Investigation of the Site Effect on Seismic Ground Motions Using Elasto-Plastic Constitutive Model

 

Hadi Khanbabazadeh

MSc of geotechnical engineering, Mazandaran University, Babol, Iran Hadi15012002@yahoo.com

Asskar Janalizadeh Choobbasti

Associate professor, department of civil engineering,
Mazandaran University, Bobol, Iran Asskr@nit.ac.ir

ABSTRACT

The seismic response of a site depends strongly on its characteristics. This issue is considered in 2800 Iran code by classifying sites to 4 groups and 4 design spectra have presented. It suggests the use of shear wave velocity of the site as a means for characterizing different soil types. The common method in earthquake engineering for modeling wave transmission in layered sites and dynamic soil-structure interaction is "equivalent-linear" method. Since this method is widely used, and the fully nonlinear method embodied in FLAC is not, in this study pointing out some of the differences between the two methodsí results in site effect estimations. The equivalent linear method uses linear properties for each element that remain constant throughout the history of shaking. Also, the interference and mixing phenomena that occur between different frequency components in a nonlinear material are missing from an equivalent linear analysis. During plastic flow, it is commonly accepted that the strain-increment tensor is related to some function of the stress tensor, giving rise to the "flow rule" in plasticity theory. However, elasticity theory (as used by the equivalent-linear method) relates the strain tensor (not increments) to the stress tensor. Plastic yielding, therefore, is modeled somewhat inappropriately. Whereas, A proper plasticity formulation is used in all the built-in models, whereby plastic strain increments are related to stresses. Because of the wide use of equivalent linear method and these deficiencies, it is necessary to compare the site response estimated by this method and fully nonlinear method. In this paper, the site response is estimated by elasto-plastic constitutive model.

Keywords: fully nonlinear method; equivalent linear method; site effect; dynamic analysis; response spectra; FLAC; Iran 2800 code.

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