۱- Introduction

۲- Shaking table test design

۳- Analysis of test results

۴- Conclusions

References

Abstract

Soil-structure interaction (SSI) has a significant effect on the earthquake response of a base-isolated structure, particularly on the rotation response of the SSI system and the isolation performance of the isolation layer, as demonstrated by previous shaking table tests (Zhuang et al., 2014). On a softer soil foundation, the SSI should have a greater influence on the seismic response of an isolated structure. To this end, a new shaking table test is conducted to estimate the effect of SSI on the dynamic characteristics of a base-isolated structure on a multi-layered soil foundation including a soft clay layer. As expected, the isolation efficiency of the isolation layer is reduced by the SSI effects, especially with increasing peak ground acceleration (PGA) of the input motion. Compared with the test results for an isolated structure on a harder soil foundation, the rotation responses of the pile cap and the isolation layer in this study are stronger. Additionally, the rotation responses of the pile cap are significantly amplified by the isolation layer. This type of amplification effect can become stronger with increasing PGA of the input motion, which differs from the results for previous tests with a base-isolated structure on a harder sand foundation. Meanwhile, when the natural isolation property of the softening soil layer is considered, the seismic responses of a base-isolated structure are reduced by the SSI effects because the natural isolation of the soft soil layer can compensate for the lost isolation ability of the isolation layer.

Introduction

Soil-structure interactions (SSI) have complex effects on the dynamic characteristics of structures situated on soils, including baseisolated structures on soft soil [23,21]. However, conventional structural design methods neglect SSI effects, which is generally reasonable for a light structure on relatively stiff soil, e.g. low-rise buildings and simple rigid retaining walls. The effect of the SSI, however, becomes prominent for a heavy structure resting on a relatively soft soil foundation, e.g. nuclear power plants, high-rise buildings, and elevated highways on soft soil [23]. The 1995 Kobe earthquake highlighted that the seismic behaviour of a structure is strongly influenced not only by the response of the superstructure, but also by the response of the foundation and the ground [15]. Hence, the response analysis method should take into consideration the whole structural system, including the superstructure, foundation, and ground, as underlined in the Standard Specifications for Concrete Structures: Seismic Performance Verification [8]. For a base-isolated structure, the isolator should increase the natural period and effective damping ratio of the structural system. However, soft soil sediments can significantly elongate the period of seismic waves. As a result, the isolation efficiency of the isolator is reduced by the effect of the SSI [27,13]. Base-isolated structures can also resonate with long period ground vibrations [16]. Accordingly, some analytical methods have been developed to address this problem [6,17,10,13,21]. In these methods, a base-isolated structure is regarded as having a single degree of freedom, and the SSI system is modelled as an elastic soil spring and a viscous damper [28,25]. A disadvantage of these simplified methods is that the soil softening during a strong earthquake cannot be considered effectively, which affects the dynamic responses of the isolator and isolated structure.