Normalizing inelastic seismic response of structures having eccentricities in plan

The potentially disastrous consequences of "plan eccentricities" on the seismic response of structures is well recognized, but the exact influence of various parameters on the inelastic response of such systems remains unclear. This paper presents the results of an extensive parametric study conducted to investigate the effect of the uncoupled translational frequency ω_x, the ratio of uncoupled frequencies Ω, the normalized eccentricity (e/r), and the level of excitation, on the inelastic response of simple structures having eccentricities in plan. A procedure is developed to ensure a logical comparison between the response of the eccentric systems and equivalent single-degree-of-freedom (SDOF) benchmark. This procedure hinges on the definition of an equivalent SDOF system, and on a relationship between the excitation levels of the eccentric and SDOF systems. The findings and observations noted from this parametric study, initially conducted on simple monosymmetric bilinear two-element single-story systems, are then shown to be equally applicable for systems with multiple elements, various types of element models, and some simple multistory systems. In consequence of these findings, a simple preliminary design methodology is proposed to estimate ductility demands on simple systems with plan eccentricities, without having to perform detailed inelastic analyses.