Vibration of Buildings to Wind and Earthquake Loads
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Therefore, most buildings are designed with lateral-force-resisting systems or seismic systems , to resist the effects of earthquake forces. In many cases seismic systems make a building stiffer against horizontal forces, and thus minimize the amount of relative lateral movement and consequently the damage. Seismic systems are usually designed to resist only forces that result from horizontal ground motion, as distinct from vertical ground motion. The combined action of seismic systems along the width and length of a building can typically resist earthquake motion from any direction. Seismic systems differ from building to building because the type of system is controlled to some extent by the basic layout and structural elements of the building.
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Basically, seismic systems consist of axial-, shear- and bending-resistant elements. In wood-frame, stud-wall buildings, plywood siding is typically used to prevent excessive lateral deflection in the plane of the wall.
In older wood frame houses, this resistance to lateral loads is provided by either wood or steel diagonal bracing. The earthquake-resisting systems in modern steel buildings take many forms. In moment-resisting steel frames, the connections between the beams and the columns are designed to resist the rotation of the column relative to the beam. Thus, the beam and the column work together and resist lateral movement and lateral displacement by bending.
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Steel buildings are sometimes constructed with moment-resistant frames in one direction and braced frames in the other. In concrete structures, shear walls are sometimes used to provide lateral resistance in the plane of the wall, in addition to moment-resisting frames. 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 year-interval wind forces differ considerably with location; in the interiors of continents they are typically about 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 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 kilograms per square metre pounds per square foot in typhoon areas.
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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 threshold of perception of lateral motion varies considerably with individuals; a small proportion of the population can sense 0.
The recommendation for motion perception is to limit acceleration to 0. 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.
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 and Mexico City in , 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. 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.
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This approached the natural frequency of many tall structures, inducing large sidesway 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. 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.
The types of structures used for high-rise buildings must meet the lateral load performance criteria outlined above, and they must be reasonably efficient in the use of material and of reasonable cost.