An iterative linear procedure using frequency-dependent soil parameters for site response analyses

Local site amplification predictions that derive from the traditional equivalent linear (EL) procedure are deemed to be reliable when weak shaking is considered, while it is well recognized that significant discrepancies can occur for stronger earthquakes where soil nonlinearity dominates the site response. To better capture nonlinearity effects, frequency-dependent equivalent linear methods, commonly referred to as FDEL methods, were proposed by modifying the EL procedure with frequency-dependent soil parameters. However, these methods have never been widely used in earthquake engineering practice. Existing FDEL methods rely on generic spectral shape fitting parameters to approximate the variation of shear strain amplitudes in the frequency domain, which can lead to substantial inconsistencies in the model prediction when applied to a particular seismic event, with a specific signature. This study investigates a range of spectral seismic intensities to identify a proxy that could be used directly to update the shear strain spectra in accordance with the simulated ground motions. A new FDEL algorithm to compute effective shear strain as a function of frequency is proposed and tested on various scenarios. When simulating strong ground motions that exhibit higher levels of soil deformation, the frequency content of ground motions processed within the FDEL procedure is substantially improved as compared to the EL method, with results more consistent with those from NL analyses.