Earthquake forces are not applied loads. They originate within the structure. Earthquake ground motion causes the base of a structure to, all of a sudden, start moving every which way. The masses, which are largely concentrated at the floor levels, because the columns and the walls of a structure account for a relatively small percentage of its weight, stay behind in their places because of inertia and then try to catch up with the movement of the base. This attempt to catch up sets the masses in motion – in terms of displacements, velocities, and accelerations (and higher derivatives of displacement such as jerk or jolt, which is the rate of change of acceleration). In the process, the masses experience forces equal to mass times acceleration (by Newton’s second law of motion); these forces are inertia forces resisting the motion of the structure (Fig. 1). Earthquake engineering, in its very essence, consists in providing (preferably multiple) load paths strong enough and stiff enough to transmit these inertia forces from where they originate, namely the floor levels, to the ground underlying the foundations where all loads ultimately belong.

In contrast to the structural response to essentially static gravity loading or even to wind loads, which can often be validly treated as static loads, the dynamic character of the response to earthquake excitation can seldom be ignored. Whereas, in designing for static loads, one would feel greater assurance about the safety of a structure made up of members of heavy sections, in the case of earthquake loading, the stiffer and heavier structure does not necessarily represent the safer design.

When a structure responds elastically to ground motions during a severe earthquake, the maximum response accelerations may be several times the maximum ground acceleration (Fig. 2) and depend on the mass and stiffness of the structure and the magnitude of the damping. Elastic response is response that is recoverable. When the earthquake eventually stops, we find our structure intact. There is no residual displacement, no damage to be repaired. If all of this is not the case, we have inelastic response, which is associated with damage. Damping is what brings a body, excited to vibration, back to rest. When the vibration is seismically induced, one of the major sources of damping is damage caused by the earthquake – initially to the nonstructural, then to the structural components.