Effects of Nonlinear Soil Behavior on Kappa (κ): Observations from the KiK-Net Database

ABSTRACT Soil nonlinear behavior is often triggered in soft sedimentary deposits subjected to strong ground shaking and has led to catastrophic damage to civil infrastructure in many past earthquakes. Nonlinear behavior in soils is associated with large shear strains, increased material damping ratio, and reduced stiffness. However, most investigations of the high-frequency spectral decay parameter κ, which captures attenuation, have focused on low-intensity ground motions inducing only small shear strains. Because studies of the applicability of the κ model when larger deformations are induced are limited, this article investigates the behavior of κ (both κr per record and site-specific κ0 estimates) beyond the linear-elastic regime. About 20 stations from the Kiban–Kyoshin network database, with time-average shear-wave velocities in the upper 30 m between 213 and 626 m/s, are used in this study. We find that the classification scheme used to identify ground motions that trigger soil nonlinear behavior biases estimates of κ0 in the linear and nonlinear regimes. A hybrid method to overcome such bias is proposed considering proxies for in situ deformation (via the shear-strain index) and ground shaking intensity (via peak ground acceleration). Our findings show that soil nonlinearity affects κr and κ0 estimates, but this influence is station dependent. Most κ0 at our sites had a 5%–20% increase at the onset of soil nonlinear behavior. Velocity gradients and impedance contrasts influence the degree of soil nonlinearity and its effects on κr and κ0. Moreover, we observe that other complexities in the wave propagation phenomenon (e.g., scattering and amplifications in the high-frequency range) impose challenges to the application of the κ0 model, including the estimation of negative values of κr.

Fragility curve modifiers for reinforced concrete dual buildings, including nonlinear site effects and soil–structure interaction

We investigate the influence of soil–structure interaction (SSI) and nonlinear soil behavior on the seismic fragility of reinforced concrete (RC) dual (frame + shear wall) buildings resting on shallow foundations. This article includes a holistic methodology to account for nonlinear soil behavior and soil–foundation–structure interaction in a modular way. Using nonlinear dynamic analyses, we derive fragility curves for a wide set of building typologies and soil profiles, showing that soil behavior during strong shaking significantly affects the vulnerability of the soil–foundation–structure system. The influence of SSI is pronounced mostly for soft soil profiles, varying in a building-specific way. Post-processing of our results evolves into a set of fragility modifiers that enable risk analysts to massively account for soil-related and/or SSI effects in large-scale risk assessments.

Mathematical Models for Nonlinear Soil Behavior

Actually, the seismic movement has an irregular cyclic character.This can be equivalent to a determined number of uniform cyclical stresses equivalent in terms of effect. Modeling the behavior of the soil to cyclical stress, is usually done, by establishing a relationship for primary loading like τ = f (γ) and after drawing the diagram “effortless strain curve”, in which τ is the stress, and γ is shear deformation. For modeling nonlinear behavior of the soil, we used like nonlinear models. The best known are the hyperbolic model and the Ramberg-Osgood model.

Nonlinear Foundation Spring and Calibration Using Measured Dynamic Response of Structure

Some offshore platforms need modifications or addition of new modules that require re-assessment of their design. An economical way of allowing for changes without major strengthening of the platform is by reducing the safety margin used in the original design by use of measured data during the operation. This paper presents application of this concept to Troll A platform in which the measured changes in the platform’s natural periods due to the nonlinear soil behavior under different storm conditions were used to calibrate the soil-foundation spring adopted in the design.