Role of Ground Motion Selection Method in Collapse Capacity Assessment of Structures

The role of ground motion selection method in assessment of collapse capacity of structural systems is evaluated in this paper. The main focus, here, is on epsilon parameter, which is the difference between the natural logarithm of the spectral acceleration of a given ground motion to the estimate of the natural logarithm of the median spectral acceleration from a given attenuation relationship. The epsilon parameter has a key effect on dynamic response of structural systems, and should be taken into account in the ground motion selection procedure in order to select appropriate ground motions which apply realistic seismic demands. For the purpose of evaluation of the effects of epsilon parameter on collapse capacity of structural systems, three categories of 10 ground motions have been selected with three different ranges of the epsilon value. Selected 30 ground motions are all large-magnitude-small-distance records that their moment magnitudes, M, vary from 6.5 to 6.9, and the closest distances to the fault rupture area, R, are in the range of 13 to 30 km. Although these ground motions are almost similar in terms of M and R values, they cause completely different structural responses during nonlinear time-history analysis. This fact shows that additional parameters should be taken into account in order to get reliable responses and also to decrease the record-to-record (aleatory) variability. Hence, the effect of epsilon parameter which can be used as an indicator of the shape of the response spectrum has been considered in this study. It is shown that the ground motions with the positive epsilon values all have small mean annual frequency of exceedance (rare events) and vise versa. In the current study, the incremental dynamic analysis has been performed using all three categories of 10 records in order to get the collapse capacity of structural systems under consideration. A comprehensive comparison has been accomplished by the authors in order to evaluate the role of ground motion selection method, specifically in terms of the epsilon parameter, in collapse capacity of different structural systems. Obtained results approve that the epsilon parameter should be considered as one of key ground motion characteristics, and neglecting this parameter can greatly affect the collapse capacity prediction of structural systems.