as the steel is taking the tensile stress, a certain amount of cracking is permissible. Thus the steel can either be taken to higher stress or be reduced in quantity. The wires may be placed in position before the concrete is poured, or they may be threaded through tubes afterwards. In the former case they may be tensioned before the concrete is poured (pre-tensioning) or afterwards (post-tensioning). In any case the phenomenon of creep must be allowed for. The shrinkage due to creep occurs over a period of a few months, the rate decreasing with time.

Concrete spans are often built segmentally, cantilevering out from the previous work. The added parts may be cast either in formwork near the site and then lifted into place, or in formwork that actually moves along the span. In either case, match-casting (using the previous segment as one side of the mould) enables the parts to fit closely. A thin film of epoxy adhesive may be used between the segments. Each segment is "stitched" into place by stressing wires, which pass through two or more of the previous parts. In principle, a long viaduct may be built with hardly any effect on the ground, by keeping all the building plant on the growing structure. This is especially valuable for preserving environmentally sensitive ground. The Linn Cove viaduct was the pioneering example of this technique.

Some links about concrete structures –

History of concrete – one

History of concrete – two

Pre-stressed concrete bridges

Concrete arches

Beams (not all concrete)

Linn Cove viaduct by Figg Linn Cove viaduct – two

Glass fibre reinforced concrete

The Concrete Society – facts about concrete

The Concrete Centre

American Concrete Institute

A few examples –

Aluminium

Aluminium alloys are extensively used in aerospace products, because of their strength and lightness, so why are they not much used in bridge building, where lightness and strength are also important? The answer is that the balance of these requirements versus costs is different.

Nevertheless, some bridges have been built in aluminium, which has the advantage of needing little maintenance, because it forms a stable oxide film. Care has to be taken where two or more metals are used together, because electrolytic action can lead to rapid corrosion. Indeed, it is not unknown for ships to be provided with sacrificial anodes which are corroded preferentially to the steel hull. It should be noted also that the fatigue properties of aluminium and its alloys are different from those of steel: there is no endurance limit below which fatigue never occurs.

The first aluminium bridge was built in 1949 at Arvida in Quebec.

Graafstroom bridge

Lockmeadow footbridge one

Lockmeadow footbridge two

Bridge deck analysis

Mobile bridges

Military bridges

Forsmo bridge

Composite materials in bridges

A composite material is one that is composed of two or more substances that are interleaved more or less intimately, but not mixed or alloyed to make a uniform substance. Of course, mixing can never be perfect, because we are mixing different atoms or molecules. If the mixing goes right down to atomic level, we probably have a solid solution, but if the material contains small domains comprising different compositions, we have a substance with two or more phases.

We will reserve the label "composite" for materials in which the regions are macroscopic rather than microscopic, though the definition has an arbitrary cut-off. Such materials may be in the form of a matrix containing fibres, as in glass reinforced plastic (GRP or fibre-glass). It may be in the form of multiple layers, an early example being plywood, which although made entirely of wood, had the grain of different layers differently orientated, and was thus not continuous in structure. Concrete was an early example of a composite material, discussed in its own section above, being so important. Reinforced and pre-stressed concrete possess a second layer of compositeness, and are discussed under the heading concrete, above.

A proposal to introduce a composite footbridge across the Thames at Lechlade was rejected on the grounds of unsuitability for an area where the architecture is largely based on Cotswold stone. Evidently for many local people, the contrast between the new and old outweighed any pleasure that might have been taken in viewing an elegant structure in an unfamiliar material.

Composite materials can offer combinations of properties which are not possible for the individual substances, though of course the weakness of the substances are also present. The design must use the best features of each material to compensate for the weaknesses of the others. Here are some links to pages about composite bridges.

Britain’s first composite bridge over a motorway was constructed during the winter of 2005 – 2006 on the M6 near Garstang.

Homestead bridge

PowerPoint presentation

Demonstrator bridge

Hybrid bridges

Carbon shell test

Hindmarsh Island bridge

Den Dungen bridge