Alternate intensity measure for maximum direction shaking and construction of the associated ground motion models

Seismic standards recommend RotD100 as the intensity measure to define the maximum direction shaking while accounting for directionality. Nevertheless, the inherent overestimation with this choice is well known. Alternatively, this paper proposes Rotated RotD100 spectrum, a single-component spectrum rotated to align with the orientation of the RotD100 spectral ordinate at the period of interest. This definition preserves the RotD100 spectral ordinate at any specific period while avoiding overestimation at other periods. Further, it is also consistent with the recent recommendation on alternate perspective critical orientation, aiming to maximise the structural response. Therefore, the proposed intensity measure for maximum direction shaking should be preferred in scenario-based seismic performance assessment. A novel framework is proposed for developing the ground motion model (GMM) with Rotated RotD100 as the intensity measure by modifying RotD100 GMM. The proposed framework is demonstrated with a subset of the NGA-West2 database. The modification factor is first numerically constructed, followed by its idealization for smoothness and practical convenience. This idealized representation also accounts for the contingency on seismological parameters: the rate of decay in modification function increases with magnitude or distance. Further, the proposed Rotated RotD100 GMM is compared with a pair of conventional GMMs (geometric-mean, RotD50 and RotD100), highlighting its unique ability to maximise response at the period of interest while still representing a realistic ground motion. Overall, the proposed framework and the Rotated RotD100 GMM are expected to be a valuable tool in several earthquake engineering applications, such as site-specific seismic hazard analysis and scenario-based performance assessment.