Earthquake Spectra

The concept of seismic or base isolation as a means of earthquake protection seems to be more than 100 years old. However, until very recently, few structures were built using this principle. Today the concept has matured into a practical reality and is taking its place as a viable alternate to conventional (fixed base) seismic resistant construction. This paper reviews some of the history of isolation and restates the basic elements of a modern isolation system. It then proceeds to review current activity, worldwide. Progress in the United States is discussed first followed by that in China, France, Greece, Italy, Japan, New Zealand and the Soviet Union. Directories of isolated structures in the United States, New Zealand and Japan are also included. Finally the performance of a selection of these structures during actual earthquakes is given.

Seismic isolation of structures has been applied in New Zealand since 1973. To date approximately 45 bridges, 3 large buildings and a few other structures have been protected with this technique. These include 40 bridges and 2 buildings designed by Works and Development Services Corporation (NZ) Ltd (WORKS). Numerous energy dissipating devices have been developed and tested by New Zealand researchers. Six of these designs have proved to be convenient and economical and have been incorporated in the seismic isolation systems of the structures built. Development work on seismic isolation devices is continuing in New Zealand and contact with specialists from other countries - in particular from Japan and the United States of America - is being maintained. Seismic isolation has been found to be a cost effective means of mitigating earthquake effects, particularly if the long term benefits of reduced seismic damage and disruption are taken into consideration.

The idea that a building can be uncoupled from the damaging effects of the ground movement produced by a strong earthquake has appealed to inventors and engineers for more than a century. Many ingenious devices have been proposed to achieve this result, but very few have been implemented and the concept now referred to as base isolation or seismic isolation has yet to be generally accepted by the engineering profession. Although most of the proposed systems are unacceptably complicated, in recent years a few practical systems have been developed and implemented. While some of these systems have been tested on large-scale shaking tables, none have to date been tested as-built by a strong earth tremor. The shake table testing and related static testing of full-scale components such as isolation bearings, however, has led to a certain degree of acceptance by the profession and it is possible that the number of practical implementations of base isolation will increase quite dramatically in the next few years. This paper describes recent implementations of base isolation and describes an approximate linear theory of isolation which can be used for the design of base isolation systems that use multilayer elastomeric isolators.

Seismic isolation is a design technique that offers significant benefits in appropriate applications, and interest in its application continues to grow. The two key issues that must be addressed early in the design phase of a project are the technical and economic feasibility issues. This paper focuses on the economic issues, and discusses the four principal cost factors that should be evaluated. These being construction costs, earthquake insurance premiums, physical damage that must be repaired and disruption costs, loss of market share and potential liability. The paper includes a summary of first cost studies on both new and existing buildings. It also discusses methods of estimating the difference in the cost of earthquake damage using different construction techniques.

Most of California's population and industry are located in zones of high seismicity, and the Federal Emergency Management Agency (FEMA) estimated that a 7.5 to 8.3 Richter magnitude earthquake in an urban area could cause up to $60 billion in damage (1). Such an earthquake could cripple the state's public and private economies, and, as California's economy is the sixth largest in the world, have a negative effect on the world market. Building practices in California offer only minimal protection from seismic damage, however new technologies, such as seismic isolation, can mitigate damage and are becoming available to government and industry. There is a need for design professionals, building officials, planners, and building owners to become aware of these new technologies, and the legal constraints to their use, and incorporate them into practice, and for engineering and architectural educators to include new seismic design technologies in undergraduate curricula.

San Bernardino Country's willingness to “experiment” with a base isolated building raises broader questions about how engineering innovations are adopted and implemented. Focusing on group decision making, this paper explores the questions that need to be examined if more rapid use of new engineering techniques is desired.

There are over 125 civil engineering structures worldwide that have been constructed using the principles of seismic isolation and 15 of these are in the United States. Although use of the technology is increasing in the United States it is significantly less than that seen in Japan and New Zealand. Some of the impediments that have been encountered, such as design codes, economics and government leadership are discussed and the paper provides a summary of the status of some of the solutions required for its more widespread use in the United States. A comparison with the Japanese implementation process is also provided.

The original Mackay School of Mines Building was constructed in 1908. It is one of the original buildings of the University of Nevada, and is situated at the north end of the main quadrangle within the campus. Prominent in its location at University of Nevada and in appearance, the building is designated as a national historic monument. During the years of 1926 and 1956, significant structural alterations were made to the original building. Phase III work at the original Mackay School of Mines Building involves adding a library at the basement, with the balance of the building being remodeled for similar-type functions. Constructed mainly of unreinforced masonry, the seismic rehabilitation of the structure warrants careful attention. During the schematic phase of the work, both conventional strengthening and Base Isolation were explored as potential techniques with which to mitigate damage from earthquakes. Cost estimate of both schemes were also developed. From a preservationist point of view, there were definite advantages in the isolation design. Since the isolation system could filter out most of the damaging forces associated with earthquakes, none of the unreinforced masonry walls required strengthening. As a result, many of the original architectural features of the original building can be salvaged, maintaining the original quality of the building and its identity. For these reasons the Base Isolation option was selected as the seismic retrofit scheme. This paper illustrates the Base Isolation design for the Mackay School of Mines, a historical structure constructed of unreinforced masonry. The isolation system consists of high-damping rubber bearings in combination with sliding elements.

