An Intensity Measure-Based Methodology for Ground Motion Orientation in Seismic Structural Assessment

In current dynamic time history assessment protocols, ground motion recordings are mostly oriented along fixed sensor directions. Consequently, the orientation of the principal resultant of rupture energy, which may control the maximum structural demand, remains undefined, precluding consistent correlation with physically detrimental directivity effects. This study introduces the Peak-Polar Rotation–Intensity Ordinates methodology as a deterministic, intensity-measure-based orientation criterion for bidirectional ground motions in nonlinear time-history analysis. The method identifies record-specific principal directions by maximizing scalar intensity measures, namely Peak Ground Acceleration, Arias Intensity, Cumulative Absolute Velocity and Specific Energy Density. This approach is evaluated through statistical analysis of over 3400 pairs of recording of Far-Field, Near-Field, and Pulse-Like seismological regimes, utilizing an eight-storey reinforced concrete frame as a representative application for nonlinear time-history analysis. The investigation reveals a regime-dependency: while Far-Field motions exhibit tendencies toward isotropy, Pulse-Like events display stronger directional polarization tendency. For the Pulse-Like subset, Peak-Polar Rotation–Intensity Ordinates based on cumulative intensity measures, particularly Cumulative Absolute Velocity and Specific Energy Density, generally produced the largest storey-drift and roof-displacement demands among the considered orientation protocols. In representative cases, the maximum storey-drift demand reached approximately twice that obtained from the fault-oriented configuration. By replacing arbitrary geometric conventions with reproducible intensity-measure-based orientations, the proposed methodology provides a complementary tool for identifying potentially more demanding ground-motion alignments in bidirectional NLTHA.