PART FIVE: TECHNOLOGY AND SOCIETY 882 He also designed a novel triple structure to give stability to his hemispherical outer dome and flatter interior one. His outer dome is constructed of wood covered with lead. Inside this is a pyramidal brick cone which supports the lantern, while the inner decoratively painted masonry dome is seen from the interior. Wren’s colonnaded drum with its continuous entablature supports the dome better than Michelangelo’s drum at St Peter’s but, even so, encircling iron chains were used, now replaced by two stainless steel girdles. Modern domes The traditional domical covering has largely been abandoned in modern building but the technical development in materials—of plastics, steel and reinforced concrete—have led to a wide variety of curved shell coverings to large structures, particularly for auditoriums, exhibition halls and sports stadiums. Such shells, made of reinforced concrete, may be used to span a space as great as Brunelleschi’s Florence Cathedral but need to be only a few inches in thickness. For example, in 1957 in Italy, Pier Luigi Nervi, who had been experimenting since 1935 with reinforced concrete, employed it to cover his Palazzo dello Sport in Rome. In this building great ‘Y’-shaped structures transmit the weight and thrust of the dome to the ground. The geodesic dome, invented by R.Buckminster Fuller and built for the United States display pavilion at the Montreal Expo 67, was almost spherical and had a maximum span of 76m (250ft). It was constructed of short struts of light metal (this is usually aluminium) and was covered by a plastic skin. The short structural elements were interconnected in a geodesic pattern, that is, by the shortest possible lines. Such domes, ideal for use as factory buildings, will withstand high winds and are very lightweight and may be quickly erected. CANTILEVER CONSTRUCTION In this type of structure the building element—block, beam or girder—is fixed securely at one end by a downward force behind a fulcrum and carries a load at the other, free, end; or the load might be distributed evenly along its whole length. When such a cantilever beam bends under its load, its surface becomes convex, unlike a similar beam secured at both ends which would curve in a concave manner under load. An early example of cantilever building is in the use of a corbel. The single corbel is in the form of a block of stone (or other permanent material) part of which is fixed into a wall. The projecting part of the block may then be used to support a vertical strut or curved brace as in a timber-trussed roof (see p. 886). A development of this was in the corbelled dome or squinch (see p. 880). In BUILDING AND ARCHITECTURE 883 this type of construction each successive layer of building stones projects further from the wall in steps; it is the weight of the stones above and behind (the fulcrum in this case) which maintains the stability of the corbelling. Such corbelled domes were being built as long ago as 2500 BC in Sardinia and this method of building was used in the famous tholos tombs at Mycenae in Greece: the so-called Treasury of Atreus there dates from about 1330 BC. The major use of large cantilever spans has been in bridge building. In the simpler of such structures the metal girders extend from the piers or towers, where they are firmly fixed, to meet and join and so produce a continuous deck for the roadway. Such a constructional method is often used where a fast- flowing river or soft subsoil makes the erection of piers difficult. In these cases a cantilever beam structure can be extended from each bank of the river to join in the centre. In a more complex design there is an anchor span between the bank and the tower, a cantilever arm extending beyond the lower and a suspended span joining this cantilever arm to another such arm which is projecting from the next tower. These metal girders form a truss framework for the bridge structure. Probably the best-known example in Britain of such a cantilever construction is the railway bridge built over the Firth of Forth near Edinburgh in Scotland between 1882 and 1890. Cantilever construction is also widely used in staircases and balconies. The beautiful, so-called geometrical staircases of eighteenth- and nineteenth-century- date, often built on circular or elliptical plan, were constructed with one end of each step built securely into the wall of the well and the other resting upon the step below. Iron balconies of the same period were supported upon cantilever brackets similarly affixed into the wall at one end. The cantilever is extensively employed in present-day structures because of its suitability for use with reinforced concrete and steel materials (see p. 900). In a steel-framed building the metal beams may be allowed to project, to carry the glass curtain-walled façades. Concrete balconies are carried on slabs extended outwards from the interior ones. ROOFING A roof is the supporting framework and the covering of