An Encyclopedia of the History of Technology part 49 pot

PART THREE: TRANSPORT 462 BRIDGES The building of bridges is almost as old as the making of roads, or at least tracks, occurring at about the same time as man’s first settlement in agricultural communities and the very beginnings of permanent villages in the Neolithic Age, some 10,000 years ago or more. At first, bridges were of very limited span: simple beams of single logs laid across gaps or streams and suited only to pedestrian traffic. The branch of a vine or similar creeper, on which a man could swing across a gap, could hardly be classed as a suspension bridge, but it was surely the antecedent of the first single rope spans. As more and more animals became more and more domesticated—the dog, the ox, the sheep, the donkey and the ass—it became increasingly necessary to widen the single log beam bridge and to give it a reasonably flat upper surface. Where long enough logs were not available to span the gap to be bridged it was but a small advance to build a pier and double the span or several piers and hence to multiply it. In the fifth century BC Herodotus describes a multi-span bridge at Babylon having a hundred stone piers joined by timber beams averaging over 1.5m (5ft) in length, the whole bridge being 200m (660ft) long. In other cases, where the piers consisted of wooden piles, the technique was possibly adopted from the Bronze Age lake dwellers of Switzerland who developed it about 2500 BC, or from other northern peoples who built similar structures towards the retreat of the last Ice Age, such as those at Star Carr in Yorkshire. To a limited extent, stone slabs were used in place of logs to make clam bridges if single span or clapper bridges if of several spans. Some have survived in Britain until today, particularly in the Cotswolds, and on Dartmoor and Exmoor. Existing since prehistoric days, they have long outlasted their timber equivalents, but they are few and stone is too brittle to be used successfully except for the smallest spans. It first appears as early as 3500 BC in Mesopotamia. The Egyptians understood the principle of the arch about the same period but appear to have used it more in buildings than in bridges. The Greeks, although they did learn the art of the arch in the fifth century BC, preferred to use the beam in the form of post and lintel construction. The Romans, great engineers, needed bridges for the vast network of roads which reached to the farthest corners of their great empire, both for trade and for military purposes. Though they were familiar with the stone and brick arch, they, too, had a preference for timber construction in bridges and used piers and lintels wherever possible. Typically, Julius Caesar in 55 BC is recorded as having built a timber bridge 550m (1800ft) long over the River Rhine in ten days. It had fifty spans. Possibly it was the speed of construction that formed the attraction of this form of construction and materials to the Romans. Roman engineering was advanced enough to involve the use of coffer dams for the construction of bridge piers over rivers. These consisted of two concentric rings of wooden piles, the space between which was packed with ROADS, BRIDGES AND VEHICLES 463 brushwood and clay. The biggest arch bridge of Roman construction that we know of was a viaduct at Narni on the Via Flaminia which had four spans making a total of 42m (138ft). The arches, like all those of Roman origin, were semi-circular in shape, making for a rather high rise at the crown of the bridge, though no worse than the later pointed Gothic arches of mediaeval days. Much effort and ingenuity was to be expended by later engineers in attempts to produce a flatter arch form. With the eclipse of the Romans in the fifth century AD, the lands once ruled by the might of Imperial Rome fell into a state of semi-anarchy governed by a myriad of small tribes with little need for and still less interest in bridge- building beyond occasional repair of a relic of Roman days. Not for another five centuries did the Christian Church take on the responsibility for the upkeep of some of the existing roads and bridges and, occasionally, the erection of new ones. The span and strength of the beam were both greatly extended by the invention of the truss, a framework of relatively light members generally arranged in triangles. The Renaissance architect Andreas Palladio is said to have constructed the first bridge of truss type in 1570, with a span of some 30m (100ft) to cross the River Cismon in the southern Dolomites, south-east of Bolzano. He used timber. The same material was much used in America with the spread of the railroads to the West in the nineteenth century when Ithiel Town, Thomas Pratt and his father, Caleb, and William Howe, as well as James Warren in Britain, all evolved different designs using wood. Also in Britain, 1847 saw the introduction of an iron truss by Squire Whipple; two years earlier, Robert Stephenson