PROCEEDINGS, STRUCTURAL ENGINEERS WORLD CONGRESS, SAN FRANCISCO, CALIFORNIA, JULY (1998) BASE ISOLATION DESIGN FOR CIVIL COMPONENTS AND CIVIL STRUCTURES F. F. Tajirian Seismic Isolation Engineering, Inc. P. O. Box 11243, Oakland, California 94611, USA ABSTRACT Seismic isolation is being used worldwide to protect buildings, new and old, and their contents from the destructive effects of earthquakes. This paper reviews applications of seismic isolation to civil components, tanks, and industrial facilities. The benefits of seismic isolation to such applications as well as differences in design requirements between building and non-building isolation are illustrated through the examples described. Seismic isolation of individual components is very beneficial in situations where existing components and their supports have to be requalified for higher seismic loads. By using seismic isolation, it may be possible to avoid expensive retrofitting of the supporting facility and the foundation. Three examples of this type of retrofit are given in the paper. INTRODUCTION It has long been recognized that power plant vessels, computers, sensitive equipment, and tanks typically found in industrial facilities are more vulnerable to earthquake damage than buildings. During the Northridge Earthquake, there was significant damage to buildings, especially hospitals, attributed to failure of contents such as tanks and pipes. Seismic isolation is a practical approach for providing seismic protection for such systems and components. This is demonstrated in this paper by reviewing several examples of seismic isolation where the primary purpose of using isolation was the protection of components. Although the acceptance of this technology for isolation of components and tanks has been slower than for buildings, future applications should increase as owners of industrial facilities realize that conventional seismic design techniques may not be adequate in protecting such equipment. In general, the rules developed for the design of isolators for buildings are also applicable to components. Major design issues that differentiate the design of seismic isolators for components from buildings include the following: • Components usually do not have sufficient mass. Consequently, the isolation system usually consists of four bearings. These systems have less redundancy than isolation systems used in buildings. It is therefore important to use high quality isolators to minimize chances of failure. The development of new isolation techniques, including softer elastomers and low friction rollers, would make it easier to isolate lighter components and possibly further lengthen the isolation period. This will make isolation of components more appropriate for soft sites with fundamental site periods between one and two seconds. • In existing facilities, isolated components may have to be located in confined spaces. Consequently, a sufficient gap may not be available around the isolated structure. • For isolated components that are located in the upper stories of a building, the displacements may be larger than the design displacements used in base isolation of buildings due to amplification of floor response. SEISMIC ISOLATION OF TANKS Seismic Vulnerability of Tank Structures Tanks have not performed well in recent earthquakes. Both concrete and steel tanks have been seriously damaged. Several types of tank failure have been observed. Tanks may be damaged for different reasons. Large shell hoop tensile stresses resulting from a combination of hydrostatic pressure and hydrodynamic pressure due to horizontal and vertical ground motions could fail the s tank. A more common type of failure is known as “elephant’ foot buckling”. This is caused by the large overturning base moments resulting from the impulsive and convective liquid loading on the tank wall during an earthquake. The high vertical compressive stresses, which develop in the s tank’ shell, may cause buckling of the shell. These forces are generally greater in tanks that are held down, thus tanks which are bolted down require thicker walls. High stresses near the hold- down bolts may results in tearing of the tank shell. If the tanks are unanchored, the tanks experience partial base uplifting which results in increased axial compressive stresses in the tank due to the reduced contact area. A new approach developed by Malhotra (1997) proposes to support the tank wall on a ring of vertically flexible rubber bearings, and the tank base plate is supported directly on the soil to limit the compressive stresses resulting from uplift. Analysis results by Malhotra have shown that overturning base moments are significantly reduced while still maintaining acceptable levels of base uplift. Unanchored tanks can be damaged if the horizontal seismic forces exceed the frictional resistance between the tank and its supporting base. Finally, tanks roofs may be damaged or the contained liquid may be spilled due to sloshing waves caused by the long period components of earthquakes. Seismic Isolation of LNG Tanks Tanks are being built in increasing numbers to store Liquefied Natural Gas (LNG). These tanks are very large and have capacities around 150,000 m3. They pose a great risk if they fail during an earthquake. The important