Estimation of inelastic displacement demands of flexible-based structures on soft soils

Yang Lu,Alec M Marshall,Iman Hajirasouliha

doi:10.1504/ijeie.2016.080034

Abstract

This study aims to develop efficient tools for performance-based seismic design of soil-structure interaction (SSI) systems on soft soils. To simulate the SSI effects, linear and nonlinear 'equivalent fixed-base single-degree-of-freedom' (EFSDOF) oscillators as well as a sway-rocking SSI model were adopted. The nonlinear dynamic response of around 10,000 SSI models and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands, and damping ratios were obtained under a total of 20 seismic records on soft soil sites. Based on the results of this study, a practical method is developed for estimating the base shear and maximum displacement demands of a nonlinear single-degree-of-freedom structure on soft soil deposits. In the proposed procedure, the effect of frequency content of ground motions is considered by normalising the period of vibration by the spectral predominant periods, while the nonlinear EFSDOF models are used to improve the computational efficiency.

Highlights

1 Introduction The preliminary design of typical building structures in current seismic design codes and provisions is mainly based on elastic spectrum analysis, where the base shear and displacement demands of nonlinear systems are estimated by using modification factors such as the ductility reduction factor R and inelastic displacement ratio C
The results indicate that for the linear equivalent fixed-base single-degree-of-freedom” (EFSDOF) oscillators, LESDOF1 provides a better estimation of displacement demands compared to LESDOF2 for soil-structure interaction (SSI) systems having a ssi=20%, whereas for lightly damped systems the trend is reversed
Based on the results of around 10,000 soil-structure interaction (SSI) systems and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands and damping ratios subjected to a total of 20 ground motions recorded on soft soil sites, the following conclusions were drawn:

Summary

Introduction

The preliminary design of typical building structures in current seismic design codes and provisions is mainly based on elastic spectrum analysis, where the base shear and displacement demands of nonlinear systems are estimated by using modification factors such as the ductility reduction factor R and inelastic displacement ratio C. Rathje et al (1998) evaluated several scalar-valued parameters that characterised the frequency content of an input motion using 306 acceleration records from 20 earthquakes in active plate-margin regions. They found that a mean period, averaged from a range of periods from 0.05 to 4sec in the Fourier spectrum of an acceleration record, was the most reliable parameter when used to normalise the period of vibration. Miranda, 2000; Ruiz-García and Miranda, 2006; Iervolino et al, 2012) for nonlinear systems All of these studies were restricted to fixed-base building systems and, the effects of soil-structure interaction (SSI) were not considered

Objectives

Methods

Findings

Conclusion