Tall Buildings

Tall buildings have fascinated humans from the beginning of civilization as evidenced by the pyramids of Giza, Egypt; Mayan temples of Tikal, Guatemala; and Kutub Minar of Delhi, India. The motivation behind their construction was primarily for creating monumental rather than human habitats. By contrast, contemporary tall buildings are primarily a response to the demand by commercial activities, often developed for corporate organizations as prestige symbols in city centers.

The feasibility of tall buildings has always depended upon the available materials and the development of the vertical transportation necessary for moving people up and down the buildings. The ensuing growth that has occurred from time to time may be traced back to two major technical innovations that occurred in the middle to the end of the nineteenth century: the development of wrought iron and subsequently steel, and the incorporation of the elevator in high-rise buildings. The introduction of elevators made the upper floors as attractive to lease as the lower ones and, as a result, made the taller building financially successful.

During the last 120 years, three major types of structures have been employed in tall buildings. The first type was used in the cast iron buildings of the 1850s to 1910, in which the gravity load was carried mostly by the exterior walls. The second generation of tall buildings, which began with the 1883 Home Insurance Building, Chicago, and includes the 1913 Woolworth Building and the 1931 Empire State Building, are frame structures, in which a skeleton of welded or riveted steel columns and beams runs through, often encased in cinder concrete, and the exterior is a nonbearing curtain wall. Most high-rises erected since the 1960s use a third type of structure, in which the perimeter structure of these buildings resembles tubes consisting of either closely spaced columns or widely spaced megacolumns with braces. Inside the perimeter structure a core, made of steel, concrete, or a combination of the two, contains many of the services such as elevators, stairwells, mechanical equipment, and toilets.

The art of designing tall buildings in windy climates is to bestow them with enough strength to resist forces generated by windstorms and enough stiffness or energy dissipation so that people working on upper floors are not disturbed by the buildings periodic swaying.

In seismic regions of the world, including the most severe areas of California, the effects of earthquakes are relatively small for tall buildings. For example, using the provisions of ASCE 7-02, the calculated base shear for a 60-story steel moment frame building located in downtown Los Angeles, CA, would be about 4% of its mass, as compared to 9% for a five-story building. However, the taller building would move considerably more than its 5-story counterpart.

The intent in seismic design then is to limit building movements, not so much to reduce perception of motion but to maintain the building's stability and prevent danger to pedestrians due to breakage and falling down of nonstructural elements.

8.1.1. Structural Concepts

The adoration that skyscrapers command lies in their apparent freedom from gravity loads: they do no just stand tall; they do so effortlessly. The key idea in conceptualizing such a bewildering and yet efficient structural system is to think of the building as a beam cantilevering from the earth (Fig. 8.1). The laterally directed force generated due to either wind or seismic action tends both to snap it (shear) and to push it over (bending). Therefore, a building must have a system to resist shear as well as bending. In resisting shear forces, the building must not break by shearing off (Fig. 8.2a) and must not strain beyond the limit of elastic recovery (Fig. 8.2b). Similarly, in resisting bending, the building must not overturn from the combined forces of gravity and lateral loads; it must not break by premature failure of columns either by crushing or by excessive tensile forces; and its bending deflection should not exceed the limit of elastic recovery (Fig. 8.3). In addition, a building in seismically active regions must be able to resist realistic earthquake forces without losing its vertical load-carrying capacity.

In a structure's resistance to bending and shear, a tug-of-war ensues that sets the building in motion, thus creating a third engineering problem: motion perception or vibration. If the building sways too much, human comfort is sacrificed or, more importantly, nonstructural elements may break resulting in damage to building contents and causing danger to pedestrians.

Wind

Wind

Building inertia forces

Building cantilevering from ground

Figure 8.1. Structural concept of a building subjected to lateral forces.

Building cantilevering from ground

Figure 8.1. Structural concept of a building subjected to lateral forces.

(a) (b)

Figure 8.2. Shear resistance of building: (a) building must not break; (b) building must not deflect excessively due to shear.

A perfect structural form to resist effects of bending, shear, and excessive vibration is a system with vertically continuous elements ideally located at the farthest extremity from the geometric center of the building. A steel or concrete chimney is perhaps an ideal, if not an inspiring, engineering model for a rational super-tall structural form. The quest for the best solution lies in translating this form into a more practical skeletal structure.

Building structural design is governed by codes that specify the minimum loads that a building must have the strength to resist. However, in planning a new building, or in retrofitting an existing facility, an owner may request enhanced requirements in its design for events that are not anticipated in the building codes. Defense facilities, nuclear power plants, and overseas embassies are just a few examples where special strengthening features are requested by building owners in the design and engineering of their facilities. Therefore, designers must consider project-specific needs and owner expectations when determining building loads. The primary loads addressed in building codes are

• Earthquake

Gravity load includes both the weight of the building and its content. The weight of the building is calculated based on material densities. The weight of the contents is not known specifically at the time of design and may vary depending upon the usage with time. Therefore, the codes

Figure 8.3. Bending resistance of building: (a) building must not overturn; (b) columns must not fail in tension or compression; (c) bending deflection must not be excessive.

Figure 8.3. Bending resistance of building: (a) building must not overturn; (b) columns must not fail in tension or compression; (c) bending deflection must not be excessive.

specify minimum floor live loads on a per square foot basis. Wind load specified by codes is based on maps of design wind speed for different regions of the country. As wind speed increases, the wind pressure on the building increases proportionally with the square of the wind velocity. The wind speed, and therefore the pressure on the building, increases with height above ground and varies dynamically (turbulance) relative to the degree of shielding provided by other buildings and geographic features. Although not usually required by building codes, wind tunnel studies are frequently performed to more accurately determine wind loads on tall buildings where standard calculations may not be adequate. Building codes in the United States do not specify allowable lateral deflections caused by wind loading, but leave those to the engineer's judgment.

The earthquake hazard is also highly dependent on the geographic region. The effects of earthquake are relatively small for very tall buildings in all regions of the world, including the seismic area of California. The flexibility of a very tall building of, say, 80-plus stories generally allows the building to sway back and forth to the ground motions without developing forces nearly as large as those produced by design wind loads. Therefore, even in a severe seismic area, tall building design is generally controlled by wind loads. However, even then the detailing of the building components and connections should conform to seismic design requirements. This is because the actual seismic forces, when they occur, are likely to be significantly larger than code-prescribed forces; hence, the material limitations and seismic detailing in addition to strength requirements. In other words, for buildings in high-seismic zones, even when wind forces govern the design, the detailing and proportioning requirements for seismic resistance must also be satisfied. The requirements get progressively more stringent as the zone factor for seismic risk gets progressively higher.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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