BASE ISOLATION DESIGN FOR CIVIL COMPONENTS AND …

1 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.

2 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.

3 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.

4 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 tank. A more common type of failure is known as elephant s 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.

5 The high vertical compressive stresses, which develop in the tank s 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.

6 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.

7 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.

8 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 Furthermore, economical tank designs developed for areas of low and moderate seismicity may be used in areas of high seismicity.

9 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 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.

10 The inner tank, consisting of nickel steel, is unanchored and has a diameter of m and a height of 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.