Dynamic response of seismically isolated buildings under long-period ground motion

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Dynamic response of seismically isolated buildings under long-period ground motion

Kuroshima Yohei, Kikuchi Masaru

HU, Hokkaido, Japan

Abstract

This paper analyzes the seismic responses of an isolated structure considering the thermal and mechanical coupling behavior of isolation devices under long- period ground motion. The paper focuses on the behavior of lead-rubber bearings. The performance of the lead-rubber bearings is reduced due to the heat generation of the lead core under cyclic loading. This phenomenon is a particularly important issue in the seismic isolation engineering field. An analytical method is constructed by combining a heat conductivity analysis and a seismic response analysis. A series of seismic response analyses are performed to evaluate the dynamic behavior of isolated buildings under long- period and long-duration ground motions.

Keywords: Seismic isolation, Lead-rubber bearing, Heat conduction analysis, Dynamic response, Coupling behavior, Long-period ground motion

Абстракт

ДИНАМИЧЕСКАЯ РЕАКЦИЯ СЕЙСМОИЗОЛИРОВАННЫХ ЗДАНИЙ ПРИ ДЛИННОПЕРИОДНЫХ ЗЕМЛЕТРЯСЕНИЯХ

Куросима Ёхэи, Кикути Macapy

УХ, Саппоро, Япония

В этой статье приведен анализ сейсмической реакции изолированной конструкции с учетом теплового и механического режимов сцепления изолирующих устройств, при длиннопериодных землетрясениях. В исследовании основное внимание уделяется состоянию свинцово-резиновой опоры. Производительность свинцово-резиновой опоры снижается за счет теплообразования в свинцовом стержне при циклической нагрузке. Это явление -- особенно важный вопрос в области инженерной сейсмоизоляции. Аналитический метод построен путем объединения анализов теплопроводности и сейсмической реакции. Для того чтобы оценить динамическую реакцию сейсмоизолированных зданий при длиннопериодных землетрясениях, был выполнен ряд анализов сейсмической реакции.

Ключевые слова: сейсмоизоляция, свинцово-резиновая опора, анализ теплопроводности, динамическая реакция, режим сцепления, длиннопериодные землетрясения

Introduction

Recently, long-period and long-duration of ground motions have been focused on in Japan1 because the 2003 Tokachi-Oki Earthquake and the 2011 Pacific Coast of Tohoku Earthquake occurred in Japan. In particular, the 2011 Pacific Coast of Tohoku Earthquake was observed to have not only long-period elements but also long duration of just less than 10 minutes. That is the reason of the long-period and long-duration ground motions have been focused on in Japan. In Japan, the Nankai Trough Earthquake is a mega-earthquake predicted to occur in the future. There is concern that the amount of damage caused would exceed that of the Pacific Coast of Tohoku Earthquake2,3. Therefore, it is essential to check the seismic safety of long period structures against that earthquake. Seismically isolated (SI) buildings have long period structures. If long-period and long-duration ground motions occur, they would cause excessive deformation and damage the isolation devices.

In case of the isolation devices damaging the cyclic loading, it is necessary to be concerned about fatigue and heat degradation. This paper focuses on the latter, which considers the effect of seismic responses of seismically isolated buildings when damaging cumulative deformation caused by cyclic loading for a long time occurs in the lead-rubber bearings (LRBs), which are one of the major isolation devices. Figure 1 shows the illustration of the LRBs4. It is an all-in-one seismic isolation device assembly of the multilayer rubber that functions as a load support and vibration isolator, and the lead plug functions as an energy absorber (damper). LRBs perform a damping effect by causing elastic deformation of the [lead-rubber | lead rubber | lead and rubber?] and converting kinetic energy into thermal energy. The study showed that the performance of LRBs is reduced due to the heat generation of the lead core under cyclic loading5.

This paper analyses the seismic response of an SI building consisting of LRBs in the isolation layer by combining a heat conductivity analysis considering the heat generation of the lead plug and a seismic response analysis under long-period and long-duration ground motions. The ground motions were designed by the Ministry of Land, Infrastructure, and Transport.

Long-Period and Long-Duration Ground Motions. Figure 2 presents the location of the Nankai Trough6. It is a 4000 meter undersea groove spreading throughout Western Japan. Recently, there is concern about the occurrence of long-period and long- duration ground motions reaching a magnitude of about 9.0 in that area. For this situation, in 2015, the Ministry of Land, Infrastructure, and Transport announced the design of long- period ground motion that is predicted to occur in Kanto, Osaka, Chukyo, and Shizuoka areas in Japan7. These ground motions would target structures that had already been hit by a previous mega-earthquake along the Nankai Trough damaging these structures in several areas.

