Influence of Soil Nonlinearities on Dynamic Soil-Structure Interaction

For moderate or strong seismic events, the maximum strains can easily reach the elastic limit of the soil behavior. Considering soilstructure interaction, the nonlinear effects may change the soil stiffness at the base of the structure and the energy dissipation into the soil. To take into account the nonlinearity of the soil in the dynamic soil-structure interaction (DSSI), a 3D constitutive model, proposed by Iwan, is used to investigate DSSI in the framework of the Finite Element Method. The model accounts for the nonlinear hysteretic behavior of soils and only needs the shear modulus degradation curve to characterize the soil behavior. This feature is very important since complex constitutive models generally involve numerous mechanical parameters difficult to characterize experimentally. A parametric study is carried out for different types of structures to characterize nonlinear effects in the time domain. Through these numerical simulations, the nonlinear behavior of the soil is shown to have beneficial or detrimental effects on the dynamic response of the structure depending on the way the interaction process is modified: change in the amplitude and frequency content of the waves propagated into the soil, fundamental frequency of the response of the soil-structure system and energy dissipation

Nonlinear Site Effects: Interest of one Directional - Three Component (1D - 3C) Formulation

Strong ground motions generally lead to both a stiffness reduction and a larger energy dissipation in the soil layers. Thus, in order to study such phenomena, several nonlinear rheologies have been developed in the past. However, one of the main difficulties of using a given rheology is the number of parameters needed to describe the model. In this sense, the multi-surface cyclic plasticity approach, developed by Iwan in 1967 is an interesting choice since the only data needed is the modulus reduction curve. Past studies have implemented this method in one-directional SH wave-propagation (1D-1C). This work, however, aims to study the local site effects by considering one-directional (1D) seismic wave propagation accounting for their three-dimensional nonlinear behavior. The three components (3C) of the outcrop motion are simultaneously propagated into a horizontal multilayer soil for which a three-dimensional constitutive relation is used. The rheological model is implemented using the Finite Element Method. The alluvial site considered in this study corresponds to the Tiber River Valley, close to the historical centre of Rome (Italy). The computations are performed considering the waveforms referred as the 14th October 1997 Umbria-Marche earthquake, recorded on outcropping bedrock. Time histories and stress-strain hysteretic loops are calculated all along the soil column. The octahedral stress and strain profiles with depth and the modulus of acceleration transfer function (surface/outcrop spectral ratios) are estimated in the cases of combining three 1D-1C nonlinear analyses and of 1D-3C conditions, evidencing the influence of threedimensional loading path

Reduced T-shaped soil domain for nonlinear dynamic soil-bridge interaction analysis

The one-directional three-component wave propagation in a T-shaped soil domain (1DT-3C) is a numerical modeling technique, in a finite element scheme, to investigate dynamic soil-structure interaction (SSI) coupled with seismic site effects, under the assumption of vertical propagation of three-component seismic motion along a horizontal multilayered soil. A three-dimensional elasto-plastic model is adopted for soils, characterized using their shear modulus reduction curve. In this research, the 1DT-3C wave propagation modeling technique is proposed as an efficient tool for bridge design to take into account directly the spatial variability of seismic loading. This approach, in the preliminary phase of bridge study and design, allows the reduction of the soil domain and the easier definition of boundary conditions, using geotechnical parameters obtained with only one borehole investigation for each pier. This leads to a gain in modeling and computational time