Building Construction, procedures involved in the erection of various types of structures. The major trend in present-day construction continues away from handcrafting at the building site and toward on-site assembly of ever larger, more integrated subassemblies manufactured away from the site. Another characteristic of contemporary building, related to the latter trend, is the greater amount of dimensional coordination; that is, buildings are designed and components manufactured in multiples of a standard module (10 cm/4 in being standard in the U.S.), which drastically reduces the amount of cutting and fitting required on the building site. A third trend is the production or redevelopment of such large structural complexes as shopping centers, entire campuses, and whole towns or sections of cities.
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Building Construction, procedures involved in the erection of various types of structures. The major trend in present-day construction continues away from handcrafting at the building site and toward on-site assembly of ever larger, more integrated subassemblies manufactured away from the site. Another characteristic of contemporary building, related to the latter trend, is the greater amount of dimensional coordination; that is, buildings are designed and components manufactured in multiples of a standard module (10 cm/4 in being standard in the U.S.), which drastically reduces the amount of cutting and fitting required on the building site. A third trend is the production or redevelopment of such large structural complexes as shopping centers, entire campuses, and whole towns or sections of cities.
Construction Industry
Building construction in the U.S. is the product of a diverse group of subindustries, with many individuals and organizations involved in the construction of a single structure, from the manufacture of necessary components to final assembly. As a general rule, state laws require a registered architect or engineer, or both, to execute the design and to make sure that the design complies with public health, zoning, and building-code requirements. The design must at the same time conform to the requirements of the owner. The architect or engineer converts these requirements into a set of drawings and written specifications that usually are sent to interested general contractors for bids. The successful bidder or bidders in turn subcontract plumbing, painting, electrical wiring, structural frame construction and erection, and other jobs to firms specializing in these crafts.
Contractors ordinarily carry out their work under the observation of an architect and or engineer, who acts as agent of the owner. State and local inspectors review the work for general compliance with the local building code. The immediate responsibility of the contractor, architect, and engineer ends when the local authorities approve the building for occupancy and the owner accepts the building. However, the contractor, architect, and engineer are legally responsible for any deficiencies in the construction or design for a period of several years after acceptance, the time depending on the terms of the contract and local laws.
Elements of a Building
The major elements of a building include the following: (1) the foundation, which supports the building and provides stability; (2) the structure, which supports all the imposed loads and transmits them to the foundation; (3) the exterior walls, which may or may not be part of the primary supporting structure; (4) the interior partitions, which also may or may not be part of the primary structure; (5) the environmental-control systems, including the heating, ventilating, air-conditioning, lighting, and acoustical systems; (6) the vertical transportation systems, including elevators, escalators, and stairways; (7) communications, which may include such subsystems as intercommunications, public address, and closed-circuit television, as well as the more usual telephone-wiring systems; and (8) the power, water supply, and waste disposal systems.
Building Loads
The loads imposed on a building are classified as either "dead" or "live." Dead loads include the weight of the building itself and all major items of fixed equipment. Dead loads always act directly downward, act constantly, and are additive from the top of the building down. Live loads include wind pressure, seismic forces, vibrations caused by machinery, movable furniture, stored goods and equipment, occupants, and forces caused by temperature changes. Live loads are temporary and can produce pulsing, vibratory, or impact stresses. In general, the design of a building must accommodate all possible dead and live loads to prevent the building from settling or collapsing and to prevent any permanent distortion, excessive motion, discomfort to occupants, or rupture at any point.
Foundations
The structural design of a building depends greatly on the nature of the soil and underlying geologic conditions and modification by man of either of these factors.
Ground Conditions
If a building is to be constructed in an area that has a history of earthquake activity, the earth must be investigated to a considerable depth. Faults in the crust of the earth beneath the soil must obviously be avoided. Some soils may liquefy when subjected to the shock waves of a quake and become like quicksand. In such cases, either construction must be avoided altogether or the foundation must be made deep enough to reach solid material below the potentially unstable soil. Certain clay soils have been found to expand 23 cm (9 in) or more if subjected to long cycles of drying or wetting, thus producing powerful forces that can shear foundations and lift lightweight buildings. Some soils with high organic content may, over time, compress under the building load to a fraction of their original volume, causing the structure to settle. Other soils tend to slide under loads.
