Civil Engineering Association

PROBABILISTIC SEISMIC DEMAND ANALYSIS FOR THE NEAR-FAULT ZONE

Ground motions close to a fault can be significantly influenced by rupture directivity effects. In particular, the effects of forward-directivity may cause severe damage to buildings. These effects have not been clearly addressed in current building codes and engineers still lack specific guidelines as to how to account for forward-directivity effects when determining the seismic hazard for structures. A methodology for probabilistic seismic demand analysis that includes the effects of forward directivity through time domain analysis is proposed in this work. First, the characteristics of forward-directivity ground motions and the structural response to these motions are studied and simplified mathematical representations for pulse-type forward-directivity ground motions are proposed. Intensity Measures for forward directivity ground motions are then proposed based on the simplified pulses. For this purpose, the non-linear dynamic response of three generic multi-story shear buildings to near-fault and ordinary ground motion ensembles was studied using Incremental Dynamic Analysis. Results show that whenever the pulse period of forward-directivity ground motions is close to the first-mode structural period, structural response is controlled by forward-directivity pulses. For these cases, structural response can be predicted using pulse-period and pulse-amplitude as intensity measures.

The principles of Probabilistic Seismic Demand Analysis are then extended to consider the effect of forward-directivity within a probabilistic framework. Structural response to pulse-type forward-directivity ground motions is quantified by means of time-domain analysis of simplified pulses that comprehensively represent all possible pulse-type ground motion scenarios. The hazard due to pulse-type motions is then coupled with conventional spectral domain seismic demand analyses for non-pulse-type ground motions. Results show that the proposed methodology captures more accurately the structural response to pulse-type ground motions than with current methods leading to the prediction of greater hazard for near-fault scenarios. In addition, the proposed method provides a clear guide for the selection of time histories for the design of near-fault structures.