Introduction

Abstract

This chapter gives a brief introduction to sliding base (SB) structures. The basic idea of SB structures is interpreted, and experimental studies on the properties of potential materials for using in the sliding interfaces and analytical studies on the responses of SB structures under harmonic and earthquake ground motions are reviewed.

1.1 Fundamentals of Sliding Base (SB) Structures

Base isolation is an effective approach for reducing damages to structures and their contents under severe earthquake excitations. It is generally implemented by using special isolators, such as laminated rubber bearings (e.g., Kelly, 1986; Kikuchi & Aiken, 1997; Skinner et al., 1993; Yamamoto et al., 2009) and friction pendulum (FP) bearings (e.g., Becker & Mahin, 2013; Castaldo & Tubaldi, 2015; Mokha et al., 1991; Roussis & Constantinou, 2006). The properties of these isolators can be elaborately designed to achieve a certain structural performance. However, they are expensive and require high construction techniques. Therefore, when the cost is a major concern, base isolation using isolators may not be an appropriate choice.

Adopting a sliding interface between the base of the superstructure and the foundation (Fig. 1.1) can also reduce the seismic response of the superstructure. The mechanism is very simple: as the friction force between the superstructure and the foundation has an upper limit, the seismic force transmitted to the superstructure is limited. Structures that adopt this type of isolation technique are called sliding base (SB) structures in this book. Since the implementation of SB structures is simple and cost effective, they are applicable to low-rise buildings in rural areas. Actually, SB structures have been used in some low-rise masonry buildings (Li, 1984; Zhou, 1997).

Fig. 1.1

Schematic plot of a building adopting SB technique

1.2 Practical Implementations of SB Structures

For the past four decades, several materials have been investigated regarding their potential use along the sliding interface of SB structures. Qamaruddin et al. (1986) conducted shaking table tests on sliding brick building models with different sliding layer materials, namely, graphite powder, dry sand, and wet sand, and obtained friction coefficients of 0.25, 0.34 and 0.41, respectively, for the corresponding interfaces. Tehrani and Hasani (1996) conducted experimental studies on adobe buildings with dune sand and lightweight expanded clay as sliding layers; the friction coefficients were reported as 0.25 for dune sand and 0.2–0.3 for lightweight expanded clay. Barbagallo et al. (2017) tested a steel-mortar interface lubricated by graphite powder; the static and dynamic friction coefficients were close to 0.19 and 0.16, respectively, and they were independent of both the sliding velocity and the superstructure properties.

Polymer materials are also suitable choices for the sliding interface. Yegian et al. (2004) investigated the frictional characteristics of four synthetic interfaces [namely, geotextile-high density polyethylene (HDPE), polypropylene (PTFE)-PTFE, ultrahigh molecular weight polyethylene (UHMWPE)-UHMWPE, and geotextile-UHMWPE] as potential candidates for sliding isolation through cyclic and shaking table tests. It was determined that the geotextile-UHMWPE interface was suitable for sliding isolation applications because the friction coefficient of this interface is insensitive to large variations in the sliding velocity and normal stress; as a result, this interface can easily be introduced into engineering design. The obtained static and dynamic friction coefficients of the geotextile-UHMWPE interface were approximately 0.11 and 0.08, respectively. Nanda et al. (2012, 2015) conducted experimental studies on four sliding interfaces with green marble against HDPE, green marble, geosynthetics and rubber sheeting, respectively. The static friction coefficients were found to be independent of the normal stress, and the dynamic friction coefficients were insensitive to variations in the sliding velocity. Moreover, the observed dynamic friction coefficients of the four investigated interfaces ranged from 0.07 to 0.18, and the relative differences between the static and dynamic friction coefficients were all below 15%. Jampole et al. (2016) adopted sliding isolation bearings consisting of HDPE sliders and galvanized steel surfaces to seismically isolate light-frame residential houses; shaking table tests showed that the friction coefficient of this sliding interface was nearly 0.18 with a slight variation between the stick and sliding phases.