design issues associated with LNG tanks are summarized in a paper by Bomhard and Stempniewski (1993). LNG tanks consist of an inner steel tank, which contains the LNG, and an outer concrete tank encasing and protecting the inner tank, with insulation placed between the two structures. The concrete tank is supported on a common concrete mat. The tank is supported by a group of closely spaced columns to allow air to circulate below. Gas leakage from the containment system can result in explosions and fires, and can cause catastrophic disaster for the environment as well as human life. Thus, the tank structures are subjected to very stringent seismic safety requirements, which can have a major impact on the design of the tank. Large tanks have a fundamental frequency between 2 and 10 Hz, placing them in the range of resonance of most earthquake ground motions. In the design of buildings, seismic energy absorption due to inelastic response is tolerated. In the case of tanks however, the requirement of tightness and the containment function stand in the way of using this approach. Normally, in the design of LNG tanks, it is preferable to avoid the use of anchor straps to prevent the inner tank from uplifting during an earthquake in order to minimize the welded attachments to the cryogenic steel. The mechanism of tank uplift is complex and not completely understood. To describe it fully, the effects of large displacements, yielding of the base plate, membrane forces in the base, phase relationship between the horizontal and vertical components and the effect of these parameters on the period of the system need to be considered. One approach to minimize the potential for tank uplift is to use tanks with large diameter to height ratios. Ratios of four have been used in some projects. This option may be a costly alternative resulting in undesirable tank shapes and inefficient use of the site area. Recently, two LNG tank projects have adopted seismic isolation to reduce seismic loads. The seismic lateral loads were significantly reduced, allowing the use of more reasonable tank dimensions, and diameter-to-height ratios as low as 2.3. Furthermore, economical tank designs developed for areas of low and moderate seismicity may be used in areas of high seismicity. Additionally, safety is enhanced by insuring that the containment capabilities of the outer tank are not impacted during a strong earthquake. Figure 1 shows a schematic section through an isolated LNG tank. The first project, is in Inchon on the west coast of Korea where three storage tanks, each having a capacity of 100,000 m3, are being constructed, Koh (1997). The inner tank height and diameter are 30 m and 68 m, respectively. Due to poor soil conditions at the site, a pile foundation system is used. The isolation system consists of steel-laminated rubber bearings with a diameter of 600mm and an overall height of 228 mm. The design isolation period is around 3 seconds. The design safe shutdown earthquake (SSE) has a maximum horizontal acceleration of 0.2 g. The second project is located on Revithoussa Island in Greece where two tanks, each with a capacity of 65,000 m3 are, being constructed. The inner tank, consisting of nickel steel, is unanchored and has a diameter of 65.7 m and a height of 22.5 m. The outer tank is made of prestressed concrete. The tanks are partially buried for reasons of aesthetics. Two preliminary designs were developed for this project. One non-isolated and the other isolated. The non- isolated alternative used the same inner tank geometry and massive anchors attached to both the outer and inner tanks. The inner tank had a thicker shell and special detailing was provided to minimize thermal effects. While it was determined to be the alternative with the least initial cost, the owner opted for the isolated alternative because it was perceived to be a safer design. In the design selected, each tank is supported on 212 isolators. The isolators consist of Friction Pendulum System (FPS) bearings with a radius of curvature of 1,880 mm and displacement capacity of 300 mm. The earthquake was defined in terms of elastic 5-percent damped spectra. Two levels were specified, the operating basis earthquake (OBE) and the safe shutdown earthquake (SSE). The SSE earthquake had a peak ground acceleration of 0.48g and spectral values of 0.61g and 0.29g at periods of 1.0 and 3.0 seconds respectively. The design called for elastic response of the tanks up to the SSE level event. It is expected that the tanks will be operational by 1999. INNER LIQUEFIED STEEL OUTER NATURAL GAS TANK PRESTRESSED (LNG) CONCRETE TANK INSULATION CONCRETE UPPER MAT CONCRETE SEISMIC PEDESTAL ISOLATOR CONCRETE FOUNDATION Figure 1: Schematic of Isolated LNG Tank Figure 2: LNG Tank in Revithoussa Island in Greece during construction (Constantinou 1997) SEISMIC ISOLATION OF ADVANCED NUCLEAR REACTORS Several countries have initiated programs to develop seismic isolation systems for advanced nuclear applications, Tajirian et al. (1990). The benefits of seismic isolation in power plant applications can be summarized as follows: • Permit standardization of the design regardless of seismic conditions. This will reduce capital costs so that future nuclear plants are competitive with those using alternate