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Seismic Isolation and How It Works
SEAONC’s TECHNICAL LEADERSHIP in SEISMIC ISOLATION
Martin Button, Ph.D., P.E.
Editor: Reinhard Ludke, S.E.
This document outlines the contributions of SEAONC members to the development of seismic isolation. Equally important to this development were contributions by members of SEAOSC. We also acknowledge the groundbreaking accomplishments in seismic isolation technology made by engineers in New Zealand, Japan, and Europe.
1. Seismic Isolation and How It Works
Seismic isolation, also known as “base” isolation, is a means of protecting a structure, building or bridge, from the devastating effects of earthquake shaking. The structure is protected by “decoupling” it from the shaking ground. Practically this is achieved by supporting the structure on several specially designed elements, isolators, that are stiff vertically – hence able to support the weight of the structure without deforming – and flexible in the horizontal direction, achieving the desired “decoupling.” The idea is for the building to remain almost stationary during an earthquake as the ground moves underneath and the isolators absorb most of the motion. Relative to a similar conventionally founded structure, deformations, accelerations and demands on the lateral force-resisting system of an isolated structure are much lower.
Isolated Structure Conventional Structure
Earthquakes generally have more energy associated with shorter periods of vibration and less energy associated with longer periods. Seismic isolation works by lengthening the horizontal period of vibration of a structure (achieved by the introduction of horizontally flexible isolators between the superstructure and the ground), thus reducing the amount of seismic energy which is transferred into the structure. Less seismic energy in the structure means less damage.
The lengthening of the horizontal period typically means that large horizontal displacements result. These displacements can be as large as several feet and occur primarily in the isolation system. Isolators are designed to safely support the weight of the structure in a horizontally offset position and to bring the structure back close to its original position at the conclusion of the earthquake. Isolators must also be designed to add energy dissipation (damping) into the response of the isolated structure to control the horizontal displacements demands on them.
For more information, see “Seismic Isolation – A Primer”, by James M. Kelly and Jiang Jun Lee  and “Design of Seismic Isolated Structures: From Theory to Practice”, by Farzad Naeim and James M. Kelly .
2. History of Seismic Isolation
Engineers have been studying how to decouple structures from the ground for 150 years. For excellent reviews of the historical development of seismic isolation technology see “Seismic Isolation: Early History” by Nicos Makris  and “Seismic Isolation: History, Application and Performance – A World View” by Ian Buckle and Ron Mayes .
Each of the two references describe the pioneering work of U.C. Berkeley Professor James Kelly, which lead directly to the first seismically isolated building in the United States. Makris  states:
“In 1976, JM Kelly, in association with CJ Derham from the Malaysian Rubber Producers’ Research Association (MRPRA) began working on the development of natural rubber bearings for the seismic protection of building and bridges. Over a period of about 5 years, this collaboration led to a series of experimental tests, both at the component level and on entire isolated structures on the shaking table of the Earthquake Engineering Research Center (EERC) at U.C. Berkeley. The results from these early tests were very promising and led to the first base‐isolated building in the United States, the Foothill Communities Law and Justice Center” in San Bernardino.
In the early 1980s, when SEAONC’s Alex Tarics was working this project, he persuaded San Bernardino County officials to include seismic isolation in the design. Reflecting on those early days of seismic isolation, Professor Kelly said, “Alex’s persuasiveness got this thing going.”
Ten years later, seismic isolation had matured, become codified, and been used as a seismic protection strategy for major buildings and bridges in the United States and other countries.
3. Seismic Isolator Types
Two types of practical seismic isolators dominate the world-wide market for seismic isolation hardware today.
The first type derives its horizontal flexibility from a natural rubber elastomer. To meet the requirement that the isolator be stiff in the vertical direction with a high load-carrying capacity, thin horizontal layers of vulcanized rubber are sandwiched between alternating thin steel shim plates in a laminated construction, shown below (graphic courtesy of Dynamic Isolation Systems, Inc. [DIS]).
DIS Seismic Isolator
Elastomeric isolators provide high initial stiffness and damping via: (a) an energy dissipating central lead core; or (b) a rubber compound formulated to have high stiffness at low deformation and low stiffness at high deformation. In isolators with lead cores, the lead core provides the high initial stiffness while it remains elastic, then dissipates energy as it cycles back and forth plastically. In isolators with deformation-dependent rubber (so-called high-damping rubber), the rubber compound itself provides the high initial stiffness and damping characteristics. (High initial stiffness is required to control displacements to imperceptible levels during windstorms.) Regardless of internal mechanism within an elastomeric isolator, the plot of horizontal force versus lateral displacement of the isolator typically looks like this (graphic courtesy of DIS):
Isolator Hysteresis Loops
The graph (a set of hysteresis loops) shows high stiffness at low lateral displacements, and each cycle of displacement encloses an area, indicative of energy dissipation. The loops have a positive slope indicating a restoring force is generated to return the supported structure to close to its original position.
