Construction
When masonry materials are stacked vertically, they are very stable; every part is undergoing compression. The real problem of construction, however, is spanning.
Ways must be found to connect walls so as to provide a roof. The two basic approaches to spanning are post-and-lintel construction and arch, vault, and dome construction. In post-and-lintel construction, lintels, or beams, are laid horizontally across the tops of posts, or columns; additional horizontals span from beam to beam, forming decks that can become roofs or be occupied as floors. In arch, vault, and dome construction, the spanning element is curved rather than straight. In the flat plane of a wall, arches may be used in rows, supported by piers or columns to form an arcade; for roofs or ceilings, a sequence of arches, one behind the other, may be used to form a half-cylinder (or barrel) vault; to span large centralized spaces, an arch may be rotated from its peak to form a hemispherical dome.
Post-and-lintel solutions can be executed in various materials, but gravity subjects the horizontal members to bending stress, in which parts of the member are in compression while others are in tension. Wood, steel, and reinforced concrete are efficient as beams, whereas masonry, because it lacks tensile components, requires much greater bulk and weight. Vaulting permits spanning without subjecting material to tension; thus, it can cover large areas with masonry or concrete. Its outward thrust, however, must be counteracted by abutment, or buttressing.
Trussing is an important structural device used to achieve spans with less weighty construction. Obviously, a frame composed of three end-connected members cannot change its shape, even if its joints could act as hinges. Fortunately, however, the principle of triangulation—attaching a horizontal tie beam to the bottom ends of two peaked rafters—can be extended indefinitely. Spanning systems of almost any shape can be subdivided into triangles, the sides of which can be made of any appropriate material—wood, rolled steel, or tubing—and assembled using suitable end connections. Each separate part is then subject only to either compressive or tensile stress. In the 18th century, mathematicians learned to apply their science to the behavior of structures, thus making it possible to determine the amounts of these stresses. This led to the development of space frames, which are simply trusses or other elements arrayed three-dimensionally.
Advances in the art of analyzing structural behavior resulted from the demand in the 19th century for great civil engineering structures: dams, bridges, and tunnels. It is now possible to enclose space with suspension structures—the obverse of vaulting, in that materials are in tension—or pneumatic structures, the skins of which are held in place by air pressure. Sophisticated analysis is particularly necessary in very tall structures, because wind loads and stresses that could be induced by earthquakes then become more important than gravity.
Architecture must also take into account the internal functional equipment of modern buildings. In recent decades, elaborate systems for vertical transportation, the control of temperature and humidity, forced ventilation, artificial lighting, sanitation, control of fire, and the distribution of electricity and other services have been developed. This has added to the cost of construction and has increased expectations of comfort and convenience.
In modern architectural terminology the word program denotes the purposes for which buildings are constructed. Certain broad purposes have always been discernible. The noblest works—temples, churches, mosques—celebrate the mysteries of religion and provide assembly places where gods can be propitiated or where the multitudes can be instructed in interpretations of belief and can participate in symbolic rituals. Another important purpose has been to provide physical security: Many of the world’s most permanent structures were built with defense in mind.
Related to defense is the desire to create buildings that serve as status symbols. Kings and emperors insisted on palaces proclaiming power and wealth. People of privilege have always been the best clients of designers, artists, and artisans, and in their projects the best work of a given period is often represented. Today large corporations, governments, and universities play the role of patron in a less personal way.
A proliferation of building types reflects the complexity of modern life. More people live in mass housing and go to work in large office buildings; they spend their incomes in large shopping centers, send their children to many different kinds of schools, and when sick go to specialized hospitals and clinics. They linger in airports on the way to distant hotels and resorts. Each class of facility has accumulated experiences that contribute to the expertise needed by its designers.
The attention of clients, architects, and users is more and more focused on the overall qualities manifested by aggregates of buildings and parts of cities as being more significant than individual structures. As the total building stock grows, conserving buildings and adapting them for changes in use becomes more important.
posted by Ronald @ 4:28 AM
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