Motives for a Multidirectional Conditional Spectrum in Seismic Design and Assessment
While the seismic design of structures has traditionally been carried out considering two perpendicular horizontal components of ground shaking, real ground motions are complex and can impose very different demands at different orientations of the building with respect to the seismic source. The relevance of this complexity and its influence on the seismic performance of different kinds of structures has, thus, been receiving increasing attention in recent years. The authors of this paper have recently focused their research efforts in studying ways to provide an encompassing definition of seismic demands that aims at designing engineering structures whose relative performance does not depend on their sensitivity to the angle of incidence of ground motion. The multidirectional conditional spectrum resulted from this framework and has been recently proposed by the authors as a tool to characterise seismic demands at different angles of incidence. This paper focuses on explaining the motives and need for such a tool, and describes in detail the three main components needed for its generation. Examples are given and the relevance of accounting for the correlation between demands at different angles is illustrated. Applications and future developments are finally discussed.