MULTI-COMPONENT GROUND MOTION RESPONSE SPECTRA FOR COUPLED HORIZONTAL, VERTICAL, ANGULAR ACCELERATIONS, AND TILT

Rotational and vertical components of ground motion are almost always ignored in design or assessment of structures despite the fact that vertical motion can be as twice much as horizontal and may exceed 2g level, while rotational excitation may reach few degrees in the proximity of fault rupture. Coupling of different components of ground excitation may significantly amplify the seismic demand by introducing additional lateral forces and enhanced P-Δ effects. In this paper, a governing equation of motion is postulated to compute the response of a SDOF oscillator under multi-component excitation. Expanded equation includes secondary P-Δ components associated with combined impacts of tilt and vertical excitations in addition to inertial forcing terms due to angular and translational accelerations. The elastic and inelastic spectral ordinates traditionally generated considering uni-axial input motion are compared at the end with those of multi-component response spectra of coupled horizontal, vertical and tilting motions. The proposed multi-component response spectrum reflects kinematic characteristics of the ground motion that are not identifiable by the conventional spectrum itself at least for near-fault region where high intensity vertical shaking and rotational excitation are likely to occur