Orientation dependency of peak ductility demand for seismic performance evaluation

The current seismic provisions specify an earthquake load level for structural design based on a uniform-hazard elastic response spectrum by addressing key uncertainties associated with seismic hazard assessment. Among various sources of uncertainties, variability regarding the orientation of horizontal ground motion measures can have important impact on reliability of structures. Despite the popularity of characterizing the orientation of elastic seismic demand, the orientation effects of inelastic seismic demand have not been examined extensively. This study investigates the orientation dependency of the peak ductility demand of inelastic single-degree-of-freedom systems with known strength by accounting for record-to-record variability of seismic effects. The probabilistic characteristics of the peak ductility demand in recording directions are compared with those in the maximum/minimum response directions and the major/minor principal directions. The analysis results indicate that the peak ductility demand depends on how input ground motions are defined in terms of record orientation. The orientation dependency of the inelastic seismic demand can be explained by the “response spectral shape effects” of ground motion records in specific directions. The results have an important implication in the current earthquake engineering practice, as they suggest that separate peak ductility demand models should be used for different ground motion measures to evaluate the seismic performance of structures consistently without bias.