The second type derives its horizontal flexibility from sliding between two surfaces. Friction pendulum isolators are the most common type of friction slider. Sliding occurs in a spherical dish and the motion mimics that of a pendulum. As lateral movement occurs, the supported structure necessarily rises slightly as the slider travels up the dish. The resulting potential energy creates a restoring force that returns the supported structure to close to its original position. The friction mechanism dissipates seismic energy as the isolator cycles back and forth, and provides high initial stiffness under non-seismic lateral loads.
Friction pendulum isolators have evolved from the original single dish with one sliding surface to the current norm: a Triple PendulumTM isolator with three sliding regimes, as shown below (graphic courtesy of Earthquake Protection Systems [EPS]).
EPS – Single Pendulum Isolator
EPS – Triple PendulumTM Isolator
The theoretical plot of horizontal force versus lateral displacement of a Triple PendulumTM isolator can take on a variety of shapes depending on the radii and friction coefficients for the various sliding surfaces. For an isolator with typical practical properties, the plot looks like this (graphic courtesy of EPS):
EPS – Graph of SHEAR vs. DISPLACEMENT
High Stiffness at Low Displacements Hysteresis Loops
EFFECTIVE STIFFNESS and TANGENT STIFFNESS
The graph (a set of hysteresis loops) shows high stiffness at low lateral displacements, with successive softening as displacements increase, and final stiffening regimes to control displacements during the most severe earthquakes. All loops have a positive slope arising from the “dish” geometry of the isolator.
SEAONC Members’ Contributions to Isolator Hardware Development
SEAONC members are pioneers in the development of seismic isolation hardware. Professor James Kelly conducted innovative research on high damping rubber isolators at the University of California, Berkeley. Dr. Ron Mayes, Dr. Lindsey Jones (co-founders of DIS) and Dr. Ian Buckle and Konrad Eriksen, took lead-rubber isolator technology developed in New Zealand and refined it to produce large-scale rubber isolators with high vertical load and lateral displacement capacity. Dr. Victor Zayas invented the Friction Pendulum isolator and with Dr. Anoop Mokha and Stanley Low, his company, EPS, has continued to develop the product and refine its manufacture.
4. Related Technology to Support the Practical Application of Seismic Isolation
The practical application of seismic isolation depended upon advances in three related technical areas. In each area, SEAONC engineers were at the forefront.
a. Computer models of isolated structures are necessary to predict their dynamic response, both globally quantifying the benefits of seismic isolation and locally computing force and displacement demands on individual isolators.
Two SEAONC members, UC Berkeley Professors Ed Wilson and Graham Powell were at the forefront of the development of software for nonlinear dynamic analysis. At Computers and Structures (CSI), a major SEAONC Sponsor, Ashraf Habibullah commercialized the UC Berkeley software and today most seismically isolated projects in the United States use CSI software. SEAONC’s Dr. Martin Button, amongst others, was an early user of CSI software, applying it to the analysis of seismically isolated structures before explicit isolator models became available. Both DIS and EPS provided technical support to many of the early projects they were involved in. At DIS Dr. Martin Button, Dr. Ian Buckle and Trevor Kelly provided nonlinear analysis and isolator design support. At EPS, Dr. Victor Zayas and Dr. Anoop Mokha provided similar support.
b. Isolator test beds with large load and displacement capabilities are required to demonstrate that physical isolators could sustain large compression loads and simultaneous horizontal displacements. SEAONC’s Professor James Kelly at UC Berkeley developed an isolator test rig that was used to demonstrate the robustness of isolators for the first US isolated building in the mid-1980s. Physical testing of isolators demonstrates their ability to simultaneously support large loads and sustain horizontal displacements. Further, it allows lateral stiffness properties of isolators to be measured and compared with properties assumed in design.
Today, reputable isolator manufactures have in-house isolator test rigs. But some projects require a combination of compression loads, lateral displacements and lateral velocities that are significantly beyond the capability of manufacturers’ rigs. As a result, Caltrans and UC San Diego jointly developed the SRMD (Seismic Response Modification Device) Test Facility which came on-line in 1999. In addition to very large capacity, this facility offers independence from manufacturers’ in-house testing. SEAONC’s Dr. Ian Aiken was a member of the design review panel for this impressive piece of equipment.
