INFLUENCE OF TOPOGRAPHIC IRREGULARITIES ON THE AMPLITUDE AND PHASE OF STRONG GROUND MOTIONS

We investigate the role of topographic irregularities on the amplitude and phase of earthquake ground motion by means of numerical analysis performed with the open-source finite element software OpenSees. We first perform a verification of the numerical model for simple 2D topographic features and harmonic motions, against: (1) numerical solutions available in the literature and (2) boundary element method analyses utilizing a code specifically developed for this study. The analytical and boundary element solutions provide verification of the OpenSees simulations for simple geometries excited at a single frequency. OpenSees, by contrast, can handle irregular geometries and broadband ground motions. We then perform a comprehensive parametric investigation for a bridge crossing a 260m-deep canyon. Our analyses are performed considering homogeneous stiff rock site conditions to emphasize the role of topographic irregularities only, without the influence of soil/rock layers of varying stiffness. Results are presented in terms of phase angle and amplitude modifications for a few critical locations along the bridge axis, corresponding to the foundations of the central piers of the bridge. We also process results by using cross-correlation measures, showing that 2D topographic irregularities can strongly modify strong ground motions.