Seismic isolation is a technology and construction strategy with a significant number of applications. Just in Japan, the number of applications exceeds 5000 large buildings and another 5000 one and two-story houses (Cilsalar et al. 2019). Worldwide applications of the technology include houses, apartment buildings, hospitals, museums, courthouses, emergency operation centers, LNG and other tanks, offshore oil and gas platforms, airport terminals, and high-rise buildings. The analysis and design of seismically isolated buildings is governed by codes and standards, and typically involves use of nonlinear response history analysis (e.g., Chap. 17 of standard ASCE 7–16; ASCE/SEI 2017). Results of the analysis used in design of seismically isolated buildings include the isolator resultant displacement, the base shear force, and story drift and shear forces. This paper compares analytical and experimental results on the isolator displacement, base shear force, and story drift that are used for the primary system design.
Codes and standards include specifications that are used for the design of secondary systems. Chapter 13 of ASCE 7–16 (ASCE/SEI 2017) presents equations for the design of non-structural components attached to floors. The equations include the peak floor acceleration that can be computed using prescriptive equations or response spectrum analysis procedures. For seismically isolated buildings, engineers often use these equations with the peak floor accelerations predicted by the nonlinear response history analysis procedures described in Chap. 17 of ASCE 7–16. Generally, the design of secondary systems is affected by many structural system response parameters, such as the structural drift (e.g., piping systems, cladding, etc.), the peak floor acceleration (e.g., rigidly attached equipment), the spectral floor acceleration (e.g., flexibly attached equipment), and combination of response parameters (e.g., cabinets and equipment that are allowed to rock, and complex systems such as sprinkler systems, gas supply systems, etc.). Accordingly, this paper also considers and compares analytical and experimental results on floor accelerations, floor total velocities and floor response spectra which are used or may be used for secondary system design.
Many past analytical studies (e.g., Kelly 1982; Chen and Soong 1988; Fan and Ahmadi 1990) and experimental studies (e.g., Kelly and Tsai 1985, 1988; Juhn et al. 1992; Soroushian et al. 2016; Ryan et al. 2016) considered the response of secondary systems attached to the primary system of seismically isolated buildings, where they established the potential benefits of seismic isolation in reducing the response of secondary systems. However, there has been little work on assessing the validity and accuracy of methods of analysis of seismically isolated buildings with emphasis on the secondary system response. Apart from a limited in scope study by Juhn et al. (1992), Wolff and Constantinou (2004) presented an extensive study on the verification of accuracy of methods of dynamic analysis of secondary systems in seismically isolated structures by comparing to experimental data. More recently, studies by Fenz and Constantinou (2008) and Sarlis et al. (2013) reported experimental results on isolated structures with double and triple friction pendulum isolators, including analytical predictions and comparison to experimental results on floor accelerations. Many more studies concentrated on validating analytical models of isolators by concentrating on the primary system response in terms, primarily, of isolators displacements and shear forces (e.g., Fenz and Constantinou 2008; Dao et al. 2013; Sarlis et al. 2013; Ryan et al. 2018).
In the study of Wolff and Constantinou (2004), on which this paper is based on, the tested model was 233kN in weight and configured as a 6-story building at quarter length scale in three building configurations (moment-frame, asymmetrically braced frame and symmetrically braced frame), and with the following seismic isolation systems: a) Low damping elastomeric bearings with and without linear or nonlinear viscous dampers, b) Single Friction Pendulum (FP) bearings with and without linear or nonlinear viscous dampers, c) Lead-rubber bearings, and d) Low damping elastomeric bearings in combination with flat sliding bearings. Response quantities compared included story drifts and shear forces, and isolator displacements for the primary system, and peak floor accelerations and velocities and floor response spectra that relate to secondary system response. A total of 227 experiments were conducted in 24 configurations (3 superstructures and 8 isolation systems) using 14 historic ground motions of far-fault and near-fault characteristics. This paper presents a sample of results from the Wolff and Constantinou (2004) study that concentrates on the structural system with the largest story drift and with contemporary configurations of isolation systems: the moment frame superstructure, elastomeric isolators with linear and nonlinear viscous dampers, friction pendulum isolators without and with linear and nonlinear viscous dampers and lead-rubber isolators. Also, results are presented for only two earthquake ground motions; the 1940 Imperial Valley, El Centro S00E record (peak acceleration of 0.34g, peak velocity of 0.34m/sec-scaled up by factor of 2 for the test and with time compressed by factor of 2), a far-field motion, and the 1994 Northridge, Sylmar NR 900 record (peak acceleration of 0.60g, peak velocity of 0.77m/sec-used as recorded for the test and with time compressed by factor of 2), a near-fault motion. These two earthquake ground motions resulted in the largest isolator displacements which were, in the length scale of the tests, about 65mm for the low damping elastomeric system and about 60mm for the single FP system. Only one directional seismic excitation was used, which simplified the complexity of the analytical prediction. All analytical models were developed in program SAP2000 (Computer and Structures 2016), which nowadays is the most commonly used program in the analysis of seismically isolated buildings. The interested reader is referred to an extensive presentation of 288 comparisons of analytical and experimental results which is available in an MCEER report (Wolff and Constantinou 2004). Another paper of the authors (Wolff et al. 2015) presented another sample of results from this study concentrating on comparisons of the experimental responses, and some analytical predictions, of the model structure with elastomeric and single friction pendulum isolators when dampers were added to the isolation system.