a building which shelters the occupants from the weather. In areas where the need is for protection from rain, wind and snow, as in northern Europe, the pitched or sloping roof was evolved. In general, the further north the region the steeper the pitch, so that snow may more easily be thrown off. There have developed a variety of different designs of sloping roof of which there are six principal types: gable, hipped, lean-to, mansard, gambrel and helm. PART FIVE: TECHNOLOGY AND SOCIETY 884 Roof shapes The simplest of the sloping roofs is the gable pattern, also known as a saddleback roof. This is the type seen in the classical temples of ancient Greece where the roof slopes down on each side from the ridge and terminates at the ends in a gable or, as in classical architecture, a pediment. In the warm Mediterranean climate of Greece, only a low pitch was necessary. Before the end of the seventh century BC the pitch had to be steeper because the roof covering was of thatch or clay but, from this time, with the adoption of roofing tiles, a shallower pitch became possible. It is notable that the Greek pediment is a lower, more elegant shape because of this roof pitch than the Roman and, particularly, the Renaissance designs of northern Europe. The hipped roof, which had also evolved by ancient classical times, is different from the gable in that the ends slope downwards to the cornice instead of finishing in gables. A lean-to is a single sloping roof built against a wall which rises above it. In Europe a gambrel roof ends in a small gable at the ridge though in America, the term is applied to a different type of roof which has a double pitch. This is more like the mansard roof where, in the double slope, the lower one is of a steeper and longer pitch than the upper; the roof is named after the seventeenth-century French architect François Mansart. The helm roof is a church steeple design. The square tower rises to four sloping faces meeting in a point at the top and gabled at the bottom. Flat roofs have been constructed since the most primitive of times. These were suited primarily to countries which had a hot dry climate and were the usual design for all kinds of structures from palaces to simple huts. In ancient times they were extensively built in Mesopotamia, the Middle East, Crete, Mycenae and classical Greece. The flat roof continued to be traditional in such areas over the centuries but was less suited to northern climes because of a tendency to leak in wet weather. In hot climates the flat roof was not only trouble-free in this respect but could provide extra sleeping space on hot nights. With the use of modern building materials and modern methods of construction and sealing, the flat roof has been widely adopted in all areas during the twentieth century, particularly for industrial and commercial buildings. Roofing materials Since early times a wide variety of materials have been in use for covering the exterior surfaces of roofs. These include thatch and wood shingles, metals such as lead or copper and slates or tiles. The Romans roofed with slate, which is particularly easy to split along its natural laminae, and it has been a common material for this purpose ever since. Sandstone and limestone of a fissile character have also been utilized for this purpose. BUILDING AND ARCHITECTURE 885 Tiles, like bricks, are a ceramic material fired in a kiln; they are fired to be harder and smoother than bricks. Roman roof tiling generally incorporated two types of tile, the tegula, the flat tile and, covering the joints between these, the rounded imbrex tile. In later tiling in Britain the flat tile was most common but the pantile, the large curving ‘S’-shaped tile, which was common in Europe especially the Mediterranean region, was imported from Flanders in the 1630s and has been manufactured in Britain since the early eighteenth century. Twentieth-century development of reinforced concrete as a building material (see pp. 891–3) has led in modern times to striking innovations in roof shapes. The parabolic or paraboloid form has been experimented with and constructed by engineers such as Pier Luigi Nervi in Italy. Such designs were based upon the form of the parabola which may be defined as ‘the path taken by an object which, when expelled or thrown, travels originally at a uniform speed to fall eventually to earth under the influence of gravity’. The parabolic curve was first defined and named by Apollonius of Perga (modern Turkey) about 225 BC. A paraboloid roof is one some of whose plane sections are parabolas. A variation upon this is the hyperbolic paraboloid roof which is constructed in a double curve of hyperbolic form, one of which is inverted. PLASTER During the Middle Ages plaster, for both exterior and interior wall covering, was made from lime, sand and water mixed with various other ingredients which, trial and error over a long period of time had shown, would help to bind the mixture and prevent cracking. These ingredients