Tall Building StructureTall buildings have fascinated mankind from the beginning of civilization, their construction being initially for defense and subsequently for ecclesiastical purposes. The growth in modern tall building construction, however, which began in the 1880s, has been largely for commercial and residential purposes.Tall commercial buildings are primarily a response to the demand by business activities to be as close to each other, and to the city center, as possible, thereby putting intense pressure on the available land space. Also, because they form distinctive landmarks, tall commercial buildings are frequently developed in city centers as prestige symbols for corporate organizations. Further, the business and tourist community, with its increasing mobility, has fuelled a need for more, frequently high-rise, city center hotel accommodations.The rapid growth of the urban population and the consequent pressure on limited space have considerably influenced city residential development. The high cost of land, the desire to avoid a continuous urban sprawl, and the need to preserve important agricultural production have all contributed to drive residential buildings upward.Ideally, in the early stages of planning a building, the entire design team, including the architect, structural engineer, and services engineer, should collaborate to agree on a form of structure to satisfy their respective requirements of function, safety and serviceability, and servicing. A compromise between conflicting demands will be almost inevitable. In all but the very tallest structures, however, the structural arrangement will be subservient to the architectural requirements of space arrangement and aesthetics.The two primary types of vertical load-resisting elements of tall buildings are columns and walls, the latter acting either independentlyas shear walls or in assemblies as shear wall cores. The building function will lead naturally to the provision of walls to divide and enclose space, and of cores to contain and convey services such as elevators. Columns will be provided, in otherwise unsupported regions, to transmit gravity loads and, in some types of structure, horizontal loads also.The inevitable primary function of the structural elements is to resist the gravity loading from the weight of the building and its contents. Since the loading on different floors tends to be similar, the weight of the floor system per unit floor area is approximately constant, regardless of the building height. Because the gravity load on the columns increases down the height of a building, the weight of columns per unit area increases approximately linearly with the building height.The highly probable second function of the vertical structural elements is to resist also the parasitic load caused by wind and possibly earthquakes, whose magnitudes will be obtained from National Building Codes or wind tunnel studies. The bending moments on the building caused by these lateral forces increase with at least the square of the height, and their effects will become progressively more important as the building height increases.Once the functional layout of the structure has been decided, the design process generally follows a well defined iterative procedure. Preliminary calculations for member sizes are usually based on gravity loading augmented by an arbitrary increment to account for wind forces. The cross-sectional areas of the vertical members will be based on the accumulated loadings from their associated tributary areas, with reductions to account for the probability that not all floors will be subjected simultaneously to their maximum live loading. The initial sizes of beams and slabs are normally based on moments and shears obtained from some simple method of gravity load analysis, or from codified mid and endspan values. A check is then made on the maximum horizontal deflection, and the forces in the major structural members, using some rapid approximate analysis technique. If the deflection is excessive, or some of the members are inadequate, adjustments are made to the member sizes or the structural arrangement. If certain members attract excessive loads, the engineer may reduce their stiffness to redistribute the load to less heavily stressed components. The procedure of preliminary analysis, checking, and adjustment is repeated until a satisfactory solution is obtained.Invariably, alterations to the initial layout of the building will be required as the client's and architect's ideas of the building evolve. This will call for structural modifications, or perhaps a radical rearrangement, which necessitates a complete review of the structural design. The various preliminary stages may therefore have to be repeated a number of times before a final solution is reached.Speed of erection is a vital factor in obtaining a return on the investment involved in such large-scale projects. Most tall buildings are constructed in congested city sites, with difficult access; therefore careful planning and organization of the construction sequence become essential. The story-to-story uniformity of most multistory buildings encourages construction through repetitive operations and prefabrication techniques. Progress in the ability to build tall has gone hand in hand with the development of more efficient equipment and improved methods of construction.Earthquake FaultsThe origin of an earthquakeAn earthquake originates on a plane of weakness or a fracture in the earth's crust, termed a "fault". The earth on one side of the fault slides or slips horizontally and /or vertically with respect to the earth on theopposite side, and this generates a vibration that is transmitted outward in all directions. This vibration constitutes the earthquake.The earthquake generally originates deep within the earth at a point on the fault where the