Practical implementation of analysis routines for nonlinear seismic ground response analysis

One-dimensional seismic ground response analyses are often performed using equivalent-linear procedures, which require few, generally well-known parameters (shear wave velocity, modulus reduction and damping versus shear strain, and soil density). Nonlinear analyses provide a more robust characterization of the true nonlinear soil behavior, but their implementation in practice has been limited, which is principally a result of poorly documented and unclear parameter selection and code usage protocols. Moreover, the benefits and potentials of nonlinear analysis relative to equivalent linear are not well defined. In this paper, we present preliminary results of a "benchmarking study" of nonlinear ground response analysis procedures. Key issues that are discussed include: (1) use of reference strain, which can be readily estimated as a function of plasticity index and effective stress, in lieu of dynamic shear strength as a parameter defining the nonlinear backbone curve; (2) strategies for managing excessive large-strain material damping that occurs when Masing's rule is applied to the backbone curve to evaluate hysteretic damping; (3) specification of input motion as "outcropping" (i.e., equivalent free-surface motions) versus "within" (i.e., motion recorded at depth in a vertical array); and (4) specification of viscous damping, specifically the target value of the viscous damping ratio and the frequencies for which the viscous damping produced by the model matches the target.