Influence of soil structure interaction effects on the real elastic axis of asymmetric buildings

Abstract Structural eccentricity plays an important role in the seismic design of buildings. According to various seismic design codes, it is one of the parameters which define whether a building may be considered as regular in plan. Structural eccentricity is defined as the distance between the center of mass and the center of rigidity. However, the center of rigidity is rigorously defined in single story buildings and in some special classes of multi-story buildings, e.g. isotropic ones, under the assumption of fixed based conditions. The present paper deals with single story and multistory asymmetric buildings that possess a real elastic axis under the assumption of fixed base condition and examines the existence or not of an elastic axis under soil structure interaction effects. The flexibility matrix and the loading vector are defined under flexible base assumption and the elastic axis is determined based on the notion of twist center. Mathematical formulas are derived which provide the coordinates of the elastic axis. Numerical examples are presented which investigate not only soil-structure interaction effects but the influence of various loading vectors as well. In general, soil-structure interaction extinct the real elastic axis; hence an optimum torsion axis must be defined. Moreover, soil-structure interaction decreases the structural eccentricity at each story level.

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