Statistical analysis of earthquake ground motion coherency loss and a simplified evaluation method of its effect on structural responses

Abstract

Almost all major seismic design codes state the necessity of considering spatially varying earthquake ground motions (SVEGM) in the analysis and design of large-dimensional structures but do not provide specific guides on modeling SVEGM, which are characterized by time lag and coherency loss. More than 100 coherency loss models have been proposed, but they exhibit significant variations, even for sites with similar conditions. Owing to the absence of a systematic study on the coherency loss ranges in current practice, the coherency loss models are rather arbitrarily selected in both research and seismic design, which may lead to inaccurate structural response predictions. This study conducted statistical analyses of available coherency loss models corresponding to three different site categories from open literature. The mean and standard deviation (STD) of lagged coherency loss functions are derived, providing a range of possible variations of coherency loss and a reference for the selection of a coherency loss for modeling SVEGM. The results are further used to derive dimensionless coherency loss effect modification coefficients as a function of structural vibration frequency, separation distance, and spatial ground motion time lag for structural response prediction. With this, the structural responses considering SVEGM can be straightforwardly obtained by multiplying the conveniently obtainable structural responses, which account for the wave passage effect only, with the corresponding modification coefficients, thus greatly reducing the calculation complexity. Analysis of application examples demonstrated that using the proposed modification functions generally improves the accuracy of structural response predictions considering SVEGM.