sources of energy. • Enhance the safety and reliability of nuclear power plants to regain public acceptance. • Facilitate decoupling of the reactor design and development, which is global in nature, from the balance of plant (BOP) design and licensing which is regional in nature. To date, six large Pressurized Water Reactor (PWR) units have been isolated in France and South Africa. The operational four unit Cruas plant in France, where the site safe shutdown earthquake (SSE) acceleration was 0.2 g, is supported on 1,800 neoprene pads measuring 50x50x6.5 cm, Postollec (1983). This design was developed for sites with moderate seismicity. For sites with higher seismicity, sliding plates are placed between the top of the neoprene pads and the upper mat, Jolivet and Richli (1977). The lower plate is made of a lead-bronze alloy and the upper plate, which is embedded in the upper mat, is stainless steel. The plate combination provides a friction factor of 0.2. When the ground accelerations exceed the friction coefficient, sliding occurs, thus limiting the shear strains in the pads and the forces in the building to the same level as that for moderate sites. This design was used by Framatome to isolate a two unit plant in Koeberg, South Africa, where the site SSE acceleration was 0.3 g. A total of 2000 pads measuring 70x70x10 cm were used. A similar design was licensed for the Karun River plant in Iran whose construction was suspended in 1978. Designs were also developed for a single 1300 MWe PWR in Laguna Verde Mexico, and a two unit 1400 MWe PWR at Le Carnet in western France. In the U.S. the Department of Energy (DOE) sponsored Advanced Liquid Metal Reactor (ALMR) has adopted seismic isolation to simplify the design, enhance safety margins, and support the development of a standardized design for the majority of the available U.S. reactor sites. The nuclear island is being designed for a safe shutdown earthquake (SSE) with a maximum horizontal and vertical acceleration (PGA) of 0.5g. Detailed seismic analyses of the ALMR have been performed and the results reported elsewhere by Tajirian and Patel (1993). The ALMR isolated structural configuration consists of a stiff rectangular steel-concrete box structure, which supports the reactor vessel, the containment dome, and the reactor vessel auxiliary cooling system stacks. The total isolated weight is about 23,000 tons and is supported on 66 high damping rubber bearings. The horizontal isolation frequency is 0.7 Hz, and the vertical frequency is greater than 20 Hz. A qualification program for the ALMR seismic isolation system was established to demonstrate that all seismic design and safety requirements are met. The essential categories are bearing tests, both reduced-scale and full-size, bearing environmental tests, and seismic isolation system tests. To date a large number of bearing tests have been performed and the results reported elsewhere by Tajirian et al. (1990), Clark et al. (1995). Another DOE-sponsored project, the Sodium Advanced Fast Reactor (SAFR) houses the reactor assembly module in a standardized shop-fabricated unit housed in a building constructed above grade with plan dimensions of 38 m by 25 m. The building, which weighs 31,000 tons, is supported on 100 seismic isolators. What makes the isolation system for SAFR unique is that it provides vertical as well as horizontal isolation. The design horizontal frequency is 0.5 Hz and the vertical frequency is 3 Hz. This is achieved by using bearings with thicker rubber layers than usual which are flexible vertically as well as horizontally. The bearing diameter is 107 cm, the total height is 41 cm, and consists of three layers of rubber each 10.2 cm thick with a shape factor of only 2.3. Reduced scale bearings were extensively tested to validate the concept of low shape factor bearings, Aiken et al. (1989). In Japan, the Central Research Institute of Electric Power Industry (CRIEPI), under contract with the Ministry of International Trade and Industry, has carried out a ten-year research and development program to develop design guidelines for base isolated Fast Breeder Plants. In 1997, the project was completed with the publication of the design guideline document. The guidelines have been revised to also make them applicable to Light Water Reactors. This means that a base-isolated nuclear plant can be licensed in Japan. More recently, a research program to apply seismic isolation to the ITER (International Thermonuclear Experimental Reactor) fusion reactor was started, Fujita (1997). SEISMIC ISOLATION OF INDIVIDUAL COMPONENTS The benefits of individual component isolation were recognized early on, leading to the isolation of 230 kV circuit breakers in Southern California, Kircher (1979). This preceded application of seismic isolation in buildings in the U.S. This was followed by shake table tests at the Earthquake Engineering Research Center (EERC), which clearly demonstrated the benefits of seismic isolation of large power plant components as well as light secondary systems, Kelly (1983). More recent applications of component isolation include the Mark II Detector at the Stanford Linear Accelerator Center which is part of the Linear Collider, isolated using four lead-rubber bearings, Buckle and Mayes (1990). The Detector