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Chapter 5

Linear Static Seismic Lateral Force Procedures

Roger M. Di Julio Jr., Ph.D., P.E.Professor of Engineering, California State University, Northridge

Key words: Code Philosophy, Design Base Shear, Design Story Forces, Design Drift Limitations, Equivalent StaticForce Procedure, Near Fault Factors, Seismic Zone Factors, UBC-97, IBC-2000, Regular and IrregularStructures, Torsion and P-delta Effects, Site Soil Factors, Importance Factors

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 forceprocedures as contaned in the 2000 edition of the International Building Code and the 1997 Edition of theUniform 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, theinertial forces are specified as static forces using empirical formulas. The empirical formulas do notexplicitly 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 aretermed "irregular". Common irregularities include large floor-to-floor variation in mass or center of massand soft stories. Such structures violate the assumptions on which the empirical formulas, used in theequivalent 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 andprocedures for dynamic analysis of structures were presented in Chapter 4.