stress that produces the slip is a maximum. This point is called the hypocenter or focus and the point on the earth's surface directly above this point is called the epicenter. The main or greatest shock is usually followed by numerous smaller aftershocks. These aftershocks are produced by slippage at other points on the fault or in the fault zone.Types of earthquake faultsFaults are classified in accordance with the direction and nature of the relative displacement of the earth at the fault plane. Probably the most common type is the strike-slip fault in which the relative fault displacement is mainly horizontal across an essentially vertical fault plane. The great San Andreas fault in California is of the type. Another type is termed a normal fault —when the relative movement is in an upward an downward direction on a nearly vertical fault plane. The great Alaskan earthquake of 1964 was apparently of this type. A less common type is the thrust fault —when the earth is under compressive stress across the fault and the slippage is in an upward and downward direction along an inclined fault plane. The San Fernando earthquake was generated on what has usually been classified as a thrust fault, although there was about as much lateral slippage as up and down slippage due to thrust across the inclined fault plane. Some authorities refer to this combined action as lateral thrust faulting. The compressive strain in the earth of the San Fernando Valley floor just south of the thrust fault was evidenced in many places by buckled sidewalks and asphalt paving.Forces exerted by an earthquakeSlippage along the fault occurs suddenly. It is a release of stressthat has gradually built-up in the rocks of the earth's crust. Although the vibrational movement of the earth during an earthquake is in all directions, the horizontal components are of chief importance to the structural engineer. These movements exert forces on a structure because they accelerate. This acceleration is simply a change in the velocity of the earth movement. Since the ground motion in an earthquake is vibratory, the acceleration and force that it exerts on a structure reverses in direction periodically, at short intervals of time.The structural engineer is interested in the force exerted on a body by the movement of the earth. This may be determined from Newton's second law of motion ' which may be stated in the following form:F=MaIn which F is a force that produces an acceleration a when acting on a body of mass M. This equation is nondimensional. For calculations M is set equal to W/g, then:F=W/g*a (1)In which F is in pounds, a is in feet per second per second, W is the weight of the body also in pounds and g is the acceleration of gravity, which is 32.2 feet per second per second.Equation (1) is empirical. It simply states the experimental fact that for a free falling body the acceleration a is equal to g and the acceleration force F is then equal to the weight W.For convenience, the acceleration of an earthquake is generally expressed as a ratio to the acceleration of gravity. This ratio is called a seismic coefficient. The advantage of this system is that the force exerted on a body by acceleration is simply the corresponding seismic coefficient multiplied by the weight of the body. This is in accordance with Equation (1) in which a/g is the seismic coefficient.Activity of faultsAll faults are not considered to present the same hazard. Some are classified as "active" since it is believed that these faults may undergo movement from time to time in the immediate geologic future. Unfortunately in the present state-of-the-art there is a good deal of uncertainty in the identification of potentially active faults. For example, the fault that generated the San Fernando earthquake did not even appear on any published geological maps of the area. This fault was discovered to be active only when it actually slipped and ruptured the ground surface. Accordingly the identification of active faults and geologically hazardous areas for land use criteria and for hazard reduction by special engineering may be of questionable value.Only in very recent years have geologists begun to try to evaluate the potential activity of faults that have no historical record of activity. By close inspection of a fault, visible in the side walls of a trench that cuts across the fault, it is sometimes possible to determine if it has been active in recent times. For example, if the trace of the fault extends through a recent alluvial material, then there must have been slippage since that material was deposited. However fault ruptures may be very difficult or impossible to see in imbedded material such as sand and gravel. Also of course the location of the fault must be known and it must reach the surface of the ground in order to inspect it by trenching.Evidence of the historical activity of a fault may sometimes be obtained by observing the faulting of geologically young deposits exposed in a trench. Such deposits are generally bedded and well consolidated so that fault rupture can easily be seen.The approximate time of formation of a fault rupture or scarp has in some cases been determined by radiocarbon analysis of pieces of wood found in the rupture or scarp.In addition to evidence of young fault activity obtained by trenching, there also may be topographic evidence of young faulting such as is obvious along the San Andreas fault. Vertical aerial photographs are one of the most important methods for finding topographic evidence of active faults. This evidence, which includes scarps, offset channels, depressions, and elongated ridges and valleys, is produced by fault activity. The age of these topographic features and therefore the time of the fault activity, can be estimated by the extent to which they are weathered and eroded.高层建筑结构高楼大厦已经着迷，从人类文明的开始，其建设是国防和最初其后教会的目的。