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

Rocking Motion Analysis Using Structural Identification Tools

This research investigates the convenience of structural identification tools to detect the rocking motion tendency, using as input the structural response to ambient vibrations. The rocking ratio and rocking spectrum are proposed as original tools to highlight the rocking motion and its frequency content. The proposed procedure allows the detection and quantification of rocking using only building vertical motion records in both cases of ambient vibration and earthquake. First, three-dimensional finite element models of reinforced concrete buildings are adopted to simulate the structural response to white noise vibration. Different low- and high-rise buildings are studied, having framed structure and frame–wall system, regular and irregular structure, shallow foundation and underground floors. The structural response obtained numerically is analyzed using different signal processing tools to obtain the dynamic features of buildings, and the rocking motion tendency is identified by comparison with a reference fixed base condition. Then, the reliability of the proposed methodology to detect rocking motion attitude, using only the structural motion, is verified and quantified using the proposed tools. Finally, the same approach is applied to real structural motion records of a high-rise reinforced concrete building

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

Nonlinear numerical simulation of the soil seismic response to the 2012 Mw 5.9 Emilia earthquake considering the variability of the water table position

This research is focused on the case study of San Carlo village (Emilia- Romagna, Italy), struck by the 20 May 2012 Mw 5.9 Emilia earthquake that caused severe damage because of widely observed soil liquefaction. In particular, it investigates the influence on nonlinearity effects of the variability of the water table depth caused by seasonal fluctuation. The one-directional propagation of a three-component seismic wave (1D-3C approach), in a multilayered soil profile, is simulated using a finite-element model and an elastoplastic constitutive behavior with hardening for the soil (Iwan’s model). The nonlinearity is described by the normalized shear modulus decay curve obtained by resonant column (RC) tests. The shear modulus is corrected during the process (Iai’s model) to consider the cyclic mobility and dilatancy of sands, depending on the actual average effective stress and the friction and dilatancy angles obtained from cyclic consolidated undrained triaxial (CTX) tests. Profiles with depth of maximum excess pore water pressure, horizontal motion, and shear strain and stress are obtained in the case of effective stress analysis (ESA), for an average position of the water table depth and for a variation of 1 m, and then compared with a total stress analysis (TSA). The variability with the water table depth of soil profile response to seismic loading is observed also in terms of hysteresis loops, time histories of the ground motion, and excess pore water pressure in the liquefiable soil layers prone to the cyclic mobility process