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Text 4. From the History of Architectural Structures
Architecture had its beginning when early humans first fashioned caves or lean-to shelters for their families. Architectural drafting and design began when these people first drew the outline of a shelter in the sand or dirt and planned the use of existing materials. As structures became more complex, the need for more complete drawings became necessary. But these architectural plans are far below today's standards. Their drawings are crude, and their measurements are not accurate.
An architect uses the knowledge gained from past centuries when designing a building today. The history of architectural design is directly related to progress in other areas of learning. For example, architecture has relied heavily upon the advancements of science and mathematics. From these advancements came new building materials and building methods. New engineering developments and new building materials have brought about more changes in architectural design in the last 30 years than had occurred in all the earlier history of architecture. Yet, many of the basic principles of modern architecture, such as bearing-wall construction and skeleton- frame construction, have been known for centuries. Even today, architectural structures are divided into two basic types, the bearing- wall and the skeleton-frame.
Bearing-Wall Construction.Bearing walls are solid and support themselves and the roof of a structure. Most early architecture, used the bearing wall for support. In fact, one of the first major problems in architectural drafting and design involved the bearing wall. The problem was how to provide openings in a supporting wall without sacrificing the needed support. One of the first solutions to this problem was the development of the post-and-lintel. In this type of construction, posts large enough to support the lintel (upper horizontal beam), wall, and roof above are used.
The ancient Greeks used post-and-lintel construction to erect many of their beautiful buildings. Most ancient people used stone as their primary building material. The great weight of the stone limited the application of post-and-lintel construction. Furthermore, stone post-and-lintel construction could not support wide openings. Therefore, many posts (columns) were placed close together, to provide the needed support. The Greeks and Romans developed many styles of columns and gave names to them. The various styles of column designs were known as orders. The orders of architecture developed by the Greeks are known as the Doric, the Ionic, and the Corinthian. Later, the Romans developed the Composite and the Tuscan orders.
Since the Greek climate was well suited to open-air construction, the Greeks used the post-and-lintel technique to great advantage. The Parthenon is a classic example of Greek use of the post-and-lintel.
Oriental architects also made effective use of the post-and-lintel. They were able to construct buildings with larger openings under the lintel because they used lighter materials, such as wood. The use of lighter materials resulted in the development of a style of architecture that was very light and graceful. The Oriental post-and-lintel designs were also used extensively for gates and entrances.
The Arch.The Romans began a new trend in the design of wall openings when they developed the arch. The arch is different from the post- and-lintel because it can span (extend over) greater areas without support. It is easier to erect because it is made from many smaller, lighter pieces of stone. The principle of the arch is that each stone is supported by leaning on the keystone in the center. The keystone is shaped like a wedge and locks the other stones in place.
The Vault.The simple arch led to the development of the vault. The vault is simply a series of arches that forms a continuous covering. This development allowed the use of the arch as a passageway rather than as just an opening in a wall. The cross vault is the intersection of two barrel vaults. The barrel vault and the cross vaultwere popular Roman construction devices.
The Dome.The dome, is a further refinement of the arch. The dome is made of arches so arranged that the bases make a circle and the tops meet in the middle of the ceiling. The Romans felt that the dome gave a feeling of power. Therefore, they used domes often in religious and governmental structures.
The Gothic Arch.Gothic architecture originated in France. It spread throughout western Europe between 1160 and 1530. Another variation of the arch, the pointed arch, was developed in Gothic architecture. The pointed arch (Gothic arch) became very popular in Gothic cathedrals because it created a sense of reaching and aspiring by its emphasis on vertical lines. Construction of the pointed arch posed the same problem as did conventional arches, that of spreading at the bottom.
To support the arch at the bottom, a new device known as a buttress was developed. Buttresses were gradually moved up the walls and resulted in the development of the flying buttress.
Text 5. New Construction Methods
The development of new materials is usually not possible without the development of new construction methods. For example, large glass panels could not have been used in the eighteenth century even if they had been available, because no large-span lintel-support system had been developed. Only when both new materials and new methods exist is the architect free to design with complete flexibility.
Present-day structures are usually a combination of old and new. In a modern building, examples of the old post-and-lintel method may be used together with skeleton-frame, curtain-wall, or cantilevered construction.
Skeleton Frame.One of the first methods developed to employ modern materials makes use of the skeleton frame. This kind of construction has an open frame to which a wall covering is attached. The frame provides the primary support, and the covering provides the needed shelter. The skeleton frame became popular with the development of framing materials and wall coverings that are light, strong, and usable in a variety of ways. The skeleton frame is now commonly used in family dwellings, and in commercial buildings. When steel is used for the skeleton, the skeleton frame is known as steel-cage construction.