had ingeniously combined a truss with an arch to form the bowstring girder bridge. The truss, taking the place of the string of the bow, tied the ends of the arch together so as to obviate the need for heavy abutments to absorb the thrust. Such a construction was used for the High Level Bridge over the River Tyne, completed in 1845, whose six 38m (125ft) spans carry the railway on the upper deck with a roadway running below it, a further innovation in its day. The truss bridge perhaps reached its zenith in the 2-mile-long, 85-span bridge designed by Sir Thomas Bouch and completed in 1878 across the River Tay—one of history’s most dramatic failures. It was largely his miscalculation of the effect of wind pressure that resulted in the loss of the central spans and a train with seventy-five passengers. One of the central spans was 245ft (74.5m) long and weighed 194 tonnes. Malleable iron was the material used. In cantilever bridges, the projecting portion of a beam is fixed in the pier or abutment at one end only—possibly opposed by a similar structure arising from the opposite side of the gap. The design appears to have originated in China in pre-Christian times. The Forth railway bridge, completed in 1889, is a prime example, and the first major bridge in Europe to be built of steel. It has two equal main spans of 521m (1710ft) that is, three double cantilever frames with PART THREE: TRANSPORT 464 short suspended trusses of 106m (350ft) each between them. Some 45,000 tonnes of Siemens-Martin steel were used in the construction. An earlier bridge, the first of the modern cantilevers, was built in 1867 by Heinrich Gerber across the River Main at Hassfurt in Germany, at first giving the name of a Gerber to a cantilever bridge. Its central span was 130m (425ft). Charles Shaler Smith followed it with the Kentucky River Viaduct with three spans of 114m (375ft) in 1876. The box girder bridge first came into prominence with Robert Stephenson’s Britannia Bridge over the Menai Straits, completed in 1850, in which the railway ran through the box girder. It consisted of a twin tubular structure with tubes of rectangular section and supported on masonry towers at each end of their 140m (460ft) span. Each tube was fabricated from wrought iron plates and sections on shore and floated out across the water on pontoons whence it was lifted 30m (100ft) on to the piers by hydraulic jacks. In 1970 the bridge was severely damaged by fire and was rebuilt with a continuous beam, the railway running below with motor traffic over the top. The spans of Stephenson’s bridge were so linked that they effectively formed cantilevers. His design, a novelty in its day, is perhaps not always recognized as a box girder today when girder construction combined with suspension cables and towers has become the accepted form for long spans. Some of the world’s longest spans are given in Table 8.4. For lesser spans, the arch bridge has remained a favourite, some 70 per cent of the 155,000 road bridges in England today being brick arches. It was also Table 8.4: The world’s longest span bridges. ROADS, BRIDGES AND VEHICLES 465 the design used for the first iron bridge in the world. The castings of which it is built were made by Abraham Darby III in 1779, at Coalbrookdale in Shropshire, and the bridge still stands today, though it is no longer in use for vehicular traffic. Its span is just over 30m (100ft) with a rise of 15m (50ft) at the centre. The main structural members are five semi-circular ribs made up of castings about 21m (70ft) long, a remarkable size and weight for the time. The structure weighs 386 tonnes. The arch bridge has become something of a standard both in steel and in reinforced concrete in motorway networks in many countries. Standardization of design has done much to cut costs, an important factor when bridge construction accounts for some 25–30 per cent of total motorway expenditure. Suspension bridges have, until recent years, been much the prerogative of American engineers, notably John Roebling and his son, Washington A.Roebling, the first to use spun cables, first at Pittsburgh and across the Niagara Gorge in 1855. John Roebling did not live to see the completion of his greatest achievement, the 470m (1539ft) span of the Brooklyn Bridge 40.5m (135ft) high over the East River in New York, opened in 1883 after fourteen years in the building. The Roeblings triumphed, but at some cost personally. The father died in 1869, three weeks after a boat crushed his right foot. His son, in 1872, was paralysed by ‘caisson disease’ and, deprived of the use of his voice, he directed the work, with his wife’s help, until its completion. The wire cables used were made of steel for the first time. The Golden Gate bridge in San Francisco, completed in 1937, was for many years the longest in the world with 1280m (4200ft) span and with towers 227m (746ft) high. In contrast, the suspension span at the Tacoma Narrows over the Puget Sound was only 853m (2800ft). Aerodynamically unstable, this bridge was also inadequately stiffened and, in 1940, only four months after it was opened, it oscillated in a gale at such a destructive amplitude that the main span fractured and fell to the water 63m (208ft) below. At the time, so great was the degree of twist, that one side of the roadway was 8.5m (28ft) above the other. The failure of this bridge, filmed at the time by an amateur cameraman who narrowly escaped being on the bridge when it failed, more than anything brought the attention of bridge engineers to focus on the stiffness of a structure and its aerodynamic stability and, as far as the box girder design was concerned, stressed the failures that occurred during building in South Wales and in Australia. It may appear surprising that James Finley had studied the stiffening of girders in suspension bridges (with chain cables) as early as 1801, a quarter of a century before Thomas Telford built his Menai suspension road bridge. Today great attention is paid to both stiffness and aerodynamic characteristics, particularly of long suspension bridges, as in the 1006m (3300ft) span of the Forth road bridge (1964), the 987m (3240ft) Severn bridge (1966) and the PART THREE: TRANSPORT 466 1410m (4626ft) of that over the Humber (1978). Inclined suspenders, alternately opposed, help to avoid motion of the deck—which in the case of the Severn bridge is only 30 (10ft) deep and in the Humber bridge only 4.5m (15ft). When completed, the Akashi-Kaikyo bridge, due in 1988, between the Japanese islands of Honshu and Shikoku will be the world’s longest span at 1780m (5840ft). Theory has shown that with present materials, techniques and knowledge, a bridge of some three times this length could be built today. Natural cement was used by the Romans and was rediscovered by a builder on the Isle of Sheppey in 1796, whence it became known as Sheppey stone. A similar discovery was made by Canvass White near Syracuse, New York, in 1818. Artificial cement was patented in 1824–5 by a Leeds bricklayer, Joseph Aspdin, and was tested and promoted by Col. Charles W.Pasley of the Royal Engineers School at Chatham. Aspdin named it Portland cement after its outward resemblance to Portland stone. However, it was of little value for bridge building as, except in compression, cement and concrete have little strength unless reinforced against tension and bending with steel. The originator of reinforced concrete was Joseph Monier, a Paris gardener, who devised it for making flower tubs in 1867. He later patented its use for railway sleepers, floors, bridges, arches and other structures. One of the main problems with reinforced concrete is that it continues to harden for up to a year after casting; this process is accompanied by some shrinkage which can involve cracking, particularly if the structure is only hinged at the abutments in the case of bridges. A third hinge is needed at the centre of the span to overcome this. The Swiss engineer Robert Maillart was one of the first to realize this and hence the potential of this material for bridge building. He built a concrete arch over the River Inn in Switzerland in 1901. Joseph Melan of the Technical University in Vienna was another pioneer of reinforced concrete which he used in his longest span of loom (427ft) over the River Ammer in Bavaria in 1929. Prestressing of concrete has greatly improved its properties for bridge building. A Frenchman, Eugène Freyssinet, conceived the idea as early as 1904, but was unable to put it to practical use until high tensile steel was developed and became readily available and this was not until after the Second World War. High tensile steel wires running through ducts in the lower flanges of the arches were pulled very tight to exert the prestressing tension, after which the ducts were filled with liquid cement to prevent corrosion of the steel. Between 1948 and 1950, Freysinnet built five identical arch bridges over the River Marne in France, each having a single span of 74m (242ft). Since the Second World War prestressed concrete has played an important role in bridge building, first in rebuilding the great number of bridges destroyed in the hostilities. Later it has been invaluable in the construction of the numerous bridges and flyovers in the networks of motorways, autobahns and autostradas that span the islands and continents on both sides of the Atlantic and the Pacific. ROADS, BRIDGES AND VEHICLES 467 A glossary of terms used in bridge building Abutment a construction that takes the thrust of an arch or vault or supports the end of a bridge Arch a curved structure, usually in the vertical plane, that spans an opening or gap Beam a long, straight piece of wood, metal or concrete used as a horizontal structural member Caisson a watertight compartment or chamber, open at the bottom and containing air under pressure, used for carrying out work under water Caisson disease Divers working at higher than normal atmospheric