Soils that have been modified in some way often perform differently, especially when other soil has been added to or mixed with existing soil, or when the soil has been made wetter or drier than normal, or when cement or chemicals such as lime have been added. Sometimes the soil under a proposed building varies so greatly over the entire site that a building simply cannot be constructed safely or economically.
Soil and geologic analyses are necessary, therefore, to determine whether a proposed building can be supported adequately and what would be the most effective and economical method of support.
If there is sound bedrock a short distance below the surface of the construction site, the area over which the building loads are distributed can be quite small because of the strength of the rock. As progressively weaker rock and soils are encountered, however, the area over which the loads are distributed must be increased.
Types of Foundations
The most common types of foundation systems are classified as shallow and deep. Shallow foundation systems are several feet below the bottom of the building; examples are spread footings and mats. Deep foundations extend several dozen feet below the building; examples are piles and caissons (Figure 1). The foundation chosen for any particular building depends on the strength of the rock or soil, magnitude of structural loads, and depth of groundwater level.
The most economical foundation is the reinforced-concrete spread footing, which is used for buildings in areas where the subsurface conditions present no unusual difficulties. The foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.
Mat foundations are typically used when the building loads are so extensive and the soil so weak that individual footings would cover more than half the building area. A mat is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls. The mat load per unit area that is transmitted to the underlying soil is small in magnitude and is distributed over the entire area. For large mats supporting heavy buildings, the loads are distributed more evenly by using supplementary foundations and cross walls, which stiffen the mat.
Piles are used primarily in areas where near-surface soil conditions are poor. They are made of timber, concrete, or steel and are located in clusters. The piles are driven down to strong soil or rock at a predetermined depth, and each cluster is then covered by a cap of reinforced concrete. A pile may support its load either at the lower end or by skin friction along its entire length. The number of piles in each cluster is determined by the structural load and the average load-carrying capacity of each pile in the cluster. A timber pile is simply the trunk of a tree stripped of its branches and is thus limited in height. A concrete pile, on the other hand, may be of any reasonable length and may extend below groundwater level as well. For extremely heavy or tall buildings, steel piles, known as H-piles because of their shape, are used. H-piles are driven through to bedrock, often as far as 30 m (100 ft) below the surface. H-piles can be driven to great depths more easily than piles made of wood or concrete; although they are more expensive, the cost is usually justified for large buildings, which represent a substantial financial investment.
Caisson foundations are used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. A caisson foundation consists of concrete columns constructed in cylindrical shafts excavated under the proposed structural column locations. The caisson foundations carry the building loads at their lower ends, which are often bell-shaped.
Groundwater Level
Foundation construction is complicated by groundwater flowing above the bottom of the proposed foundation level. In such cases the sides of the excavation may be undermined and cave in. Lowering the groundwater level by pumping the water out of the excavation usually requires the installation of braced sheathing to shore up, or retain, the sides of the excavation to prevent any cave-ins. When the amount of water within the excavation is excessive, ordinary pumping methods, which bring to the surface loose soil mixed with the water, can undermine the foundations of buildings on adjoining property. To prevent damage caused by soil movement, wellpoint dewatering is often used. Wellpoints are small pipes with a perforated screen at one end. They are driven or jetted into the ground so that the screen, which prevents soil from flowing in with the water, is below groundwater level. These pipes are linked to a common manifold (pipe) that is connected to a water pump. In this way the groundwater is removed from below the excavation without damaging nearby property. Dewatering may also make it unnecessary to sheathe the sides of the excavation, providing the soil will not slide into the excavation because of its composition or because of vibrations from nearby heavy traffic or machinery.
Structure
The basic elements of any ordinary structure are the floors and roof (including horizontal supporting members), columns and walls (vertical members), and bracing (diagonal members) or rigid connections used to give the structure stability.
