Revisiting Earthquake Lessons - Base Isolated Buildings
Wednesday, April 3, 2019
Authors: Ronald L. Mayes (SGH), Ian Aiken (SIE) and Andrew Taylor (KPFF)
This is one in a series of short articles revisiting lessons learned in past earthquakes. This series is being presented to share past lessons with newer members of our profession who were not there to experience them first hand. Contact Dave McCormick or Kelly Cobeen with your comments and suggestions.
Structure Type: Base Isolated Buildings
Earthquakes: 1994 Northridge; 1995 Kobe; 2010 Chile; 2011 Christchurch and 2011 Tohoku
Lesson: Building owners, engineers and public officials can learn much from the observed good performance of base isolated buildings and bridges in recent significant earthquakes. This highlights the potential advantages of implementing these systems more widely in the United States and beyond.
One of the early impediments to the implementation of base isolation technology was the lack of field observations on the performance of base isolated buildings and bridges in significant earthquakes. The 1994 Northridge and 1995 Kobe earthquakes provided full-scale tests of two base-isolated buildings. The 1994 Mw 6.7 Northridge event resulted in property losses estimated as high as $50 billion ($85 billion in today’s dollars) and caused significant damage to 31 Los Angeles area hospitals, forcing nine to partially or fully evacuate. The USC University Hospital – the world’s first base isolated hospital - had no damage at all (Asher et. al 1995). The Los Angeles County hospital complex, 1 km away from the isolated USC University hospital, suffered $389 million in damage and was eventually replaced. (EERI 1994)
In the 1995 Mw 6.9 Kobe earthquake that resulted in $200 billion in damage, the world’s largest base-isolated computer center (6 stories and 500,000 sq. ft.) suffered no damage at all and, like the USC University Hospital, was instrumented to provide a wealth of recorded data. A similar, but fixed-base, six-story reinforced concrete building located a few hundred meters away, was also instrumented. The data from these two buildings provided one of the best side-by-side comparisons of actual earthquake response of conventional and isolated structures. The base isolated building reduced the earthquake forces across the isolators by a factor of 3.5 with almost no amplification of forces up the height of the building. The fixed-base building amplified the ground forces by a factor of 3 at the roof. Thus, compared with the fixed-base building the isolated building reduced the earthquake forces by a factor of 3.5 at the ground level and by a factor of 10 at the roof. It is this dramatic reduction in earthquake forces that provides for the safety of not only the structure itself, but also the building contents and all of its architectural, mechanical and electrical components. Eighty percent or more of the total cost of a building is in the non-structural components: architectural details, cladding system, mechanical/electrical/plumbing (MEP) components, telecommunications/data systems, as well as contents. Protection of all of these components from earthquake damage is the most difficult part of the challenge that design professionals face in providing an owner with a fully operational building.
Following the 1995 Kobe earthquake the use base isolation in Japan increased significantly. The total number of seismically isolated buildings in the United States today is approximately 200. This contrasts sharply with the number of buildings in Japan as of 2017 were approximately 4,300 commercial and multifamily residential buildings, and approximately 5,600 single family homes have base isolation systems.
The Great East Japan Earthquake and tsunami occurred on March 11 2011 and caused massive and widespread damage in the Tohoku region, the east of Honshu, the main island of Japan. The Mw 9.0 event had approximately 4 minutes of strong shaking. Many technical papers have provided detailed information on the performance of both base isolated buildings and other structural control systems. These include papers from the EERI reconnaissance team (Taylor 2012), Taylor and Aiken 2012 and many Japanese authored papers (e,g. Kasai et. al. 2013) as well papers that provide details on the recorded response of many of the base isolated buildings: Kashima et.al. 2012; Takayama and Morita 2012; Miwada2012, Okawa et. al. 2013 and Saito et. al. 2013 are examples.
