A Clarification of the Orthogonal Effects in a Three-Dimensional Seismic Analysis

Abstract

For response spectra analyses it is shown that the “design of elements for 100 percent of the prescribed seismic forces in one direction plus 30 or 40 percent of the prescribed forces applied in the perpendicular direction” is dependent on the user's selection of the reference system. These commonly used “percentage combination rules” have no theoretical basis and can underestimate the design forces in certain members and can produce a member design which is relatively weak in one direction. It is proven that the alternate code approved method, in which a SRSS combination of two 100 percent spectra analyses with respect to any user defined orthogonal axes, will produce design forces which are not a function of the reference system and results in a structural design which has equal resistance to seismic motions from all directions.