One- to Three-Story Buildings
With low buildings the variety of possible shapes is much greater than with taller buildings. In addition to the familiar box shape, which is also used in very tall buildings, low buildings may use cathedral-like forms, vaults, or domes. A simple single-story structure might consist of a reinforced-concrete slab laid directly on the ground, exterior masonry walls supported by the slab (or by a spread footing cast continuously around the perimeter of the building), and a roof. For low buildings, the use of interior columns between masonry load-bearing walls is still the most common construction method. Spaced columns supported by the slab or by individual spread footings may be used, however; in that case the exterior walls can be supported by or hung between the columns. If the roof span is short, abutting planking made of wood, steel, concrete, or other material can be used to form the roof structure.
Each structural material has a particular weight-to-strength ratio, cost, and durability. As a general rule, the greater the roof span, the more complicated the structure supporting the roof becomes and the narrower the range of suitable materials. Depending on the length of the span, the roof may have one-way framing beams (Figure 2a and 2b) or two-way framing (beams supported on larger girders spanning the longest dimension). Trusses can be substituted for either method. Trusses, which can be less than 30 cm (12 in) or more than 9 m (30 ft) deep, are formed by assembling tension and compression members in various triangular patterns. They are usually made of timber or steel, but reinforced concrete may be used.
The structure of a simple one-story building may also consist of the wall and roof framing combined by being either fastened together or shaped in one piece. The possible structural shapes are almost infinite and include the three sides of a rectangle fastened together into a unit called a bent (Figure 2c), the familiar church form of vertical sides and sloping roof (Figure 2d), the parabola (Figure 2e), and the semicircle or dome.
The supporting structure and exterior walls, floor, and roof may also be made as a unified whole, much like a rectangular pipe with closed or open ends. These forms may be cast in reinforced plastic.
Multibay and Multistory Buildings
By far the most common form of building structure is the skeleton frame, which consists essentially of the vertical members shown in Figure 2a, 2b, and 2c, combined with a horizontal framing pattern. For tall buildings, the use of load-bearing walls (as in Figure 2a) with horizontal framing members has declined steadily; nonload-bearing curtain walls are used most frequently.
The skeleton frame most often consists of multiples of the construction shown in Figure 2c. For structures up to 40 stories high, reinforced concrete, steel, or composite-reinforced concrete and steel can be used in a variety of ways. The basic elements of the steel skeleton frame are vertical columns, horizontal girders spanning the longer distance between columns, and beams spanning shorter distances (Figure 3a). The frame is reinforced to prevent distortion and possible collapse because of uneven or vibratory loads. Lateral stability is provided by connecting the beams, columns, and girders; by the support given the structure by the floors and interior walls; and by diagonal bracing or rigid connections between columns, girders, and beams. Reinforced concrete can be used in a similar way, except that concrete shear walls would be used instead of diagonal bracing to provide lateral stability.
Newer techniques of constructing moderately high buildings include inserting prefabricated units within the skeleton frame; cable hanging; and stacking.
For the insertion technique, a stable skeleton frame may be constructed with a utility core that includes fire stairs, elevators, plumbing, piping, and wiring. Prefabricated boxlike units then can be inserted in the openings between the horizontal and vertical framing. Major changes in the future use of the building could then be made by removing and replacing the boxlike units.
In hanging (Figure 3b), a vertical utility core is built, and strong horizontal roof framing is anchored to the top of the core. All floors below, except at ground level, are supported by attaching them to the core and to tension members hung from the roof framing. After the core is complete, the floors are built from the top down.
Stacking (Figure 3c) is a construction technique in which prefabricated, boxlike units are raised by cranes and placed on top of and alongside each other and then are fastened together.
For buildings over 40 stories, typically steel had been considered the most appropriate material. However, recent advances in the development of high-strength concretes have made concrete competitive with steel. Tall buildings often require more sophisticated structural solutions to resist lateral loads, such as wind, and earthquake forces. One of the more popular structural systems is the exterior structural tube, which was used in the construction of the World Trade Center (411 m/1350 ft) in New York City. Here, closely spaced columns connected rigidly to the horizontal spandrel beams on the perimeter of the building provide sufficient strength to resist loads and the stiffness to minimize lateral deflections. The structural tube has now been used with concrete and with composite construction consisting of structural steel members encased in reinforced concrete.
