Seismic buildings codes usually specify lateral design forces in a formula that involves the seismicity of the region; the importance of the occupancy; the type, period, and weight of the structure; and sometimes the soil properties of the site. The intent of the code procedures is to achieve a structure that will withstand a minor earthquake without damage, withstand a moderate earthquake with negligible structural damage, and withstand a major earthquake without endangering human life.


Reinforced concrete has some characteristics that enhance its ability to resist earthquake forces, and others that are detrimental. Perhaps the greatest advantage of reinforced concrete construction is continuity. If one component fails, the load is transferred to other components and the structure stands. To achieve the benefits offered by continuity, a structure must have ductility. Ductility means the ability of individual members within a structure to deform permanently without suffering a significant loss of strength.


Knowledge gained from the 1971 San Fernando earthquake led to two major findings. First, if the soft first story concept is used, large permanent displacements are likely during major earthquakes. Second, when flexural hinging is anticipated in columns or beams, closely spaced and properly sized transverse reinforcement--ties, stirrups, spirals--is required to maintain a member's axial load and shear capacities.


Three general types of structures now recognized by the Uniform Building Code are: bearing wall or box-type structure; moment resisting frame; and dual bracing system--a combined frame and wall system. Each structural system has its advantages and disadvantages, and the design standards for each system reflect the behavior of the major structural elements.