pressure breathe in increased nitrogen, which is dissolved in the bloodstream. When they return to normal pressure, the nitrogen forms bubbles which, if collected in the capillary vessels, causes cramps (‘the bends’). If it collects in the joints, damage to nerve endings can cause paralysis, temporary or permanent Cantilever a beam or girder fixed at one end only; a part of a beam or structure which projects beyond its support Centring a temporary structure, usually of timber, which serves to support an arch under construction Coffer dam a watertight structure enclosing an area below water level, pumped dry so that construction work can take place Corbel a projection of timber or stone etc., jutting out from a wall to support a weight Falsework a temporary framework used during building Girder a substantial beam, usually made of iron or steel Keystone the central stone forming the top of an arch or a dome or vault (also called a quoin or headstone) Lintel a horizontal beam as used over a door or a window Pier a vertical member or pillar that bears a load Pile a column of timber, iron, steel or concrete, driven into the ground or river-bed to provide a foundation for a structure Soffit the underside of a structure such as an arch, beam or stair Spandrel the space between the shoulder of an arch and the surrounding rectangular moulding or framework, or between the shoulders of adjoining arches and the moulding above Springer part of an arch where the curve begins; the lowest stone of this Starling an arrangement, usually of piles, that surrounds a bridge pier to protect it from erosion caused by scouring, debris etc. Truss a structural framework of wood or metal, usually arranged in a formation of triangles, forming a load-bearing structure Voussoir a wedge-shaped stone or brick used in the construction of an arch or vault. TUNNELS Tunnelling, for canals, roads and railways, is a relatively modern technique where any sort of mechanization is involved, although though irrigation tunnels, called qanats, are found in Iran and have existed for many hundreds PART THREE: TRANSPORT 468 of years. These tunnels, dug by the Persians and Armenians to bring water to the towns, are ten, twenty or more miles long and date from the eighth century BC in some parts. A number of vertical shafts were dug and then joined by horizontal tunnels. Similarly, manual labour was used in digging the canal tunnels in the systems created in Britain as well as on the continent of Europe from 1750 to 1850, even earlier in France. A total of some 69km (43 miles) of Britain’s canals are tunnelled, the longest tunnel being at Standedge on the Huddersfield Narrow Canal, this being 4951m (5415 yards) long. Mining tunnels date at least from Roman times and existed all over the empire wherever there were minerals to be exploited. Modern tunnelling, using more or less complicated machinery, originated in 1818 when Marc Isambard Brunel designed a machine copying the principle of working of the shipworm, Teredo navalis. The head of this mollusc is protected by two jagged concave triangular shells, between which a proboscis protrudes, like the centre pin of a carpenter’s bit. This equipment enables the shipworm to grind the hardest oak into a nourishing paste, while the petrified excreta, having passed through the animal, forms a smooth lining to the tunnel behind it. In Brunel’s machine, thirty-three miners working in cells in the box-shaped iron casing which was advanced by screw jacks, the tunnel being lined with bricks as the face moved forwards. The boring beneath the Thames between Rotherhithe and Wapping was the world’s first underwater tunnel, completed in 1841. It is 1093m (1200yds) long. The twin tunnels, originally used as roads, are still in use today as a railway. A second tunnel between Rotherhithe and Wapping was built in 1865 by Peter Barlow and James Henry Greathead, using a similar shield advanced hydraulically. Cast iron segments were used to line the bore, rather than the brickwork used by Brunel and in most of the earlier canal tunnels. Greathead’s pioneering work formed the basis for most of the subsequent London Underground Railway. Rotating cutting heads, hydraulically-operated arms for lifting and holding the cast iron (later steel) lining segments while they were bolted up together, and conveyors to remove the excavated spoil from the face were added subsequently to make the tunnelling machine almost entirely automatic, as it is today. Different companies, too, have gone into the manufacture of specialist machines for tunnelling through chalk, rock or sand in addition to the clay of Brunel’s and Greathead’s early work. The first British tunnel of any length was for a railway under the River Severn. Built between 1880 and 1893, it was over 6.9km (4.3 miles) long, 3.7km (2.3 miles) of which were under the water, and 76 million bricks were used in the tunnel lining. An alternative to boring a tunnel through the solid ground for