Structural performance evaluation in near-source conditions

Abstract

The present thesis confronts the problem of evaluating the seismic performance of structures in near-source conditions, when said structures are designed for inelastic response to strong ground motion. What sets near-source seismic input apart and causes it to merit particular attention, is the fact that NS ground motions often contain significant wave pulses. Therefore, a dataset of previously identified impulsive near-source records is used to derive an analytical-form relationship for the inelastic displacement ratio by means of regression analysis. It is found that a double-opposite-bumps form is required to match the empirical data as function of the structural period over the pulse period ratio, similar to what has been proposed in the literature for soft soil sites. The relationship consistently builds on previous studies on the topic, yet displays different shape with respect to the most common equations for static structural assessment procedures. This reveals that inelastic seismic demand of near-source pulse-like ground motions can exhibit different trends than ordinary records i.e., records not identified as pulse-like. Subsequently, the extension of non-linear static procedures for seismic design and assessment is discussed, with respect to the inelastic demand associated with forward directivity. In this context, a methodology is presented for the implementation of the Displacement Coefficient Method towards estimating near-source seismic demand. This method makes use of the results of near-source probabilistic seismic hazard analysis and the semi-empirical equation for pulse-like inelastic displacement ratio. An illustrative application of the Displacement Coefficient Method, with explicit inclusion of near-source, pulse-like effects, is given for a set of typical, plane, reinforced concrete frames, designed under Eurocode provisions. Different scenarios are considered in the application and non-linear dynamic analysis results are obtained and discussed with respect to the static procedure estimates. Conclusions drawn from the results help to assess the importance of incorporating near-source effects in performance-based seismic design. Finally, the seismic demand of oscillators with more complex, trilinear, backbone curves to near-source pulse-like ground motions is examined. This study is motivated by the need for seismic demand estimates by nonlinear static procedures that delve deeper into the inelastic range and arrive at quantifying dynamic collapse capacity, which has already set researchers on this path for ordinary ground motions. Thus, this chapter closely follows the methodology of Vamvatsikos and Cornell (2006), employing incremental dynamic analysis and the suite of one hundred and thirty pulse-like-identified ground motions, in order to develop an elaborate reduction factor-ductility-period relation for pulse-like near-source motions and oscillators characterized by generic trilinear backbones. The resulting analytical model captures both central tendency and dispersion of near-source pulse-like seismic demand. The model also makes the important inclusion of pulse period as a predictor variable, whose importance is demonstrated in an illustrative application.