The structural engineer’s task is to design structures such that they can resist earthquake the ground shaking associated with an earthquake without collapsing. Since an earthquake may occur anytime during the design life, the first task is to identify the magnitude of peak ground acceleration (rga) that has a specified probability of occurrence during the design life. A common value is 2 % probability of occurrence in 50 years, which corresponds to a return period of 2,500 years. Earthquake ground motion is site specific in that it depends on the location and soil conditions for the site. Sites near known faults and sites on soft soils such as soft clay experience more intense ground motion. Factors such as the importance of the building, the geographic location of the site, and the type of soil must be taken into account when specifying the design magnitude for rga.

In order to understand how buildings respond to ground motion, one needs to examine the dynamic response. Consider the three-story frame We approximate it with the simple beam/mass system defined This approximation, known as a single degree-of-freedom model, provides useful information concerning the influence of certain structural properties on the response.