a river crossing is the immersed tube tunnel in which tubular sections are built up on land, towed out and sunk into position before the ends are removed. ROADS, BRIDGES AND VEHICLES 469 Rotherhithe, again, was the site of the first experiment in this method by Robert Vazie and Richard Trevithick between 1802 and 1808. They built 319gm (1046ft) of 1.5m (5ft) high by 0.9m (3ft) wide brick tunnel across the River Thames, but ran out of money before the work could be completed. However, they did establish the practicability of the system which has been much used since, notably for the John F.Kennedy tunnel under the River Scheldt at Antwerp. Five prefabricated reinforced concrete sections averaging 102.2m (335.3ft) long make up the 512m (1680ft) of the prefabricated part of the two three-lane roadways and the two-track railway tunnel, completed in 1969. The sections of an immersed tube tunnel are usually laid in a trench dug in the river bed. Hydraulic jacks are sometimes used to draw them together before they are connected to each other. The immersed tube has been likened to the underwater equivalent of the cut-and cover method of construction on land such as was used for some of the first underground railway lines in London as in other cities. Compressed air plays an important role in the tunnelling engineer’s equipment. In hard rock tunnelling it often provides the power for driving face- cutting tools or the percussive tools used for drilling holes in which to place explosive charges for blasting. In drilling the Mont Cenis tunnel, from 1860 onwards, a 170cu.m/min (6000cfm) air supply at 6 atmospheres (87 psi, 6kg/ cm 2 ) was obtained from water descending 50m (164ft) through cast-iron pipes, the entrained air being separated out through valves at the bottom of the fall. In this case ten hydraulic air compressors were used to obtain the supply, these being based on the principle of the Trompe or Trombe, originating in Italy in the sixteenth century, then much used for providing air supply to brass or iron smelting furnaces. A similar arrangement was used in drilling the St Gotthard tunnel in 1873. Hydraulic rather than pneumatic power was used when work commenced on the Ahrlberg tunnel in 1880. Thomas Doane’s mechanical air compressor, patented in 1866, was used on the Hoosac tunnel and started the seeds of an industry in which the Americans have long been among the leaders, together with Sweden where rock drilling is very much a part of life. When, in the early 1960s, the Mont Blanc road tunnel was being constructed between Aosta and Chamonix, air-operated drills were used mounted on a three-deck gantry, the drills being supported on hydraulically- operated booms, while the pumps which supplied pressure hydraulic fluid were driven by compressed air motors. Compressed air motors are often preferred to electric motors in mining and similar situations where there is a risk of fire or explosion. Where compressed air drills are used, the exhaust air is often used as an air supply for the operators as well as to ventilate and cool the tunnel while work is proceeding. Pressurizing a tunnel which is being dug under water also performs the useful function of keeping out the water, should there be a leak in the lining. PART THREE: TRANSPORT 470 The maximum pressure that can be safely used is about 50 psi (3.5kg/cm 2 ) equivalent to 33.5m (110ft) of water, without damaging effects to those working in the tunnel by ‘the bends’ (compression sickness or caisson disease). Today, the world’s longest traffic tunnel, i.e. not for water or sewage, is the 54km (33.5 mile) railway tunnel that runs between the islands of Honshu and Hokkaido in Japan. This even exceeds the probable length of the proposed Channel Tunnel. This project is hardly new. It started as early as 1802 when Albert Mathieu, a French mining engineer, first put forward plans for a tunnel under the English Channel between France and England. In spite of the interest of Napoleon Bonaparte this came to nothing, as did the plans of another Frenchman, a civil engineer named Aimé Thome de Gamond who devoted himself to the matter for thirty-four years from 1833. At the 1851 Great Exhibition in the Crystal Palace in Hyde Park he tried to raise capital for the venture but abandoned it on account of its cost, at that time estimated as £33,600,000. The debate has persisted through the years, many more schemes being put forward. A Labour government approved the project in the 1970s, but cancelled the contract after a tunnelling machine had been positioned below ground on the English side. In 1986, the governments of Great Britain and France signed a treaty for the building of a 50km (31 mile) double rail tunnel, financed by money from the private sector. Initial estimated costs were something over £2000 million. Much has been made of the difficulty of adequately ventilating a road tunnel of the usual noxious fumes, largely carbon monoxide, emitted by the internal combustion