Moreover, in 2016 June 24th, that idea was notified to the municipality of measures against long-period ground motions for this earthquake5.

Analytical Model and Input Ground Motions. Figure 3 presents the flowchart of the analysis of this paper. An analytical method is constructed by combining a heat conductivity analysis and a seismic response analysis to reproduce the mechanical coupling behavior of LRBs. In this flowchart, first, it does the seismic response analysis and calculates the plastic energy increment of the lead plug. Next, it calculates the energy for heat generation by calculating the energy and performing the thermal conductivity analysis. Finally, it calculates the temperature change from the result of the thermal conductivity analysis and repeats all the steps. Figure 4 presents the SI building model. This paper was used for the MDOF model for a typical 15-story building constructed with 14 LRBs.

Figure 3 Flowchart of analysis

The restoring force characteristics in the isolation layer were applied to the Kikuchi and Aiken model. Shear springs for the superstructure were applied to the Takeda model. This paper was used to design long-period ground motion that is predicted in Osaka (OS1) and Chukyo (CHI) infrastructure and transport that was announced by infrastructure and transport. Figure 5 shows the time history of each input ground motion. Figure 6 shows the pseudo response velocity when h = 5% and response spectra when h = 10%. It was concerned about the heat degradation of LRBs under cyclic loading because CHI and OS1 are long-duration characteristics.

Analysis Results

seismically isolated buildings ground

Figures 7 to 11 present the load-displacement relation of LRB, temperature change of the lead core surface and center at that height, peak response acceleration, peak response displacement, and interstory drift angles of each layer. Moreover, the dotted line indicates the yield point, and the gray line shows the crack in Figure 11. By considering the heat generation, the shear strain in the isolated layer increased 1.8 times compared to that of non-heat generation. Furthermore, the surface of the lead core reached a temperature reached of 180°C. The crack in the superstructure occurs due to the reduction of the LRB damping effect caused by heat generation. In the OS1 results, considering heat generation, the yield stress in the lead plug layer decreased. The maximum temperature of the lead plug was lower than that of the CHI result. The seismic response of the superstructure was increased, and the crack occurred due to the heat generation. But the difference of the peak response of heat generation and non-heat generation was lower than that of the CHI result.

Conclusion

From the above results, it is essential to consider the phenomenon of reducing the performance of the LRBs due to the heat generation of the lead core under long-duration ground motions. Moreover, the isolation layer caused excessive deformation under long-period ground motions. Therefore, these things showed that it is important for SI buildings to consider the thermal and mechanical coupling behavior for seismic response analysis. Additionally, it is also necessary to apply the restoring force characteristics that enable to adapt to over 250% shear strain.

Reference

1. Architectural Institute of Japan, ` Structural Response and Performance for Long Period Seismic Ground Motions',2007

2. K.Dan, Y.Ishii, J.Miyakoshi, H.Takahashi, M.Mori, N.Fukuwa, `Modeling of Fault Rupturing of Subduction Plate-Boundary Earthquakes with Magnitude 9 for Predicting Strong Motions : Application to the Nankai trough and examples of strong motions predicted in Tokai region', Journal of structural and construction engineer- ing.Vol.72,692, 1685-1694, 2013

3. J.Miyakoshi, K.Dan, Y.Ishii, A.Oana, `Modeling of Fault Rupturing of Subduction Plate-Boundary Earthquakes with Magnitude 9 for Predicting Strong Motions : Application to the Nankai trough and examples of strong motions predicted in Tokai region', Summaries of technical papers of annual meeting Architectural Institute of Ja- pan(Kanto), 145-146, 2015

4. Homepage of OILES CORPORATION, http://www.oiles.co.jp/ (access date: Jan. 31,2017)

5. Y.Tekenaka, A.Kondo, E.TakaokaM.Hikita, H.Kitamura, T.Nakamura `Experimental Study on Heat-Mechanics Interaction Behavior of Laminated Rubber Bearings, Journal of structural and construction engineering'.Vol.74,646, 2245-2253, 2009

6. Homepage of Cabinet Office, Government of Japan, http://www.cao.go.jp/ (access date: Jan. 31, 2017)

7. Homepage of Ministry of Land, Infrastructure, Transport and Tourism, http:// www.mlit.go.jp/ (access date: Jan. 31, 2017)

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