consists of fragile equipment and weighs about 1500 tons. Its dimensions are 7.6m W x 10.5m L x 9.25m H. The seismic criteria consisted of US Nuclear Regulatory Guide RG 1.60 spectra scaled to 0.6g. The bearings are 74 cm square and 33 cm high and include a lead plug with a diameter of 24 cm. The maximum allowable horizontal displacement at 0.6g was 33 cm. Fragile art objects at the J. Paul Getty Museum in Malibu, California have also been isolated using a sliding isolation device Yaghoubian (1991). The seismic criterion was a magnitude 6.5 earthquake occurring one mile from the site. Such an event was estimated to produce a peak acceleration of 0.7g in the horizontal direction and a vertical acceleration of 0.45g. The isolators consist of off-the-shelf ball transfer units which roll on steel plates placed on the floor. The centering mechanism consists of a steel bowl and a centering post. The bowl has a highly polished surface to reduce friction. The post consists of a telescoping mast whose movement is restrained by a coil spring. Shake table tests were carried out to demonstrate the efficiency of the isolation system. Several reports are also available of seismic isolation of electric circuit breakers in Japan and Italy, Bonacina et al. (1994). Raised floor systems are widely used around the world in computer rooms and clean room facilities. Most electrical or mechanical equipment are rigidly secured to the floor or are supported on wheels that are locked from movement. In some cases equipment cabinets are supported on wheels that are free, thus allowing sliding or rocking to occur. To prevent overturning during a seismic event, the cabinets are tied to the floor with bungee cords. In Japan, it is now common practice to isolate raised floor systems. Several systems utilizing a combination of springs, pneumatic isolators, multi-stage rubber bearings, sliders, and dampers, have been developed and applied by major construction companies, Fujita (1991). To date, isolated raised floor systems have not been implemented in the USA. A number of shake table tests at EERC have been performed on floor isolation systems developed by IBM, Tajirian (1990). In these tests, the isolation system consisted of elastomeric bearings with Teflon elements sliding on polished stainless steel plates. Restoring force was provided by a steel-laminated elastomeric bearing. More recently shake table tests were performed at NCEER, Lambrou and Constantinou, (1994). The isolation system in these tests consisted of FPS bearings and dampers. Both tests demonstrated that the isolation system is effective in limiting forces transferred to equipment supported on the floor. REDUCTION OF FOUNDATION LOADS WITH SEISMIC ISOLATION Seismic isolation is commonly used to reduce seismic loads in buildings and other major components. However, another use of isolation is to limit the seismic forces exerted by the isolated structure on the supporting foundation. This is especially useful when new large masses are introduced in facilities were adequate seismic loads have not previously been considered and a costly upgrade would be required before the increased seismic loads could be accommodated. In this section, three such examples are given. Upgraded Titan IV launch vehicles are used to send payloads into space from Vandenberg Air Force Base, California. Each Titan rocket consists of a liquid-fueled core vehicle, and two strap- on solid rocket motor units (SRMU). Each SRMU is divided into three segments: Aft, Center, and Forward. The weights of each unit are 167, 144, and 67, ton (metric) respectively. Before launch, the SRMU segments are stored and checked in an existing facility near the launch site. This facility required modifications to accommodate the increased loads caused by the SRMUs while they are in storage. There was concern that personnel could be endangered due to movement or failure of SRMU segments during an earthquake. Several conventional design schemes to restrain the SRMU segments were evaluated, including rigidly attaching the bases of the segments to their foundations and using existing steel platform stands to provide lateral support for the SRMUs. It was concluded that the Forward segments, which are lighter than the other segments could be supported at the base without requiring major additional modifications to the supports. However, for the Aft and Center segments the implementation of rigid foundations would have required the excavation of the existing foundation and installation of new foundations underneath the segments’ support stands. These modifications would have required an extended construction period and thus would not have been compatible with program objectives. Seismic isolation was therefore selected for the Aft and Center Segments. This minimized the requirement for major modifications to the existing foundations and structures. A site specific response spectrum with a peak ground acceleration of 0.6 g was used as input. Each segment was supported on a steel frame supported on four high damping rubber bearings. A medium stiffness compound with a shear modulus of 0.55 MPa at 100 percent shear strain was used. The bearings had a diameter of 38 cm, and a total height of 25.4 cm. The rubber stack consisted of 23 layers with a