engine, so the idea of an electrified railway, carrying road vehicles has prevailed. EARTHMOVING AND ROADBUILDING MACHINERY Man has performed prodigious feats of construction without the help of any machinery. The pyramids of Ancient Egypt were built entirely with slave labour. Silbury Hill, in Wiltshire, England, is some 40m (131ft) high and covers an area of 2ha (5 acres). Archaeologists estimate (using radio-carbon dating methods) that it was built about 2600 BC with nothing but picks made from the antlers of deer, shovels and scrapers from the shoulder blades of oxen, and woven baskets to remove or shift the spoil, very nearly 14,000 tonnes of which had to be moved. Throughout most of the railway age, cuttings, embankments and tunnels were entirely man-made, sometimes with horses to remove the spoil. A railway navvy was expected to shift 20 tonnes of earth a day in a 14-hour shift—and did so. The mechanical digger or steam shovel originated in the United States in the 1830s, William Otis exporting his second and third machines to Russia and a fourth to England in 1840. The American invention made little impact on British contractors, although it was accepted from the start in its country of ROADS, BRIDGES AND VEHICLES 471 origin. Probably the first British contractor to use a mechanical digger was Thomas Brassey, who had contracted in 1852 to build the Grand Trunk Railway of Canada, from Quebec to Lake Huron. To make up for the shortage of local labour, 3000 navvies were imported from England but cholera and frostbite took a high toll and many returned home in spite of the high wages that Brassey was paying. In about 1854, Brassey resorted to the American invention which enabled him to complete the line by 1859. Trunk road and motorway construction today involve vast investment in mechanical plant, yet the speeds of construction differ remarkably little from those of the railway era. The wage bill is, however, greatly reduced. Diesel- powered internal combustion engines have superseded the steam engine. The variety of machines employed is extensive, from a number of different types of excavator to quite small back-hoes and front-end loaders and the serviceable dump-truck. Both caterpillar-tracked and wheel-mounted versions of most machines are in use, depending on the nature of the terrain. Like the steam shovel, most types of earth-moving machinery have originated in the United States where, historically, there has long been a shortage of labour. This is a field in which the USA excels in both production and usage as well as generally building the largest and most powerful machines in the world, although these are often used for other purposes, such as opencast mining, rather than road-building. The world’s largest land machine, for instance, ‘Big Muskie’, weighing some 12,000 tonnes, is a dragline excavator with six 5000hp motor generator sets and a bucket of 168 cubic metres (220 cubic yards) capacity. The biggest tractor shovel is the Clark 675 with twin turbo- charged diesel engines, each of nearly 450kW (600hp). Its 18 cubic metre (24 cubic yard) bucket reaches a height of 6.5m (21.3ft). The dump truck record, the Terrex Titan, will carry 350 short tons (324 tonnes) and is powered by a 3300hp engine. Its ten wheels have 3.35m (11ft) diameter tyres. The Marion Power Shovel Company make the biggest bulldozer, weighing 136 tons (126 tonnes) and having an engine of g84kW (1320hp). In the last 25–30 years a new earth-moving device has come into service. The bucket wheel excavator is a product of Europe rather than America, Orenstein & Koppel of Lübeck in West Germany being the foremost makers. At the front of this track-mounted machine is a long boom carrying a rotating wheel which has buckets spaced around its periphery. Along the boom runs a conveyor belt, while a further boom and conveyor, able to be independently swung, carries the spoil back from the central tractor mounting. The largest bucket wheel excavator O & K make is reputed to be able to shift 198,900 cubic metres (260,600 cubic yards) a day with a 21.5m (71 ft) diameter wheel, 5 cubic metre (6.5 cubic yard) capacity buckets and a drive of 2.5MW (3350hp). This machine is generally too powerful for road construction. Early earth-moving machines were mostly employed on railway works and hence were rail-mounted. Attempts were sometimes made to spread the load of . centuries did the Christian Church take on the responsibility for the upkeep of some of the existing roads and bridges and, occasionally, the erection of new ones. The span and strength of the beam. forward plans for a tunnel under the English Channel between France and England. In spite of the interest of Napoleon Bonaparte this came to nothing, as did the plans of another Frenchman, a civil. between the shoulder of an arch and the surrounding rectangular moulding or framework, or between the shoulders of adjoining arches and the moulding above Springer part of an arch where the curve