For very tall buildings, the mixing of steel and concrete is becoming more popular. The high strength-to-weight ratio of steel is excellent for the horizontal spanning members. High-strength concretes can economically provide the compression resistance needed for vertical members. In addition, the mass and internal damping properties of the concrete assist in minimizing vibration effects, which are potential problems in very tall buildings.
Exterior Walls (Facades) and Roofs
The curtain wall, the most common type of nonload-bearing wall, may be assembled either on or off the site. It consists of an exterior skin backed with insulation; a vapor barrier; sound-deadening materials; and an interior skin that may be part of the curtain wall or may be attached separately. The exterior skin may be made of metal (stainless steel, aluminum, bronze), masonry (concrete, brick, tile), or glass. Limestone, marble, granite, and precast concrete panels are also used for facades.
The traditional method of constructing a roof is to lay down, over a steel or concrete deck spanning the framing members, rolls of roofing felt laminated with tar and topped with gravel. Synthetic materials are also being used increasingly in place of felt and tar. New grasslike and ruglike materials made of plastic enable recreation areas to be built on top of roofs at little expense.
Interior Partitions
Traditional methods of partitioning a building interior include the use of masonry walls 10 to 15 cm (4 to 6 in) thick made of concrete, gypsum, or pumice block, painted or plastered; or wood or metal frames covered with lath over which plaster is spread. Plasterboard and wallboard are increasingly used.
To provide for greater flexibility within buildings, movable or easily disassembled partitioning systems are used, the only restriction to their placement being the spacing of the interior columns. Such partitions may be metal, prefabricated plasterboard components, accordionlike rolling curtains, or, if sound transmission is a problem, leaded curtains that move either horizontally or vertically. Lightweight materials usually mean an increase in transmitted sound and a loss of privacy. Nevertheless, the trend is toward lighter partitions and increased use of sound-absorbing materials. In many buildings, the only walls still made of masonry are fire walls, which enclose elevator shafts, stairs, and main corridors.
Environmental Control
Perhaps the greatest improvements in building construction have been in heating, cooling, ventilation, lighting, and sound control. In most large buildings complete, year-round air conditioning is now standard. Some areas in a building may need to be cooled even in winter, depending on the distance from exterior walls and the heat generated by lighting, electrical equipment, or human occupancy. The level and quality of lighting have improved greatly. Largely as a consequence of these changes, the cost of the mechanical and electrical systems in buildings has increased at a greater rate than other individual building costs; such costs currently account for a quarter to a third of total construction expenditure. Increasingly since the late 1970s all these systems are automatically operated and controlled by computers that are programmed to maximize efficiency and minimize waste and energy consumption.
Communications and Power Systems
The growing use of power, telephone, and facsimile transmission equipment and of closed circuit television, intercommunication, and security and alarm systems has increased the amount of wiring that must be installed in buildings. Main cables run vertically in open shafts, with branches at each floor running through conduits located either in the hung ceiling space or embedded in the floor slab itself.
The electrical power required in buildings has increased with the number and complexity of environmental systems in operation. Because a power outage cannot be tolerated, emergency power generators are installed in an increasing number of buildings. Some buildings, particularly in remote locations, are equipped with their own primary electrical generating systems. Diesel and gas-turbine generators are used. The heat generated by these engines, instead of being wasted, is sometimes utilized for other purposes within the building.
Vertical Transportation
Elevators, especially high-speed, automatically controlled, cable-operated elevators, are the major form of vertical transportation in high-rise structures (see Elevator). Low-rise buildings and the lower floors of commercial buildings may also have escalators. For fire protection, it is necessary to provide at least two means of egress from every major space in a building. Therefore, in addition to elevators and escalators, all buildings, even the tallest, have two enclosed and protected stairways for their entire height.