thickness of 6.4 mm. The isolation frequencies were 0.52 Hz for the Aft segment and 0.56 Hz for the Center segment at the design displacement. The total maximum displacements for the Aft and Center segments were 22.4 cm and 20 cm respectively. In the second example, a situation arose in which it was necessary to provide a back-up emergency system in case of a major malfunction of the main turbine generator exciter in the Diablo Canyon Power Plant in California. The exciter is not a safety-related piece of equipment, but is needed for the electric power generation. One solution that was considered was the use of two existing mobile exciters which would be placed in the Turbine building to replace the malfunctioning exciter until it could be repaired and put back in service. The mobile exciters consist of transformers and switch gear mounted on a truck trailer, each weighing about 32 tons. Pacific Gas and Electric needed to demonstrate that the mobile exciters would not fail during the design-basis earthquake in such a manner as to compromise other nearby safety-related structures. Calculations showed that the exciters would exert large seismic reaction forces; as a result: (1) strengthening of the truck trailers and equipment anchorages would be required, (2) an excessively large mounting skid would be required, and (3) the number of foundation bolts would be excessive. These modifications were undesirable because it would lengthen the installation time of the mobile exciters, increasing the duration of a forced outage. An alternate approach using seismic isolation was developed in which four high-damping rubber bearings were used to support each trailer. The design-basis earthquake consisted of horizontal and vertical floor response spectra with a maximum horizontal acceleration of 0.75 g. The peak spectral acceleration between 2 and 9 Hz was 1.8 g, and a peak velocity of 127 cm/sec between 0.5 and 2 Hz. Three preliminary isolator bearing designs were developed giving isolation frequencies of 0.5 Hz, 0.75 Hz, and 1.0 Hz. Because of the relatively low weight of the trailer, the 0.5 and 0.75 Hz conventional bearing designs using the available high damping rubber compound at the time (shear modulus of about 0.76 MPa at 100 percent shear strain) would have required a very tall and slender bearing which would not have had the required stability during large horizontal displacements. Consequently, a stacked multi-stage bearing design was developed. Each bearing unit was composed of a stack of small bearings bolted to the corner of stabilizing steel plates. Alternatively, by selecting the 1.0 Hz system, the displacements were smaller and allowed the use of a conventional bearing design, while reducing forces sufficiently to eliminate the need for strengthening the supporting floor. Four high damping rubber bearings with an outside diameter of 42 cm and a total rubber height of 30.5 cm were used. The maximum design horizontal displacement was 30.5 cm. In the third example, a partially completed offshore structure, situated in the Caspian Sea, was retrofitted with seismic isolation, Medeot and Infanti (1997). The original structure, located in 120 m of water, consisted of two, back-to-back steel jackets, each supported on ten piles. Construction of the original jacket was completed in 1993. The structure was not designed for seismic loads. A detailed seismic analysis of the structure as built demonstrated that the piles could not sustain the level of loads associated with the MCE event (1000 year return period). Two alternatives were considered for strengthening the structure. The first alternative considered strengthening the foundation by installing additional skirt piles. This option was eliminated as too costly and schedule-intensive. The second option was to reduce the seismic load on the foundation by isolating the top deck structure from the jacket. The isolation system comprised of spherical PTFE sliding bearing equipped with steel hysteretic dampers. The isolation system was equipped with sacrificial restraints, which were designed to break during an earthquake. This prevents the movement of the energy dissipators during wind and wave loading. References Aiken, I. D., Kelly, J. M., and Tajirian, F. 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K., Method for seismic base isolation of liquid-storage tanks, Journal of Structural Engineering, ASCE, Vol. 123, No. 1, January, 1997. Medeot, R., and Infanti, S., Seismic retrofit of Chirag 1 offshore platform, Int. Post-SMiRT Conference Seminar on Seismic Isolation, Passive Energy Dissipation and Active Control of Seismic Vibrations of Structures, Taormina, Italy, August 25-27, 1997. Postollec, J-C, Les foundations antisismiques de la Centrale Nucleare de Cruas-Meysse, notes du service etude geni civil d'EDF-REAM, 1983. Tajirian, F. F., Seismic isolation study final report, Technical Report, Bechtel Corporation, 1990. Tajirian, F. F., Kelly, J. M., and Aiken, I. D., Seismic isolation for advanced nuclear power stations, Earthquake Spectra, 1990, Vol. 6, no. 2, May. Tajirian, F. F., and Patel, M. R., Response of seismic isolated facilities, a parametric study of the ALMR, 12th SMiRT Conference, Stuttgart Germany, August 1993. 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