Estimating seismic response to bidirectional excitation per unidirectional analysis: A reevaluation for motions with fling-step using SDOF systems

Within the context of near-fault motions with fling-step signature, the present paper reevaluates the applicability of a straightforward alternative to estimate the response to bidirectional shaking per unidirectional analysis. The method originally evolved for motions with forward-directivity, uses unidirectional analysis in a single incidence angle. The performance of the method is examined using reinforced concrete (RC) bridge piers as the repre-sentative single-degree-of-freedom (SDOF) oscillators. Synthetic fling has been introduced to two horizontal components of processed (base-line corrected and filtered) strong motions. It has been shown that the response to bidirectional excitation can be closely estimated by means of a unidirectional analysis using the ground motion components rotated to the most preferred orientation. Existence of the least preferred orientation in which the unidirectional analysis results in the maximum difference from the bidirectional analysis appears to reinforce the development. For a ground motion pair, these orientations can be identified in advance through characteristic ground motion parameters, viz., energetic length and characteristic intensity for peak and cumulative demand, respectively. The usefulness of the said orientations to estimate the Park-Ang damage index is also established. The efficacy of the simple methodology has also been demonstrated through fragility analysis. The straightfor-ward method can hence be used in the near-fault regime regardless of the ground motion characteristics.