Water Supply and Waste Disposal
Buildings must have a piped-in water supply for a variety of purposes: drinking, washing, cooking, waste disposal, internal fire fighting (either through standpipes and hoses or through automatic sprinklers), and service to air-conditioning systems or boilers.
Disposal of wet and dry wastes in buildings is accomplished by a variety of devices, such as incinerators, shredders, and garbage compactors. There are also devices that assist waste-pickup and disposal systems. The usual method of carrying away waterborne waste is through piping connected to the sewer system outside the building. New technology is aimed at recycling water to reduce waste and pollution.
See Also Concrete; House.
Contributed By:
Richard L. Tomasetti
Construction Industry
Building construction in the U.S. is the product of a diverse group of subindustries, with many individuals and organizations involved in the construction of a single structure, from the manufacture of necessary components to final assembly. As a general rule, state laws require a registered architect or engineer, or both, to execute the design and to make sure that the design complies with public health, zoning, and building-code requirements. The design must at the same time conform to the requirements of the owner. The architect or engineer converts these requirements into a set of drawings and written specifications that usually are sent to interested general contractors for bids. The successful bidder or bidders in turn subcontract plumbing, painting, electrical wiring, structural frame construction and erection, and other jobs to firms specializing in these crafts.
Contractors ordinarily carry out their work under the observation of an architect and or engineer, who acts as agent of the owner. State and local inspectors review the work for general compliance with the local building code. The immediate responsibility of the contractor, architect, and engineer ends when the local authorities approve the building for occupancy and the owner accepts the building. However, the contractor, architect, and engineer are legally responsible for any deficiencies in the construction or design for a period of several years after acceptance, the time depending on the terms of the contract and local laws.
Elements of a Building
The major elements of a building include the following: (1) the foundation, which supports the building and provides stability; (2) the structure, which supports all the imposed loads and transmits them to the foundation; (3) the exterior walls, which may or may not be part of the primary supporting structure; (4) the interior partitions, which also may or may not be part of the primary structure; (5) the environmental-control systems, including the heating, ventilating, air-conditioning, lighting, and acoustical systems; (6) the vertical transportation systems, including elevators, escalators, and stairways; (7) communications, which may include such subsystems as intercommunications, public address, and closed-circuit television, as well as the more usual telephone-wiring systems; and (8) the power, water supply, and waste disposal systems.
Building Loads
The loads imposed on a building are classified as either "dead" or "live." Dead loads include the weight of the building itself and all major items of fixed equipment. Dead loads always act directly downward, act constantly, and are additive from the top of the building down. Live loads include wind pressure, seismic forces, vibrations caused by machinery, movable furniture, stored goods and equipment, occupants, and forces caused by temperature changes. Live loads are temporary and can produce pulsing, vibratory, or impact stresses. In general, the design of a building must accommodate all possible dead and live loads to prevent the building from settling or collapsing and to prevent any permanent distortion, excessive motion, discomfort to occupants, or rupture at any point.
Foundations
The structural design of a building depends greatly on the nature of the soil and underlying geologic conditions and modification by man of either of these factors.
Ground Conditions
If a building is to be constructed in an area that has a history of earthquake activity, the earth must be investigated to a considerable depth. Faults in the crust of the earth beneath the soil must obviously be avoided. Some soils may liquefy when subjected to the shock waves of a quake and become like quicksand. In such cases, either construction must be avoided altogether or the foundation must be made deep enough to reach solid material below the potentially unstable soil. Certain clay soils have been found to expand 23 cm (9 in) or more if subjected to long cycles of drying or wetting, thus producing powerful forces that can shear foundations and lift lightweight buildings. Some soils with high organic content may, over time, compress under the building load to a fraction of their original volume, causing the structure to settle. Other soils tend to slide under loads.
Soils that have been modified in some way often perform differently, especially when other soil has been added to or mixed with existing soil, or when the soil has been made wetter or drier than normal, or when cement or chemicals such as lime have been added. Sometimes the soil under a proposed building varies so greatly over the entire site that a building simply cannot be constructed safely or economically.
