Seismic performance of nonlinear soil-structure systems located on soft soil considering foundation uplifting and soil yielding

doi:10.1016/j.istruc.2020.09.046

Structures

Volume 28, December 2020, Pages 973-982

https://doi.org/10.1016/j.istruc.2020.09.046 Get rights and content

Abstract

In most of researches on soil-structure systems, it is assumed that the foundation is bonded to the ground where no foundation uplift is allowed. Uplifting makes changes in force–displacement behavior of the soil-structure systems, which in turn alters structural demands. In this research, a set of non-dimensional parameters is considered which controls the behavior of uplifting systems. The effects of foundation uplift on response of soil-structure system are investigated parametrically through time history analysis for a wide range of systems subjected to harmonic excitation and also ground motions recorded on soft soil with predominant period. It is seen that the response of systems with more dominant soil-structure interaction effects will influenced more by foundation uplift. It is shown that the period of soil-structure systems to the predominant period of ground motion has an important rule on the response of uplifting systems. While this period ratio is greater than one, the total displacement of uplifting soil structure systems almost equals to elastic soil-structure system when uplifting is prevented but the drift of the structure as a part of soil-structure system while uplifting is permitted decreases in comparison with corresponding systems which foundation uplift is prevented. If the period ratio becomes smaller than one, systems which are allowed to uplift experience larger displacements in comparisons with corresponding case which foundation uplift is prevented. It is shown that neglecting the influence of foundation uplifting in seismic performance assessment procedure of stiff and squat structures located on soft soil is not conservative, because in this situation structures can experience larger drift due to foundation uplift in comparison with the case that uplifting is prevented while subjected to excitation with long predominant period. Also for nonlinear soil-structure systems it is shown that foundation uplifting and soil yielding can reduce the ductility of the structure as a part of soil structure system. It is observed that traditional structures with longer period of vibration which are designed based on conventional spectra will be influenced less by foundation uplifting and soil yielding.

Introduction

It is known that the soil affects the structural response in both elastic and inelastic ranges through both site effect and soil-structure interaction (SSI). Many researchers studied the SSI effect on elastic response of the structures [1], [2], [3]. From these researches it can be concluded that the main effect of SSI is to increase the natural period of the system and, usually, to increase its effective damping ratio. Then Veletsos and Verbic [4] and Bielak [5] studied the response of inelastic soil-structure systems. Müller and Keintzel [6] and Ghannad and Ahmadnia [7] showed that the ductility demand of structures, as a part of the soil-structure system, could be different than that of the fixed base system. Aviles and Perez-Rocha [8] studied the SSI effect on inelastic displacement ratios and strength reduction factors. The site-dependent strength reduction factors were studied by Ghannad and Jahankhah [9] for soil-structure systems.

In most of researches on soil-structure systems it is assumed that the foundation is bonded to the ground or in the other words foundation uplift is not allowed. However, there are many reports which show that several structures experienced separation from the supporting soil during earthquakes [10]. Uplifting can lead to increase or decrease in structural demands by changing the force–displacement behavior of the soil-structure system. Therefore, the effects of this phenomenon should be considered in any rational study on response of soil-structure systems.

There are two main categories which investigated the response of uplifting systems. First, there are studies which investigate the rocking response of rigid blocks. For instant, the studies of Housner [11], Psycharis and Jennings [12], Makris and Konstantinidis [13], Gazetas and Apostolou [14], Apostolou et al. [10] and Yim, et al. [15] can be mentioned in this category.

In second category the response of flexible structures on rigid or flexible base were studied. Meek [16] analyzed a flexible single degree of freedom system resting on soil and showed that the base shear of the structure due to uplift decreases and this reduction becomes significant for slender structures. Yim and Chopra [17] considered a flexible SDOF system with rigid foundation located on distributed springs. They showed reduction in the base shear of structures with short periods. Also studies of Oliveto et al. [18] showed the same results while considering large displacement behavior of uplifting systems under earthquake records. The results of studying seismic response of reactor structures were the same by Kennedy et al. [19] and Wolf and Skrikerud [20] while systems are allowed to uplift. Harden et al. [21] showed that inelastic displacement ratios in current seismic provisions could be highly un-conservative for systems allowed to uplift. Acikgoz and Dejong [22] revealed that flexible systems are more resistant to toppling while experiencing excessive deformation because of foundation uplift. Ghannad and Jafarieh [23] studied inelastic displacement ratios and ductility demand of soil-structure systems allowed to uplift. They considered elastic behavior for soil and described the response of systems through introducing new non-dimensional parameters.