Shake table tests at UC Berkeley (Professor James Kelly, Professor Steve Mahin), SUNY Buffalo (Professor Mike Constantinou), UC San Diego (Professor Tara Hutchinson) and at the E-Defense facility in Japan (Professor Steve Mahin, Professor Keri Ryan, UNR) have all played an important role in the understanding of the overall response of isolated structures.
c. Developments in engineering seismology, particularly in characterizing ground motions at a given site and at longer periods, has been vital to the acceptance of seismic isolation technology. Advances in seismology since the 1980s have enabled the generation of site-specific ground spectra and sets of site-specific ground acceleration records, which consider fault types and distances, site characteristics, and return period. These data are basic input to the analysis of seismically isolated structures. Use of site-specific ground motions gives greater confidence that the computed response of an isolated structure, including demands on the isolators, are soundly based. SEAONC members Dr. Neville Donovan and Maury Power were early contributors to the understanding of long-period ground motions. SEAONC members Dr. Ramin Golesorkhi, John Egan and Stanford Professor Jack Baker follow in their footsteps with current contributions.
5. Building Codes Addressing Seismic Isolation
The availability of a design code is a key step in the implementation of any new building technology. SEAONC was a leader in developing the first U.S. code that addressed seismic isolation.
In 1986 the Base Isolation Subcommittee, chaired by Eric Elsesser, of the SEAONC Seismology Committee published Tentative Seismic Isolation Design Requirements. The SEAOC State Seismology Committee then refined these requirements in Design and Construction of Seismically Isolated Buildings. In 1989, California’s Office of Statewide Health Planning and Development adopted a modified version of the SEAOC provisions in An Acceptable Method for Design and Review of Hospital Buildings Utilizing Base Isolation. The International Conference of Building Officials (ICBO) adopted the provisions developed by SEAOC as part of the 1991 Uniform Building Code, appearing as an Appendix to Chapter 23, Earthquake Regulations for Seismic-Isolated Structures.
The first edition of ASCE 7, Minimum Design Loads for Buildings and Other Structures, was published in 1998. Provisions for Seismically Isolated Structures appeared as a sub-section of Chapter 9, Earthquake Loads. By the 2005 edition of ASCE 7, Seismic Design Requirements for Seismically Isolated Structures appeared as Chapter 17, where these requirements remain today in ASCE 7-22. There are also provisions for seismic isolation as a retrofit strategy, codified as Chapter 14 in ASCE 41-17.
Since Eric Elsesser’s pioneering efforts in the mid-1980s, SEAONC members have continued to play a major role in the evolution of code provisions for seismic isolation. Dr. Charlie Kircher, Dr. Ron Mayes, Dr. Ian Aiken, Dr. Martin Button and Dr. Victor Zayas, amongst others from SEAONC, have made significant contributions. Bob Bachman of SEAOSC, and SUNY Buffalo Professors Mike Constantinou and Andrew Whittaker, a former SEAONC member, have also been at the forefront of seismic isolation code development.
6. Isolated Projects by SEAONC Firms and Members
SEAONC Seismic Isolation Projects Tables
The Seismic Isolation Projects Tables provide a list of seismically isolated projects engineered by SEAONC firms and members. Not all projects are in Northern California. The projects are about evenly split between new construction, including three new hospitals, and seismic retrofit of existing, mainly historic, structures. In addition, there are bridge and lifeline retrofit projects, demonstrating the range of applications of seismic isolation. Each project was Peer Reviewed as required by code. Most of the Peer Reviewers are SEAONC members.
List of Referenced Books and Publications
1. James M. Kelly and Jiang Jun Lee, Seismic Isolation – A Primer: The Theoretical Basis of a Protective System for New and Existing Buildings, Computers and Structures.
2. Farzad Naeim and James M. Kelly, Design of Seismic Isolated Structures: From Theory to Practice, Wiley, 1999.
3. Nicos Makris, “Seismic Isolation: Early History,” Earthquake Engineering and Structural Dynamics, 2019; 48:269-283
4. Ian G. Buckle and Ronald L. Mayes, “Seismic Isolation: History, Application and Performance – A World View,” Earthquake Spectra, Vol. 6, No. 2, 1990, 161-201
List of Referenced Websites
Dynamic Isolation Systems, Inc.: http://www.dis-inc.com/
Earthquake Protection Systems: https://www.earthquakeprotection.com/
Computers and Structures, Inc.: https://www.csiamerica.com/
UCSD SRMD Test Facility: https://se.ucsd.edu/facilities/laboratory-listing/srmd
PEER (U.C. Berkeley) Shake Table: https://apps.peer.berkeley.edu/laboratories/earthquake_simulator_lab.html
SUNY Buffalo Shake Table: https://www.buffalo.edu/seesl/services/shake_tables.html
UCSD Shake Table: http://nheri.ucsd.edu/
E-Defense Shake Table (Japan): https://www.bosai.go.jp/hyogo/ehyogo/profile/profile.html
Forell | Elsesser Engineers: https://forell.com/
Nabih Youssef Associates: https://www.nyase.com/
Rinne & Peterson, Inc.: https://rpse.com/
Rutherford + Chekene: https://www.ruthchek.com/
SOHA Engineers: https://www.soha.com/
- 1989 Loma Prieta Earthquake
- Dynamic Analysis and Earthquake Engineering