Soil and geologic analyses are necessary, therefore, to determine whether a proposed building can be supported adequately and what would be the most effective and economical method of support.
If there is sound bedrock a short distance below the surface of the construction site, the area over which the building loads are distributed can be quite small because of the strength of the rock. As progressively weaker rock and soils are encountered, however, the area over which the loads are distributed must be increased.
Types of Foundations
The most common types of foundation systems are classified as shallow and deep. Shallow foundation systems are several feet below the bottom of the building; examples are spread footings and mats. Deep foundations extend several dozen feet below the building; examples are piles and caissons (Figure 1). The foundation chosen for any particular building depends on the strength of the rock or soil, magnitude of structural loads, and depth of groundwater level.
The most economical foundation is the reinforced-concrete spread footing, which is used for buildings in areas where the subsurface conditions present no unusual difficulties. The foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.
Mat foundations are typically used when the building loads are so extensive and the soil so weak that individual footings would cover more than half the building area. A mat is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls. The mat load per unit area that is transmitted to the underlying soil is small in magnitude and is distributed over the entire area. For large mats supporting heavy buildings, the loads are distributed more evenly by using supplementary foundations and cross walls, which stiffen the mat.
Piles are used primarily in areas where near-surface soil conditions are poor. They are made of timber, concrete, or steel and are located in clusters. The piles are driven down to strong soil or rock at a predetermined depth, and each cluster is then covered by a cap of reinforced concrete. A pile may support its load either at the lower end or by skin friction along its entire length. The number of piles in each cluster is determined by the structural load and the average load-carrying capacity of each pile in the cluster. A timber pile is simply the trunk of a tree stripped of its branches and is thus limited in height. A concrete pile, on the other hand, may be of any reasonable length and may extend below groundwater level as well. For extremely heavy or tall buildings, steel piles, known as H-piles because of their shape, are used. H-piles are driven through to bedrock, often as far as 30 m (100 ft) below the surface. H-piles can be driven to great depths more easily than piles made of wood or concrete; although they are more expensive, the cost is usually justified for large buildings, which represent a substantial financial investment.
Caisson foundations are used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. A caisson foundation consists of concrete columns constructed in cylindrical shafts excavated under the proposed structural column locations. The caisson foundations carry the building loads at their lower ends, which are often bell-shaped.
Groundwater Level
Foundation construction is complicated by groundwater flowing above the bottom of the proposed foundation level. In such cases the sides of the excavation may be undermined and cave in. Lowering the groundwater level by pumping the water out of the excavation usually requires the installation of braced sheathing to shore up, or retain, the sides of the excavation to prevent any cave-ins. When the amount of water within the excavation is excessive, ordinary pumping methods, which bring to the surface loose soil mixed with the water, can undermine the foundations of buildings on adjoining property. To prevent damage caused by soil movement, wellpoint dewatering is often used. Wellpoints are small pipes with a perforated screen at one end. They are driven or jetted into the ground so that the screen, which prevents soil from flowing in with the water, is below groundwater level. These pipes are linked to a common manifold (pipe) that is connected to a water pump. In this way the groundwater is removed from below the excavation without damaging nearby property. Dewatering may also make it unnecessary to sheathe the sides of the excavation, providing the soil will not slide into the excavation because of its composition or because of vibrations from nearby heavy traffic or machinery.
Structure
The basic elements of any ordinary structure are the floors and roof (including horizontal supporting members), columns and walls (vertical members), and bracing (diagonal members) or rigid connections used to give the structure stability.
One- to Three-Story Buildings
With low buildings the variety of possible shapes is much greater than with taller buildings. In addition to the familiar box shape, which is also used in very tall buildings, low buildings may use cathedral-like forms, vaults, or domes. A simple single-story structure might consist of a reinforced-concrete slab laid directly on the ground, exterior masonry walls supported by the slab (or by a spread footing cast continuously around the perimeter of the building), and a roof. For low buildings, the use of interior columns between masonry load-bearing walls is still the most common construction method. Spaced columns supported by the slab or by individual spread footings may be used, however; in that case the exterior walls can be supported by or hung between the columns. If the roof span is short, abutting planking made of wood, steel, concrete, or other material can be used to form the roof structure.
