without collapse, but must also remain operational after an earthquake. Therefore, in addition to life safety, damage control is an important design consideration for structures deemed vital to postearthquake functions.
In general, most earthquake code provisions implicity require that structures be able to resist
1. Minor earthquakes without any damage.
2. Moderate earthquakes with negligible structural damage and some nonstructural damage.
3. Major earthquakes with some structural and nonstructural damage but without collapse. The structure is expected to undergo fairly large deformations by yielding in some structural members.
It is important to distinguish between forces due to wind and those induced by earthquakes. Earthquake forces result directly from the distortions induced by the motion of the ground on which the structure rests. The magnitude and distribution of forces and displacements resulting from ground motion is influenced by the properties of the structure and its foundation, as well as the character of the ground motion.
An idea of the behavior of a building during an earthquake may be grasped by considering the simplified response shape shown in Fig. 2.1. As the ground on which the building rests is displaced, the base of the building moves with it. However, the building above the base is reluctant to move with it because the inertia of the building mass resists motion and causes the building to distort. This distortion wave travels along the height of the structure, and with continued shaking of the base, causes the building to undergo a complex series of oscillations.
Although both wind and seismic forces are essentially dynamic, there is a fundamental difference in the manner in which they are induced in a structure. Wind loads, applied as external loads, are characteristically proportional to the exposed surface of a structure, while the earthquake forces are principally internal forces resulting from the distortion produced by the inertial resistance of the structure to earthquake motions.
The magnitude of earthquake forces is a function of the mass of the structure rather than its exposed surface. Whereas in wind design, one would feel greater assurance about the safety of a structure made up of heavy sections, in seismic design, this does not necessarily produce a safer design.
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