Seismic isolation for buildings is an evolving state-of-the-art concept that is gaining acceptance for reducing seismic response of structures as well as the equipment housed therein. As there are no standards or codes covering this concept, owners are using independent engineer reviewers to help ensure that the isolation concept is viable for their buildings and the resulting design and construction meets the agreed-upon project needs. The responsibilities and scope of review, types of data reviewed, and questions asked by the reviewer are important ingredients of a successful project.

An innovative seismic isolation system, the Friction Pendulum System (FPS), offers improvements in strength, versatility and ease of installation as compared to previous systems. Moreover, the approach offers several inherent performance benefits not available before. The FPS uses geometry and gravity to achieve the desired seismic isolation results. It is based on well known engineering principles of pendulum motion, and is constructed of materials with demonstrated longevity and resistance to environmental deterioration. The desirable isolation characteristics exhibited by FPS components hold the promise of an effective and practical system for significantly increasing the seismic resistance of new and existing buildings. This paper summarizes results of a comprehensive research and testing program to assess the technical performance of the FPS. In addition, an example building design using the FPS is given.

The seismic performance of hypothetical low and high-rise steel framed structures founded on both soft and stiff soils in Mexico City and equipped with (i) friction damping devices, (ii) base isolators and (iii) a combination of base isolators and friction damping devices is compared. The response of the three structural systems, including soil- structure interaction, is examined for two specific sites in Mexico City: the stiff hills zone and the soft lake bed zone. The results of the study show that although soil-structure interaction can be beneficial for some base isolated structures, friction damping alone provides a more consistent way of protecting structures in Mexico City against earthquakes.

The dynamic response characteristics of a slip surface isolation system having restraints which produce a bilinear hysteresis are investigated. Design issues pertaining to base isolated structures are reviewed with regard to slip surface systems. The proposed restraint mechanism results in three parameters which can be selected so as to produce the optimal behavior for a given structure. Results show that the proposed system can be effective in reducing accelerations in the structure and that it may offer certain advantages compared to other isolation devices.

Seismic isolation offers an attractive approach for reducing seismic loads in nuclear structures, and more significantly, in reactor components. Isolation will lead to a simplification of designs, facilitate standardization, enhance safety margins, and may potentially reduce cost. To date, six large Pressurized Water Reactor units have been isolated in France and South Africa and several advanced nuclear concepts in the U.S., Japan, and Europe have incorporated this approach. It is recognized that to qualify and license an isolation system in the U.S. and in Japan, a comprehensive testing program of isolation components and systems would be required. A major seven year program was initiated in Japan in 1987 with the objective of establishing a qualified seismic isolation design for a large fast breeder reactor to be constructed at the end of this decade. In the U.S., two concepts which use steel laminated elastomeric bearings for seismic isolation have been developed. One of these concepts is a novel system which provides three-dimensional isolation. An extensive test program of scaled prototype bearings to demonstrate their feasibility and effectiveness has been carried out.

Earthquake simulator tests were performed on a 1/5-scale, 6-story reinforced concrete shear-wall structure and a 1/4-scale, 9-story braced steel frame structure. The structures were supported by five different base isolation systems which consisted of various types and combinations of elastomeric bearings. The main objective of this study was to compare the peak experimental displacements of the base isolation systems tested with values given by the tentative base isolation design provisions proposed by the Seismology Committee of the Structural Engineers Association of Northern California (SEAONC). Comparisons of experimental results and values from the SEAONC base isolation design formula for displacements indicated that the formula is generally conservative, even for predominantly low frequency earthquake motions, provided the ground motion coefficient A _v (based on the effective peak velocity as defined by ATC 3-06) is used in the design equation for base-isolated structures with periods greater than 1 second.

Seismic isolation was selected as a potential method of increasing the structural integrity margins for liquid metal reactor power plants. Analyses indicated that seismic isolation would reduce by 90 to 95% the acceleration experienced by the reactor vessel at its fundamental frequency. A cooperative development program was established by electric utility organizations in Japan, the United Kingdom and the United States. Alternative seismic isolator concepts were compared and the laminated elastomer/steel with lead plug concept was selected to be the first concept tested. Sixteen half-sized units were tested, the results compared with predictions and potential isolator design improvements inferred. The cooperative program is continuing through testing of other isolator concepts.