Seismic fragility analysis of subway tunnel under horizontal and vertical ground motions

Seismic safety of subway tunnels, considering both horizontal and vertical ground motion and the effects of uncertainty, is insufficiently researched. To delicately consider the seismic safety assessment of subway tunnels under the joint effect of horizontal and vertical ground motion, this paper developed a joint horizontal and vertical ground motion selection method based on a multi-objective optimization algorithm, which is targeted on the horizontal and vertical code spectra of the tunnel structure, and the horizontal and vertical ground motion records were jointly selected to match the both horizontal and vertical code spectra. Based on the criteria of correlation, efficiency, practicability, and profitability, the optimal intensity measure of tunnel structures under horizontal and vertical ground motion was assessed. Based on the optimal intensity measure, the seismic fragility of tunnel structures under horizontal and vertical ground motions was analyzed, and the seismic safety assessment of subway tunnels under horizontal and vertical ground motions was assessed. The analysis results have indicated as follows: the effect of vertical ground motion is relatively insignificant on the assessment of the optimal intensity measures, and the optimal intensity measure of the ground motion is the Velocity Compound Intensity, IV; the structural safety margin of the tunnel structure under the joint of horizontal-vertical ground motion is significantly less than that under the horizontal ground motion, and the structural safety of subway tunnels is significantly affected by the vertical ground motion. The results of this study would provide a reference for seismic design and assessment of subway tunnels under multi-dimensional ground motions.