Benchmarking of Nonlinear Geotechnical Ground Response Analysis Procedures

摘要

One-dimensional seismic ground response analyses are often performed using equivalent-linear procedures, which require few, generally well-known parameters. Nonlinear analyses have the potential to more accurately simulate soil behavior, but implementation in practice has been limited because of poorly documented and unclear parameter selection and code usage protocols, as well as inadequate documentation of the benefits of nonlinear modeling relative to equivalentlinear modeling. Regarding code usage/parameter selection protocols, we note the following: (1) when input motions are from ground surface recordings, we show that the full outcropping motion should be used without converting to a within condition; (2) Rayleigh damping should be specified using at least two matching frequencies with a target level equal to the small-strain soil damping; (3) the target soil backbone curves used in analysis can be parameterized to capture either the soils dynamic shear strength when large-strain soil response is expected (strains approaching 1 percent), relatively small-strain response (i.e., approx 3 percent) as inferred from cyclic laboratory tests, or a hybrid of the two; (4) models used in nonlinear codes inevitably represent a compromise between the optimal fitting of the shapes of backbone and hysteretic damping curves, and we present two alternatives for model parameterization. The parameter selection and code usage protocols are tested by comparing predictions to data from vertical arrays. We find site amplification to be generally underpredicted at high frequencies and overpredicted at the elastic site period where a strong local resonance occurs that is not seen in the data. We speculate that this bias results from overdamping.