Each structural material has a particular weight-to-strength ratio, cost, and durability. As a general rule, the greater the roof span, the more complicated the structure supporting the roof becomes and the narrower the range of suitable materials. Depending on the length of the span, the roof may have one-way framing beams (Figure 2a and 2b) or two-way framing (beams supported on larger girders spanning the longest dimension). Trusses can be substituted for either method. Trusses, which can be less than 30 cm (12 in) or more than 9 m (30 ft) deep, are formed by assembling tension and compression members in various triangular patterns. They are usually made of timber or steel, but reinforced concrete may be used.
The structure of a simple one-story building may also consist of the wall and roof framing combined by being either fastened together or shaped in one piece. The possible structural shapes are almost infinite and include the three sides of a rectangle fastened together into a unit called a bent (Figure 2c), the familiar church form of vertical sides and sloping roof (Figure 2d), the parabola (Figure 2e), and the semicircle or dome.
The supporting structure and exterior walls, floor, and roof may also be made as a unified whole, much like a rectangular pipe with closed or open ends. These forms may be cast in reinforced plastic.
Multibay and Multistory Buildings
By far the most common form of building structure is the skeleton frame, which consists essentially of the vertical members shown in Figure 2a, 2b, and 2c, combined with a horizontal framing pattern. For tall buildings, the use of load-bearing walls (as in Figure 2a) with horizontal framing members has declined steadily; nonload-bearing curtain walls are used most frequently.
The skeleton frame most often consists of multiples of the construction shown in Figure 2c. For structures up to 40 stories high, reinforced concrete, steel, or composite-reinforced concrete and steel can be used in a variety of ways. The basic elements of the steel skeleton frame are vertical columns, horizontal girders spanning the longer distance between columns, and beams spanning shorter distances (Figure 3a). The frame is reinforced to prevent distortion and possible collapse because of uneven or vibratory loads. Lateral stability is provided by connecting the beams, columns, and girders; by the support given the structure by the floors and interior walls; and by diagonal bracing or rigid connections between columns, girders, and beams. Reinforced concrete can be used in a similar way, except that concrete shear walls would be used instead of diagonal bracing to provide lateral stability.
Newer techniques of constructing moderately high buildings include inserting prefabricated units within the skeleton frame; cable hanging; and stacking.
For the insertion technique, a stable skeleton frame may be constructed with a utility core that includes fire stairs, elevators, plumbing, piping, and wiring. Prefabricated boxlike units then can be inserted in the openings between the horizontal and vertical framing. Major changes in the future use of the building could then be made by removing and replacing the boxlike units.
In hanging (Figure 3b), a vertical utility core is built, and strong horizontal roof framing is anchored to the top of the core. All floors below, except at ground level, are supported by attaching them to the core and to tension members hung from the roof framing. After the core is complete, the floors are built from the top down.
Stacking (Figure 3c) is a construction technique in which prefabricated, boxlike units are raised by cranes and placed on top of and alongside each other and then are fastened together.
For buildings over 40 stories, typically steel had been considered the most appropriate material. However, recent advances in the development of high-strength concretes have made concrete competitive with steel. Tall buildings often require more sophisticated structural solutions to resist lateral loads, such as wind, and earthquake forces. One of the more popular structural systems is the exterior structural tube, which was used in the construction of the World Trade Center (411 m/1350 ft) in New York City. Here, closely spaced columns connected rigidly to the horizontal spandrel beams on the perimeter of the building provide sufficient strength to resist loads and the stiffness to minimize lateral deflections. The structural tube has now been used with concrete and with composite construction consisting of structural steel members encased in reinforced concrete.
For very tall buildings, the mixing of steel and concrete is becoming more popular. The high strength-to-weight ratio of steel is excellent for the horizontal spanning members. High-strength concretes can economically provide the compression resistance needed for vertical members. In addition, the mass and internal damping properties of the concrete assist in minimizing vibration effects, which are potential problems in very tall buildings.
