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The two primary types of vertical load-resisting elements of tall buildings are columns and walls, the latter acting either independently as shear walls or in assemblies as shear wall cores. The building function will lead naturally to the provision of walls to pide 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 end span 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 Faults
The origin of an earthquake
An 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 the opposite side, and this generates a vibration that is transmitted outward in all directions. This vibration constitutes the earthquake.