The reported performance of buildings with seismic isolation systems was generally excellent. All of the isolated buildings visited by the EERI/AIJ team had responded to the Great East Japan Earthquake as expected, and there were no reports of structural or non-structural damage in these buildings (Taylor, A & Aiken, I 2012). In a few cases the team observed minor damage to moat covers and joint covers, but this damage was not consequential and easily repairable. The observed damage to moat and joint covers highlights the importance of close design coordination between the structural engineer, who understands the expected movements of the isolated structure, and the architect, who generally is responsible for the detailed design of moat and joint covers. Furthermore, ongoing maintenance and inspection of the moat covers and surrounding areas is needed so the gaps are not restricted for some aesthetic reason as happened in a project in Chile where the moat was filled beautifully shaped river stones just a few years after it was constructed.
Ishinomaki City was one of the most severely damaged areas in the Great East Japan Earthquake and subsequent tsunami, where approximately 4,000 people were killed or reported missing. The Ishinomaki Red Cross hospital is located in Ishinomaki, approximately 130 km from the epicenter of the Mw 9.0 main shock. The hospital building was seismically isolated and was also protected from the tsunami inundation by an embankment.. The hospital experienced a maximum isolation movement of 26 cm (Terashima et. al. 2014). As a result of the base isolation, the operation and function of the hospital was maintained after the earthquake and the tsunami, which reached within a few hundred meters of the hospital. Many victims were treated and lives were saved. A video clip of the interior of the Ishinomaki hospital during and after the earthquake can be seen at (http://www.youtube.com/watch?v=Pc1ZO7YwcWc). In nearby Tomei City, a fire station building experienced 41 cm, the largest movement ever observed in an isolated building.
Building owners, engineers and public officials can learn much from the observed performance of response control systems in the Great East Japan Earthquake, and the potential advantages of implementing these systems more widely in the United States. The earthquake confirmed that base isolation technology provides excellent earthquake protection for both structural and non-structural elements of buildings. In both Japan and the United States a high percentage of the economic loss from earthquakes is the result of damage to nonstructural components and building contents. Base isolation can effectively and significantly reduce these economic losses. Since the Kobe earthquake in 1995 Japan has recognized the performance potential of base isolation and other structural control technologies and has widely adopted the technologies as a means of reducing earthquake losses
Morgan and Jampole, 2016 reported on the performance of base isolated buildings in the 2010 and 2011 earthquakes in Chile, New Zealand and Japan and identified numerous lessons regarding the design, behavior and expected performance of isolated structures. Key among these was the expected performance of an isolated superstructure. When an owner decides to target superior performance objectives for their facility during an earthquake, the options are often categorized as either: 1) base isolating the structure; or 2) other conventional design solutions. Given the substantial performance benefits of isolation, its selection and implementation may lead to the false expectation by the owner that their structure will be wholly undamaged following a major earthquake. While the superstructure and non-structural elements of isolated buildings have been entirely undamaged, damage to expansion/moat joints is not uncommon, and there may be some limited contents damage. Though isolated structures are undoubtedly much less susceptible to damage than their fixed-base counterparts, the expectation of perfect performance (e.g., no evidence whatsoever that the earthquake even affected the structure) should be discouraged. Building occupants should certainly expect to “feel” the earthquake, as was reported in the Christchurch Women’s Hospital and which can clearly be seen in the video from the Ishinomaki hospital.
Beyond the specific earthquakes and observations that have been mentioned, there is an ever-growing number of examples of good performance of the thousands of isolated buildings throughout Japan that frequently experience strong earthquake shaking. These earthquake observation lessons, and an extensive body of research that now spans over 50 years, provide structural engineers and building owners with the confidence that we can move to minimize future economic losses by more widely adopting base isolation and other structural control technologies.
EERI 1994 Northridge Earthquake Reconnaissance Report, Earthquake Engineering Research Institute, Oakland California, 1994.