In recent years there are some researches [24], [25], [26] which study the effects of foundation uplift and soil yielding on the seismic response of soil-structure systems. But it can be seen that there is no parametric study on the subject which considers the effects of foundation uplift, soil yielding and nonlinear behavior of superstructure, simultaneously. In this research the response of soil-structure systems allowed to uplift is investigated while subjected to the ground motions recorded on soft soil which have significant predominant period. Moreover, it should be noted that, in the seismic performance-based design philosophy inelastic displacements are used as a key parameter for design of new buildings and also evaluation of existing structures. It is known that all inelastic displacement ratios which have been proposed in seismic provisions [27] are based on fixed base models. While it is obvious that soil-structure interaction especially foundation uplift and soil yielding can affect the response of systems. Therefore, there is a need to investigate the simultaneous effects of SSI, foundation uplift and soil yielding on total displacement of soil-structure systems and ductility of the structure as a part of soil-structure system and describe the variations considering different non-dimensional parameters.

Section snippets

Soil-structure model

A shown in Fig. 1 a simplified model is used as the soil-structure system. A single degree of freedom model is considered as structure with the period, T_str, and damping ratio, ξ_str. The lumped mass, m_str, and the height, h, are the effective mass and the effective height of the equivalent structure, respectively. The structure is constructed on a rigid foundation mat which has been rested on distributed springs and dampers of soil elements. The translational stiffness of soil is modeled by a

Key parameters of uplifting soil-structure systems

Consider the model of Fig. 1 for soil-structure systems. The deformation of soil springs is not uniform and varies with time during excitation. Uplift starts when the springs of soil at one edge of the foundation become unstressed. This is called incipient uplift of foundation from the supporting soil. If it is assumed that the structure behaves elastic, thus, the only source of nonlinearity comes from uplift of foundation. As shown in Fig. 2 for lateral static loading, when the applied

Methodology of parametric study

To explore the effect of foundation uplift on response of soil-structure systems, a parametric study is conducted. Considering a wide range for non-dimensional key parameters, the soil-structure model of Fig. 1 is subjected to a collection of ground motions recorded on soft soil. Using open-source software OpenSees (Open System for Earthquake Engineering Simulation) [32], the analysis is conducted directly in time domain. The model is verified by the results which were presented in the studies

Elastic soil-structures allowed to uplift

As explained above the behavior of soil-structure systems can be described through a number of non-dimensional parameters. Also Ghannad and Jafarieh [23] showed that the concept can be extended to uplifting soil-structure systems by introducing two new parameters. Considering all the mentioned issues, the assessment of the response of uplifting soil-structure systems located on soft is done in two stages. First the response of uplifting soil-structure systems is studied while they are subjected

Response of nonlinear soil-structure systems

In this section the response of nonlinear soil-structure systems is assessed under excitations recorded on soft soil (Table 1). For inelastic structures allowed to uplift Ghannad and Jafarieh [23] introduced strength ratio (R_f) whose value defines the level of nonlinear behavior in the structure while uplift is allowed. R_f is the ratio of the strength of the structure (F_y) to the lateral load needed for initiation of uplift (P_iu) as mentioned below: $R_{f} = \frac{F_{y}}{P_{iu}}$

According to R_f three scenarios can be

Conclusions

For systems subjected to excitations which are recorded on soft soil, the effects of soil-structure interaction and foundation uplift on elastic response of the superstructure were investigated simultaneously. In order to conduct a parametric study, a set of non-dimensional key parameters was considered. These parameters are the ratio of the period of the soil-structure system to that of the corresponding fixed-base structure, T_ssi/T_str, the aspect ratio of the super-structure, h/b, uplift

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Seismic performance of nonlinear soil-structure systems located on soft soil considering foundation uplifting and soil yielding

Abstract

Introduction

Section snippets

Soil-structure model

Key parameters of uplifting soil-structure systems

Methodology of parametric study

Elastic soil-structures allowed to uplift

Response of nonlinear soil-structure systems

Conclusions

Declaration of Competing Interest

Soil Dyn Earthquake Eng

Soil Dyn Earthquake Eng

Nucl Eng Des

Nucl Eng Des

Soil Dyn Earthquake Eng

Soil Dyn Earthquake Eng

Dynamics of buildings-soil interaction

Bull Seismol Soc Am

Dynamic behavior of building–foundation systems

Earthquake Eng Struct Dyn

Effect of soil on structural response to wind and earthquake

Earthquake Eng Struct Dyn

Dynamics of elastic and yielding structure-foundation systems

Proceedings of the 5th World Conference on Earthquake Engineering

Dynamic response of non-linear building-foundation systems

Earthquake Eng Struct Dyn

Ductility requirements for flexibility supported antiseismic structures

The effect of soil-structure interaction on inelastic structural demands

Eur Earthquake Eng

Influence of foundation flexibility on Rμ and Cμ factors

J. Struct Eng (ASCE)

The behavior of inverted pendulum structures during earthquakes

Bull Seismol Soc Am

Rocking of slender rigid bodies allowed to uplift

Earthquake Eng Struct Dyn

The rocking spectrum and the limitations of practical design methodologies

Earthquake Eng Struct Dyn

Proceedings of the 5^th World Conference on Earthquake Engineering

Influence of foundation flexibility on R_μ and C_μ factors