included feathers, hair, blood, dung, straw and hay. Plaster of Paris, the best quality plaster obtainable, was introduced into England about 1255–60. It was made by burning gypsum (calcium sulphate) and mixing it with water; this gave a finer, harder material. It was called plaster of Paris in England because at that time in France the chief source of supply of gypsum was in the Montmartre district of Paris. Plaster of Paris was reserved, in England, for fine finishes in important buildings because, as an imported material, it was expensive. However, before long, gypsum deposits were discovered and worked in England, chiefly on the Isle of Purbeck, Dorset, and in the valleys of the River Trent in Nottinghamshire and the Nidd in Yorkshire. The beautiful decorated plaster ceilings and walls of English interiors of the sixteenth to eighteenth centuries were created by craftsmen who learnt their trade from Italian stuccoists. Such Renaissance plasterwork had evolved in fifteenth-century Italy where the craftsmen had long experimented with the medium and had developed a type of plaster which had been used by the Romans. This was malleable and fine, yet set slowly, giving time for the design to be worked; when set it was very hard. The Italians called it stucco duro. It PART FIVE: TECHNOLOGY AND SOCIETY 886 contained lime and some gypsum but also powdered marble. Elizabethan stuccoists in England also experimented with this plaster and empirically found that the addition of certain other ingredients would improve the mix. Such ingredients, which varied from area to area, included beeswax, eggs, ale and milk. During the eighteenth century, when decorative plasterwork became more delicate and lower in relief, a number of patent stucco compositions were marketed. The makers of such plasters claimed that the materials were finer, harder and easier to work. Mr David Wark patented one such plaster and Monsieur Liardet (a Swiss clergyman) another. The firm of Adam Brothers purchased both these patents and in 1776 obtained an Act of Parliament which authorized them to be the sole makers and vendors of the material. They called it ‘Adam’s new invented patent stucco’. By 1800 improvements were being made to stucco to make it more robust for exterior work. The fashion was to build in brick, then face this material with stucco and paint it to imitate a costlier stone building. John Nash, the designer of the Regent’s Park Terraces, developed such a stucco made from powdered limestone sand, brickdust and lead oxide. Later cement was added to plaster for exterior work. By 1840 several patent gypsum plasters were being sold: Keene’s Cement was one such. In the twentieth century, soon after the First World War, a plasterboard was being made for interior use, chiefly for ceilings. There was an acute shortage of plasterers immediately after the war and this board became very popular. It was made in panels which consisted of a layer of gypsum plaster sandwiched between sheets of strong paper. As the plasterers returned to work the use of this plasterboard declined. The Second World War brought an even more acute shortage of both plaster and craftsmen. Plasterboard reappeared on the market and this time was a greatly improved product so leading to its widescale use. The new material could be applied easily and directly to brick or concrete surfaces, joints were almost invisible and improved thermal insulation was achieved by adding a backing of aluminium foil. CONCRETE AND CEMENT Concrete is one of the oldest materials made by man. It is composed of four major ingredients: sand, stone, cement and water. The first two of these are comparatively inert and it is the last two which are the active ingredients. When all four are mixed together a wet plastic substance results and this can be applied and moulded as desired. The cement and the water cause a chemical reaction which gradually transforms the mix into a rock-hard conglomerate. The world concrete derives from the Latin concretus, grown or run together. BUILDING AND ARCHITECTURE 887 The stone content may be in the form of pebbles or flints, crushed rock or broken-up materials of many kinds: clinker, cinder, slag, bricks, old concrete. Especially suitable are granite, hard sandstone and rounded pebbles found in natural gravel beds. In early times the word cement referred to a mixture of broken stone held loosely together by a binding material of lime, clay, burnt gypsum and sand. The word derives from the Latin caementum, rough stones, rubble, building material. This ‘almost concrete’ substance preceded the making of true concrete in which the source of the cement was primarily quicklime (calcium oxide), usually obtained by burning pulverized limestone. This process was called calcining. It is not precisely known when concrete was first made as the early development of cementing agents produced a mortar which was insufficiently strong to bind the whole mass durably together and, being friable, these