Structural system of high-rise buildings

Wind loads

The structural systems of tall buildings must carry vertical gravity loads, but lateral loads, such as those due to wind and earthquakes, are also a major consideration. Maximum 100-year-interval wind forces differ considerably with a location; in the interiors of continents, they are typically about 100 kilograms per square metre (20 pounds per square foot) at ground level. In coastal areas, where cyclonic storms such as hurricanes and typhoons occur, maximum forces are higher, ranging upward from about 250 kilograms per square metre (50 pounds per square foot). Wind forces also increase with building height to a constant or gradient value as the effect of ground friction diminishes. The maximum design wind forces in tall buildings are about 840 kilograms per square metre (170 pounds per square foot) in typhoon areas.
The effect of wind forces on tall buildings is twofold. A tall building may be thought of as a cantilever beam with its fixed end at the ground; the pressure of the wind on the building causes it to bend with the maximum deflection at the top. In addition, the flow of wind past the building produces vortices near the corners on the leeward side; these vortices are unstable and every minute or so they break away downwind, alternating from one side to another. The change of pressure as a vortex breaks away imparts a sway, or periodic motion, to the building perpendicular to the direction of the wind. Thus, under wind forces, there are several performance criteria that a high-rise structure must meet. The first is stability—the building must not topple over; second, the deflection, or sideways at the top, must not exceed a maximum value (usually taken as 1/500 of the height) to avoid damage to brittle building elements such as partitions; and, third, the swaying motion due to vortex shedding must not be readily perceptible to the building occupants in the form of acceleration, usually stated as a fraction of gravity. The threshold of perception of lateral motion varies considerably with individuals; a small proportion of the population can sense 0.003 g or 0.004 g. The recommendation for motion perception is to limit acceleration to 0.010 g for wind forces that would recur in 10-year intervals. The fourth criterion involves the natural period of the building structure. This is the vibration period at which the swaying cantilever motions of the building naturally reinforce and enhance each other and could become large enough to damage the building or even cause it to collapse. The natural period of the building should be less than one minute, which is the period of vibration due to the shedding of wind vortexes.

Earthquake Loads

Earthquake or seismic forces, unlike wind forces, are generally confined to relatively small areas, primarily along the edges of the slowly moving continental plates that form the Earth’s crust. When abrupt movements of the edges of these plates occur, the energy released propagates waves through the crust; this wave motion of the Earth is imparted to buildings resting on it. Timber frame buildings are light and flexible and are usually little damaged by earthquakes; masonry buildings are heavy and brittle and are susceptible to severe damage. Continuous frames of steel or reinforced concrete fall between these extremes in their seismic response, and they can be designed to survive with relatively little damage.
In two major earthquakes involving large numbers of high-rise buildings, in Los Angeles in 1971 and Mexico City in 1985, lateral accelerations due to ground motions in a number of tall buildings were measured with accelerometers and were found to be of the order of 0.100 to 0.200 g. In Los Angeles, where the period of the seismic waves was less than one second, most steel-frame high rises performed well with relatively little damage; continuous concrete frames also generally performed well, but there was considerable cracking of concrete, which was later repaired by the injection of epoxy adhesive. In Mexico City, however, the period of the seismic waves was quite long, on the order of a few seconds. This approached the natural frequency of many tall structures, inducing large sides way motions that led to their collapse. Based on this experience, determination of the seismic performance criteria of buildings involves the lateral resistance of forces of 0.100 to 0.200 g and consideration of the natural period of the building in relation to the period of seismic waves that can be expected in the locality. Another important factor is the ductility of the structure, the flexibility that allows it to move and absorb the energy of the seismic forces without serious damage. The design of buildings for seismic forces remains a complex subject, however, and there are many other important criteria involved.





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