Asher, J.W., Hoskere, S. N., Ewing, R.D., Van Volkinburg, D. R., Mayes, R.L. and Button, M., 1995. “Seismic performance of the base isolated USC University Hospital in the 1994 Northridge Earthquake,” Seminar l (TTS) on Isolation, Energy Dissipation and Control of Vibration of Structures, Santiago, Chile, August 21-23, 1995.
Taylor, Andrew W.,2012. “Response Control Systems in the United States and Lessons Learned from the Tohoku Earthquake,” Proceedings, One Year After the 2011 Great East Japan Earthquake – International Symposium on Engineering Lessons Learned from the Giant Earthquake, Session HA2, Damage of Buildings, Kenchiku-kaikan, Tokyo, Japan, March 1-4, 2012.
Taylor, A. & Aiken, I, 2012, “What’s happened to seismic isolation of buildings in the U.S.?” Structure Magazine, March 2012.
Kashima, T., Koyama, S., Okawa, I. and Iiba, M., 2012. “Strong motion records in buildings from the 2011 Great East Japan earthquake.” IJMA, 139, p.20.
Takayama, M. and Morita, K., 2012. “Seismic response analysis of seismically isolated buildings using observed records due to 2011 Tohoku earthquake,” in15th world conference on earthquake engineering, Lisbon, Portugal.
Miwada, G., Yoshida, O., Ishikawa, R. and Nakamura, M., 2012. “Observation records of base-isolated buildings in strong motion area during the 2011 off the Pacific Coast of Tohoku earthquake,” in Proceedings of the international symposium on engineering lessons learned from the 2011 Great East Japan earthquake (pp. 1017-1024).
Kasai, K., Mita, A., Kitamura, H., Matsuda, K., Morgan, T.A. and Taylor, A.W., 2013. “Performance of seismic protection technologies during the 2011 Tohoku-Oki earthquake,” Earthquake Spectra, 29(s1), pp.S265-S293
Okawa, I., Kashima, T., Koyama, S. and Iiba, M., 2013. “Recorded responses of building structures during the 2011 Tohoku-oki earthquake with some implications for design practice,” Earthquake Spectra, 29(s1), pp.S245-S264.
Saito, T., Iiba, M., and Kani, N., 2013. “Performance of Seismically Isolated Buildings at March 11, 2011, Tohoku Earthquake,” Proceedings of the 19th CIB World Building Congress, Brisbane, Australia, May 2013.
Terashima, M., , Kawamura, N., Konishi, Y., Someya, M., Kishiki, S., Yamada, S. 2014 “Contribution To Emergency Action And Post-Earthquake Evaluation Of A Seismically Isolated Hospital Building Experienced The Great East Japan Earthquake,” Second European Conference on Earthquake Engineering, Istanbul, Turkey, August 2014.
Morgan T. and Jampole, E. 2016. ”Performance of Base Isolated Structures in Recent Pacific Rim Earthquakes: Lessons Learned and Implications for US Practice,” Proceedings of 2016 SEAOC Annual Convention, September 2016.
The USC University Hospital – the world’s first base isolated hospital - had no damage at all in the 1994 Northridge Earthquake. 31 Los Angeles area hospitals had significant damage forcing nine to partially or fully evacuate. The Los Angeles County hospital complex, 1 km away from the isolated USC University hospital, suffered $389 million in damage.
The world’s largest base-isolated computer center (6 stories and 500,000 sq. ft.) suffered no damage at all in the 1995 Kobe Earthquake. A similar, but fixed-base, six-story reinforced concrete building located a few hundred meters away, was also instrumented. Compared with the fixed-base building the isolated building reduced the earthquake forces by a factor of 3.5 at the ground level and by a factor of 10 at the roof.
Displacement scratch plate gauge, MT Building, Sendai, Japan, showing isolation system 23 cm maximum displacement in the 2011 Mw 9.0 Great East Japan Earthquake (photo: A. Taylor).
Special flexible detailing of utility connections between the isolated superstructure and the foundation accommodates the large isolation system movements and ensures functionality during and after earthquake shaking. (photo: A. Taylor).