Linear Static Seismic Lateral Force Procedures

Abstract

The purpose of this chapter is to review and compare the sections of current seismic design provisions,which deal with the specification of seismic design forces. Emphasis will be on the equivalent static force procedures as contaned in the 2000 edition of the International Building Code and the 1997 Edition of the Uniform Building Code. There are two commonly used procedures for specifying seismic design forces: The “Equivalent Static Force Procedure” and “Dynamic Analysis”. In the equivalent static force procedure, the inertial forces are specified as static forces using empirical formulas. The empirical formulas do not explicitly account for the “dynamic characteristics” of the particular structure being designed or analyzed. The formulas were, however, developed to adequately represent the dynamic behavior of what are called “regular” structures, which have a reasonably uniform distribution of mass and stiffness. For such structures, the equivalent static force procedure is most often adequate. Structures that do not fit into this category are termed “irregular”. Common irregularities include large floor-to-floor variation in mass or center of mass and soft stories. Such structures violate the assumptions on which the empirical formulas, used in the equivalent static force procedure, are based. Therefore, its use may lead to erroneous results. In these cases, a dynamic analysis should be used to specify and distribute the seismic design forces. Principles and procedures for dynamic analysis of structures were presented in Chapter 4.