Exterior Walls (Facades) and Roofs
The curtain wall, the most common type of nonload-bearing wall, may be assembled either on or off the site. It consists of an exterior skin backed with insulation; a vapor barrier; sound-deadening materials; and an interior skin that may be part of the curtain wall or may be attached separately. The exterior skin may be made of metal (stainless steel, aluminum, bronze), masonry (concrete, brick, tile), or glass. Limestone, marble, granite, and precast concrete panels are also used for facades.
The traditional method of constructing a roof is to lay down, over a steel or concrete deck spanning the framing members, rolls of roofing felt laminated with tar and topped with gravel. Synthetic materials are also being used increasingly in place of felt and tar. New grasslike and ruglike materials made of plastic enable recreation areas to be built on top of roofs at little expense.
Interior Partitions
Traditional methods of partitioning a building interior include the use of masonry walls 10 to 15 cm (4 to 6 in) thick made of concrete, gypsum, or pumice block, painted or plastered; or wood or metal frames covered with lath over which plaster is spread. Plasterboard and wallboard are increasingly used.
To provide for greater flexibility within buildings, movable or easily disassembled partitioning systems are used, the only restriction to their placement being the spacing of the interior columns. Such partitions may be metal, prefabricated plasterboard components, accordionlike rolling curtains, or, if sound transmission is a problem, leaded curtains that move either horizontally or vertically. Lightweight materials usually mean an increase in transmitted sound and a loss of privacy. Nevertheless, the trend is toward lighter partitions and increased use of sound-absorbing materials. In many buildings, the only walls still made of masonry are fire walls, which enclose elevator shafts, stairs, and main corridors.
Environmental Control
Perhaps the greatest improvements in building construction have been in heating, cooling, ventilation, lighting, and sound control. In most large buildings complete, year-round air conditioning is now standard. Some areas in a building may need to be cooled even in winter, depending on the distance from exterior walls and the heat generated by lighting, electrical equipment, or human occupancy. The level and quality of lighting have improved greatly. Largely as a consequence of these changes, the cost of the mechanical and electrical systems in buildings has increased at a greater rate than other individual building costs; such costs currently account for a quarter to a third of total construction expenditure. Increasingly since the late 1970s all these systems are automatically operated and controlled by computers that are programmed to maximize efficiency and minimize waste and energy consumption.
Communications and Power Systems
The growing use of power, telephone, and facsimile transmission equipment and of closed circuit television, intercommunication, and security and alarm systems has increased the amount of wiring that must be installed in buildings. Main cables run vertically in open shafts, with branches at each floor running through conduits located either in the hung ceiling space or embedded in the floor slab itself.
The electrical power required in buildings has increased with the number and complexity of environmental systems in operation. Because a power outage cannot be tolerated, emergency power generators are installed in an increasing number of buildings. Some buildings, particularly in remote locations, are equipped with their own primary electrical generating systems. Diesel and gas-turbine generators are used. The heat generated by these engines, instead of being wasted, is sometimes utilized for other purposes within the building.
Vertical Transportation
Elevators, especially high-speed, automatically controlled, cable-operated elevators, are the major form of vertical transportation in high-rise structures (see Elevator). Low-rise buildings and the lower floors of commercial buildings may also have escalators. For fire protection, it is necessary to provide at least two means of egress from every major space in a building. Therefore, in addition to elevators and escalators, all buildings, even the tallest, have two enclosed and protected stairways for their entire height.
Water Supply and Waste Disposal
Buildings must have a piped-in water supply for a variety of purposes: drinking, washing, cooking, waste disposal, internal fire fighting (either through standpipes and hoses or through automatic sprinklers), and service to air-conditioning systems or boilers.
Disposal of wet and dry wastes in buildings is accomplished by a variety of devices, such as incinerators, shredders, and garbage compactors. There are also devices that assist waste-pickup and disposal systems. The usual method of carrying away waterborne waste is through piping connected to the sewer system outside the building. New technology is aimed at recycling water to reduce waste and pollution.
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