A Top Down Approach to Achieve Full System Modeling in Seismic Analysis and Design

Since the introduction of ANSI A58.1-1972, provisions for seismic load analysis have evolved from simple 2-dimensional static analysis to complex 3-dimensional procedures that may require the representation of diaphragms as semi-rigid, incorporate physical mass offsets to account for accidental torsion, directly include P-Delta effects, and include bidirectional loading. The determination of the complexity of the analysis arises from a variety of rules that trigger decisions based on system features that may be determined a priori, or which may necessitate side analyses that are themselves complex. These rules were introduced incrementally over several code cycles, due in part to the increased availability and capacity of computers and analysis software. This “bottom up” approach in which rules are applied to increase analysis complexity is still used in ASCE 7-16. In this paper, an alternate “top down” approach is recommended wherein the basic requirement is to develop a detailed 3-dimensional analytical model, and then use exceptions where appropriate to simplify the analysis. It is believed that this approach may pave the way to more rational analysis in future editions of ASCE 7, with the goal of eliminating modal response spectrum analysis in favor of linear response history analysis, and eventually, to provide a design-based nonlinear response history analysis procedure in Chapter 12.