materials have not survived. The oldest example of concrete discovered to date was made about 5600 BC. Concrete was certainly made and used in ancient Egypt: wall paintings exist which depict these processes and fragments have been found in a number of places where it acted as an infill mortar in stone walling. The Greeks also made a lime or burnt gypsum mortar for walling with sun-dried bricks. It was the Romans who developed the use of concrete in a structural manner, not merely as an agent for binding and covering. It appears likely that their first use of the material was in emulation of Greek builders and examples of their work have been found which date from about 300 BC. Towards the end of the second century BC the Romans experimented with a volcanic earth which they found in the region of the town of Pozzuoli near Mount Vesuvius. Because the earth was like a reddish-coloured sand, the Romans took it for sand and mixed it with lime to make concrete. This concrete was of exceptional strength and hardness because the pozzolana, as it is called after its town of origin, is in reality a volcanic ash which contains alumina and silica and this combines chemically with the lime to produce a durable concrete. It is probable that the Romans were unaware of the reason why their new concrete was so superior, but doubtless their engineers and architects quickly comprehended its value. They experimented with the material and discovered that it would set as well under water as in air. They mixed it with an aggregate of broken stone and brick and built structures of concrete and brick faced with marble. By the first century AD the Romans were experts at handling concrete and were taking full advantage of its possibilities as a structural medium. It enabled them to span and roof spaces of great magnitude, achievements which were not equalled until the introduction of steel construction in the nineteenth century. They built aqueducts, breakwaters, bridges and massive foundation rafts, and vaulted their great building interiors. They experimented with the reinforcement of concrete with bronze strips and dowels in order to improve its tensile strength, but these experiments were not very successful because the higher rate of thermal expansion of the metal caused cracking in hot weather. PART FIVE: TECHNOLOGY AND SOCIETY 888 The Romans built walls and vaults of great thickness in this hard, durable pozzolana concrete which would resist any stress, but these walls were so heavy that the engineers needed to develop a system of lightening the structure by the introduction of relieving arches and earthenware jars into the wall. They also mixed a quantity of light porous volcanic rock into the aggregate. Concrete was utilized for all possible needs, from floors upon which mosaic was laid to the construction of Hadrian’s Wall. Pozzolana concrete was widely used for building in Europe for some 800 years. It is due in no small measure to the extent of building in this material that so much of the work survives. The Romans had developed the use of concrete as an immensely important structural medium yet, after the collapse of the western half of the empire in the fifth century AD, the greater part of this knowledge and experience was lost for about 1300 years. The making of concrete did not die out, but its use was mainly confined to infilling in walling and as a foundation material. The Normans built concrete foundations to many castles and medieval masons did the same in ecclesiastical architecture. The thirteenth-century Salisbury Cathedral is a notable example. It was in the mid-eighteenth century that builders began to search for a means to make a cement which would set quickly and be strong and durable. Pozzolana and lime mixtures had been imported from Italy by English builders since the sixteenth century but the secret of Roman concrete had not been understood. The English engineer John Smeaton was successful in his researches to find a satisfactory cement for the new Eddystone lighthouse situated on a rocky reef in the English Channel some 22.5km (14 miles) off Plymouth. The two previous lighthouses here had been constructed of wood and Smeaton was commissioned in 1756 to build a stronger one of stone. His problem was to make a hydraulic cement (one which would set under water) as Roman pozzolana cement had done; this was essential to cement the stone blocks together in order to withstand the onslaught of the sea in such an exposed position. Smeaton found that the best results could be obtained by using lime obtained from a limestone containing an appreciable quantity of clay. The limestone he used came from Wales and he added to this some Italian pozzolana. The lighthouse, built in 1759, lasted until replaced in 1876. Smeaton published his researches, A Narrative of the Eddystone Lighthouse, in 1791. Further experimentation was carried out in a number of European countries in the succeeding 60 years. James Parker, an Englishman, patented his Roman