The use of the skeleton frame, as opposed to bearing-wall construction, has given architects new opportunities. They can now design a structure without direct vertical-line outside base support. In this new type of construction, called cantilever, the loads are supported at only one end. Steel is well suited to cantilever construction because loaded steel beams, supported at only one end, can be extended farther without sagging than can any other material.
Since loads in steel-cage construction are not supported by the outside wall, curtain walls are possible. In this type of building, known as curtain-wall construction, a steel cage is erected, forming the shape of the building. The curtain wall, or skin, is added last. This curtain has no structural relationship to the stability of the building; it acts only as a protection from the weather. Therefore, the curtain wall can be made of materials with little or no structural value, such as glass, sheet metal, or plastic.
Shapes.For centuries, architectural development has been restricted by the use and overuse of the square and the cube (right angles) as the basis for most structures. Architects are now using other shapes such as the, octagon, pyramid, pentagon, circle, and sphere. This has come about with the development of materials that are stronger, lighter, and have a variety of uses. New construction methods also enable architects to design buildings that are completely functional (able to fulfill all needs) without reference to any basic geometric form. Many forms are now possible, and even the basic shapes of floor plans can be drawn to meet a variety of needs.
Sizes. New technology uses knowledge gained from advances in science. One of the most striking results has been the use of new materials and new methods to design and build structures of size greater than ever before. The Sears Tower in Chicago is now the tallest building in the world. But as technology develops even more, buildings can increase to sizes previously thought impossible. Frank Lloyd Wright once proposed a mile- high skyscraper. Ten such structures would house the working office staff of all New York City. Six would suffice for Chicago. The proposed skyscraper would tower far above the largest structures of today.
Who can say what will be possible? The idea of building a geodesic dome over central Manhattan, in New York City, certainly seems impossible at the moment. But remember that landing humans on the moon, and flights to and landings on distant planets, also seemed impossible not many years ago.
Location. Today, architects not only design buildings of enormous size but can also choose locations for buildings that were unthought of years ago. Further advances in transportation and architectural engineering will make even more difficult locations not only possible but workable.
Text 6. Portland Cement
Portland cement is made by heating an intimate mixture of chalk and clay to a white heat (temperature of incipient fusion) and, after the resultant clinker has cooled, grinding it to extremely fine powder.
Portland cement is usually made by the wet process. The chalk and clay mixed with water are reduced to a creamy consistency in washmills, circular tanks in which a central vertical spindle carries a rotating steel framework to which are suspended heavy harrows with projecting teeth. The circumference is fitted with gratings, and the process continues until the creamy liquid called slurry is able to pass the screen. It passes then to a second and third washmill, fitted with screens of ever smaller mesh, until after passing the last washmill only 5 per cent is retained on a sieve of 32,400 meshes to the square inch. During this time the chemists are making periodic tests to ensure the correct proportion of lime to chalk and adjusting when necessary. These proportions are vital.
The slurry is then passed to much larger tanks known as mixers, where it is kept stirred by rotating arms with vertical paddles until the kilns are ready to receive it.
The calcining or burning is generally done in rotary kilns. They are lined with firebrick and set at an angle of about 8 degrees to the horizontal.
The slurry is introduced at the top end by a rotating spoon feed, and gradually works its way down the kiln owing to the combined action of the slope and rotation. In so doing it meets the hot flames (the fuel, generally powdered coal, is introduced into the lower end) which pass up through the kiln and then to the chimney. In this way the slurry gets hotter as it descends and reaches the zone of maximum temperature (about 2,800 °F) some distance from the lower end, when chemical combination of the constituents takes place; all the water having, of course, long since been driven off.
At this stage the cement has formed itself into extremely hard nodules about the size of walnuts known as clinker, which now drops into a lower but parallel rotating and inclined tube where it is cooled from a white heat by meeting a current of air. This air is thus heated to about 600 °F and used for blowing with the powdered coal into the kiln, so economising in fuel.
The next process is the grinding of the clinker in tube mills (horizontally rotating cylinders) divided into three or four compartments. Each compartment contains exceptionally hard steel balls, which, when the mill rotates and clinker is introduced, are lifted and fall on to the clinker and so crush it. The cement passes from one compartment to another, and grinding continues. During grinding about 2 per cent to 3 per cent of gypsum is ground in to make the cement slow setting. The cement then goes to large circular silos, or into sheds where it is stored. The cement then has to be tested.
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