Time Domain Implementation of Transmitting Boundaries in ABAQUS for Discrete Soil-structure Interaction Systems

Soil-structure-interaction (SSI) analyses are essential to evaluate the seismic performance of important structures before finalizing their structural design. SSI under seismic condition involves much more complex interaction with soil compared to the dynamic loads having source on the structure. Seismic SSI analysis requires due consideration of site-specific and structure-specific properties to estimate the actual ground motion (scattered motion) experienced at the base of the structure, and subsequently the effects of the scattered motion on the structure. Most challenging aspect of seismic SSI analysis is to implement transmitting boundaries that absorb the artificial reflections of stress waves at the truncated interface of the finite and infinite domains, while allowing the seismic waves to enter the finite domain. In this paper, the time domain implementation of seismic analysis of a soil-structure system is presented using classical discrete models of structure and interactive force boundary conditions for soil. These models represent typical SSI systems- a single Degree of Freedom (DOF) of a spherical cavity with mass attached to its wall, a two DOF system consisting of a mass attached by a nonlinear spring to a semi-infinite rod on elastic foundation, and a three DOF system with additional DOFs for modelling the structural stiffness and damping. The convolution integral representing the force boundary condition on the truncated interface, is evaluated interactively using UAMP user-subroutine in ABAQUS and applied as concentrated forces at the interface (truncated interface) nodes of the bounded domain or generalized-structure domain. The verification problems presented in the paper show the satisfactory performance of the developed MATLAB code and ABAQUS implementation with FORTRAN user-subroutines. The classical phenomena associated with the dynamic soil-structure systems are discussed through the present work.

Effect of Transmitting Boundary on Soil – Slope - Foundation Interaction

In recent years, extensive research has been performed when the foundation is placed on the crest of slope. Besides of so many researches less number of researches has been done for foundation is placed on slope considering Soil-Structure Interaction (SSI) effect. Construction on slopes poses more challenges especially under seismic load due to an earthquake in addition to the forces of sliding slope itself. The failure of slope not only affects any structure but also has damaging consequences on the environment in general. Therefore, transmitting boundaries should be adequate to absorb the seismic energy at the boundaries. In this paper, effect of transmitting boundary on soil-slope-foundation interaction (SSFI) is studied. Two cases have analysed for SSFI when foundation is placed at various position on the crest and slope itself. El-Centro earthquake (1940) with three different PGA viz. 0.25g, 0.5g and 1g is applied as input motion for both the cases. It is observed that the foundation placed near the slope is more susceptible to damage. Responses of slope (acceleration and displacement) have also been observed at three different nodal points on slope. Results show that the amplification in soil mass leads to settlement of foundation. Shear stress and equivalent plastic strain distribution for SSFI is discussed for different load conditions. It is found that the slope-foundation system shows the local and global failure. Further Post earthquake peak settlement for foundation is plotted and it shows the true behavior of soil.

Viscoelastic Boundary Conditions for Multiple Excitation Sources in the Time Domain

Transmitting boundaries are important for modeling the wave propagation in the finite element analysis of dynamic foundation problems. In this study, viscoelastic boundaries for multiple seismic waves or excitations sources were derived for two-dimensional and three-dimensional conditions in the time domain, which were proved to be solid by finite element models. Then, the method for equivalent forces’ input of seismic waves was also described when the proposed artificial boundaries were applied. Comparisons between numerical calculations and analytical results validate this seismic excitation input method. The seismic response of subway station under different seismic loads input methods indicates that asymmetric input seismic loads would cause different deformations from the symmetric input seismic loads, and whether it would increase or decrease the seismic response depends on the parameters of the specific structure and surrounding soil.

Nonlinear Amplification of Seismic Ground Motion by Alluvial Valley

This paper studies nonlinear amplification of seismic ground motion by alluvial valley in layered sites. The equivalent linear method is used in dynamic analysis and transmitting boundaries are added at boundaries of the computation region. It is shown that, soil nonlinearity has significant effect on seismic ground motion around alluvial valley. The amplitudes in the case of linear alluvium and soil layers are the largest, those in the case of nonlinear alluvium and soil layers are the smallest, and those in the case of nonlinear alluvium and linear soil layers fall in between. The periods in the case of nonlinear alluvium and soil layers are the longest, those in the case of nonlinear alluvium and soil layers and those in the case of linear alluvium and linear soil layers are almost the same.