cement in 1796. This was made in Kent at Northfleet by calcining stones found on the beach on the Isle of Sheppey. The stones, limestone with a high clay content, were burnt at a high temperature then powdered. In France the engineer Louis J.Vicat began his researches into French limestone in 1812. His publication of these show that he discovered that the best hydraulic lime derived from limestone containing clay, so producing alumina and silica. He experimented with the use of different clays, adding these to slaked lime and BUILDING AND ARCHITECTURE 889 burning the mixture. Other investigators followed in experimentation into making artificial cements from clay and chalk. One of these was the Englishman James Frost who patented his British cement, made at Swanscombe in Kent, in 1822. Joseph Aspdin, an English bricklayer, patented a better, more reliable product in 1824; he called it Portland cement. This cement was also made from a mixture of chalk and clay but was calcined at a higher temperature before being ground. Aspdin used the name Portland because he thought that it resembled Portland stone in colour. His cement works were in Wakefield in Yorkshire but his son set up another works at Rotherhithe on the Thames. Modern Portland cement is quite different from Aspdin’s, having resulted from many later improvements. A major one of these was achieved in 1845 by Isaac Charles Johnson, manager of the White & Sons Cement Works at Swanscombe, in Kent. In his process the raw materials were burnt at a much higher temperature until the mass was almost vitrified. The resulting clinker was then finely ground to produce a cement far superior to Aspdin’s original product. Between 1850 and 1900 concrete was gradually employed more widely for building purposes, but greater use of the material was inhibited by the high cost of making Portland cement. Production methods were slowly adapted to reduce this cost. One of these was to replace in the 1880s the millstones used for grinding the clinker by iron mills. Later, these in their turn were replaced by tube mills filled with pebbles and, in a further advance, steel balls replaced the pebbles. Early nineteenth-century kiln design was also a handicap in reducing the costs of making the cement. Parker had made his Roman cement in a bottle or dome kiln of the type which had traditionally been used to burn lime. Isaac Johnson introduced a horizontal chamber kiln which was an improvement and this was bettered by a shaft kiln which enabled the raw materials to be burned continuously in a vertical shaft. However, the cost of manufacture was most sharply reduced by the introduction of the rotary kiln which is the basis of modern design. An early example of rotary kiln was introduced in 1887 at Arlesey in Hertfordshire to the designs of Frederick Ransome. It was 8m (26 ft) long, small indeed compared to a modern kiln which would probably exceed 90m (300 ft) in length. Reinforced concrete Plain concrete is strong in compression and so resists the weight of a load placed upon it, but weak in tensile strength: it has a poor resistance to forces attempting to pull it apart. As a result if such concrete is used to manufacture beams, arches or curved roof coverings, the outward and downward thrusts will cause the material to fail. When the Romans used concrete to construct PART FIVE: TECHNOLOGY AND SOCIETY 890 their immense vaults and domes they built massively and the concrete was poured into brick compartments which provided the necessary strength. Similarly arch voussoirs were of brick. If concrete is reinforced by metal bars, wires or cables embedded in it, the elastic strength of the metal will absorb all tensile and shearing stresses and so complement the high compression resistance of the concrete to provide an immensely strong constructional material; Roman experimentation with bronze for this purpose had little success (see p. 887) but similar trials in the late eighteenth century using iron were very satisfactory because, over the normal range of temperatures, the iron and the concrete displayed the same coefficients of thermal expansion and contraction. In modern construction steel has replaced iron. Using the steel as reinforcement economizes in the utilization of this expensive material. The concrete, which shrinks around the reinforcement as it sets, grips the metal firmly, so protecting it from rusting and weathering. It also reduces the fire hazard, for the steel, which would warp and bend if it became sufficiently hot, is protected within its concrete casing. The theory of reinforcing concrete with iron to increase its tensile strength was advanced in late eighteenth-century France. Rondelet, Soufflot’s collaborator in building the Church of Geneviève in Paris (the Pantheon), experimented by using a metal-reinforced mortar and rubble aggregate. In England, Sir Marc Isambard Brunel constructed an experimental arch in building his Thames tunnel of 1832. He incorporated strips of hoop iron and