The aforementioned sliding interfaces were basically insensitive to the variations in the sliding velocity and pressure. Therefore, the Coulomb friction model can be used to model the behavior of the sliding interface of an SB structure.

1.3 Review of Analytical Studies on SB Structures

1.3.1 Studies on 2DOF SB Systems Subjected to Harmonic Ground Motions

The simplest model for an SB structure contains two masses, one for the superstructure and the other for the sliding base; thus, this model is referred to as a 2-degree-of-freedom (2DOF) SB system. Westermo and Udwadia (1983) and Mostaghel et al. (1983) first studied the dynamic responses of 2DOF SB systems under harmonic excitations, and both groups developed the governing equations of motion and numerical implementations needed to perform response history analyses of such systems. Westermo and Udwadia (1983) pointed out that the response of this system under harmonic excitations converged rapidly to a periodic response after several cycles. Three different periodic responses were observed, namely stick-stick, stick-sliding, and sliding-sliding cases, depending on the amplitude of the input accelerations and the structural characteristics of the system. They also derived the explicit equations for the condition of the initiation of the stick-sliding case. Mostaghel et al. (1983) conducted parametric studies for the critical responses of the 2DOF SB system under harmonic ground motions. Iura et al. (1992) followed the work of Westermo and Udwadia (1983), and Mostaghel et al. (1983). They derived the analytical expressions for the condition of the initiation of the sliding-sliding case. Therefore, combined with the work of Westermo and Udwadia (1983), the explicit equations for the occurrence conditions of three periodic response cases were obtained. More recently, Hu and Nakashima (2017) conducted a comprehensive parametric study on the maximum responses of 2DOF SB systems under harmonic ground motions and derived a theoretical solution for the response corresponding to the sliding-sliding case.

1.3.2 Studies on SB Structures Under Earthquake Excitation

Mostaghel and Tanbakuchi (1983) studied the seismic responses of 2DOF SB systems subjected to the N-S component of the El-Centro record from the 1940 Imperial Valley earthquake and the S86E component of the Olympia record from the 1949 Western Washington earthquake. Response spectra of the absolute acceleration and sliding displacement were developed. It was found that the SB isolation can effectively control the level of the superstructure response. Qamaruddin et al. (1986b) conducted a study similar to that of Mostaghel and Tanbakuchi (1983), with emphasis on using the SB system in masonry buildings. The N-S component of the El Centro record and the longitudinal component of the Koyna record were considered. The findings of their study were similar to those of Mostaghel and Tanbakuchi (1983). Yang et al. (1990) and Vafai et al. (2001) studied the responses of multiple-degree-of-freedom (MDOF) SB structures. Their focus was to develop efficient numerical methods for response history analyses, whereas the response characteristics of these structures were not sufficiently addressed.

Jangid (1996a) compared the responses of single-story SB structures subjected to two components and a single component of the El-Centro record; the numerical results indicated that the former increased the sliding displacement and reduced the absolute acceleration of the superstructure in comparison with the latter. Shakib and Fuladgar (2003a) adopted the same model as Jangid (1996a) but included the vertical component in the ground motion input, and three ground motion records were considered, namely, the El-Centro record, the Tabas record from the 1978 Tabas earthquake, and the Renaldi record from the 1994 Northridge earthquake. The effect of the vertical component was highly dependent on the superstructure period and the input ground motions; additionally, the responses of low-period structures could be strongly affected by the vertical component of the ground motion, while this influence was insignificant when the superstructure period exceeded 0.7 s. Jangid (1996b) and Shakib and Fuladgar (2003b) also studied the responses of asymmetric single-story SB structures under multidimensional inputs. Both studies indicated that the bidirectional interaction between the frictional resistance at the sliding interface and the vertical component of the earthquake excitation could significantly affect the responses of torsionally coupled systems with SB isolation. More recently, Hu et al. (2020, 2022) investigated the peak superstructure response and peak sliding displacement of SB structures subjected to three-component earthquake excitation using a large number of ground motion records. The influence of various structural and ground motion characteristics on these two response quantities was comprehensively studied, and simplified design equations were also developed.