wood into a brick and cement arch. Experimentation with metal reinforcement was carried out in several countries during the nineteenth century. In England William B.Wilkinson took out a patent in 1854 for embedding wire ropes or hoop iron into concrete blocks for building fireproof commercial and industrial premises. It was, however, the French who were the chief pioneers in this field. Joseph Monier experimented with large garden tubs reinforced with wire mesh, then developed a system (1877) of reinforcing concrete beams. He also took out patents for reinforcing bridges and stairways. François Coignet exhibited a technique of reinforcing with iron bars at the Paris Exposition and François Hennébique developed hooked connections for reinforcing bars in 1892. By the late nineteenth century reinforced concrete construction was developing speedily in America, Germany and Austria. As early as 1862 a patent had been taken out for filling a cast-iron column with concrete. In 1878 steel rods were being inserted into concrete columns. Soon after 1900 a stronger column was being produced by inserting spiral steel hoops. Reinforced concrete was being widely used for constructing bridges: an early example was built in Copenhagen in 1879. Longer spans were being handled in the USA and France; the bridge of 1898 at Châtellerault in France is an example. In Britain reinforced concrete was being developed more slowly and plain concrete was more widely used during the nineteenth century. Pre-cast BUILDING AND ARCHITECTURE 891 concrete had been available since about 1845 for paving and balustrades. In 1875, William Lascelles patented a system for building houses from pre-cast concrete slabs fixed to a wooden framework; panels for walls and ceilings could be cast with relief decoration. Plain concrete was being used for building factories and bridges. The Glenfinnan Viaduct in the Scottish Highlands, which carried the railway from Fort William to Mallaig, is a survivor of this type of design. It was built in 1897 by Robert McAlpine and has 21 massive concrete arches. J.F. Bentley also used plain concrete in the massive Roman manner in the domes of his Westminster Cathedral in London. In the early years of the twentieth century Britain slowly adopted reinforced concrete construction, a process speeded up by London offices being opened for business by the two French pioneers, Coignet and Hennébique. Most of the early building was industrial. The first reinforced concrete frame building was the Royal Liver Building begun in Liverpool in 1908. Pre-stressed concrete The next advance in the development of concrete as an important structural material came just after the First World War. This was in the field of prestressed concrete which greatly increased the material’s potential. The purpose of pre-stressing is to make more efficient and economic use of materials. It makes possible the creation of slender, elegant forms which are at the same time immensely strong. The theory is that the metal reinforcement should be stretched before the concrete is poured into position and the pull maintained until the concrete is hard. The reinforcement is in the form of wire cables stretched through ducts and these are placed so as to create preliminary compressive stresses in the concrete in order to avoid cracking under conditions of load. To be successful pre-stressing requires high quality concrete and high-tensile steel. In the late nineteenth-century experimentation with the process, results were not very satisfactory owing to the quality of the materials. The pioneer of the process was the French engineer Eugène Freyssinet. As an army engineer he had experimented with concrete bridge building and, after 1918, he formed his own company to continue his development. In 1934 he achieved fame when he saved the Ocean Terminal at Le Havre from sinking into the mud under the sea by building new foundations of pre-stressed concrete. After 1945 steel was in limited supply and architects turned more and more to reinforced and pre-stressed concrete. Improved technical understanding had shown this to be a material with almost unlimited structural possibilities. It was not costly and the needs of an individual project could be exactly calculated in terms of reinforcement and strength of concrete. It could be cast into any form and this gave a greater freedom of architectural expression. . building is in the use of a corbel. The single corbel is in the form of a block of stone (or other permanent material) part of which is fixed into a wall. The projecting part of the block may then be. join in the centre. In a more complex design there is an anchor span between the bank and the tower, a cantilever arm extending beyond the lower and a suspended span joining this cantilever arm to another. lower, more elegant shape because of this roof pitch than the Roman and, particularly, the Renaissance designs of northern Europe. The hipped roof, which had also evolved by ancient classical