Shaking Table Test of Vertical Isolation Device - IJESDijesd.org/papers/440-A1003.pdf · SHAKING TABLE TEST The performance of the isolator was further verified via a series of shaking

Abstract—In this paper, the performance verification study

on a recently developed vertical vibration isolator is presented. The vertical seismic isolation system consists of four wedge-shape steel parts which provide friction surface at their interfaces, a center and two side springs, and a lateral bar which functions as a guide to side springs. The theoretical model for the isolator derived from the force equilibrium relationship is verified via numerical analyses and prototype tests. The performance of the isolator is further verified using the shaking table test. The analytical and experimental studies show that the isolator can reduce the vertical vibration effectively.

I. INTRODUCTION During the past decade, the occurrence of strong

earthquake has become more frequent and violent. Since 2010, earthquakes with the magnitude of over 7 have been occurred 66 times worldwide, and, among these, 6 earthquakes have recorded the magnitude greater than 8. Most notably, the Great East Japan earthquake occurred in 2011 had the magnitude of 9. Large earthquakes often result in massive casualties and property loss. For example, the 2004 Indian Ocean earthquake and tsunami claimed more than 200,000 casualties. To mitigate the damage induced by earthquakes, technological measures have been researched, and the seismic isolation has been accepted as one of the most effective means [1].

A core technic in seismic isolation is applying isolation bearings to the interface between a structure and its foundation. To date, various types of isolation bearings have been developed [2]. However, most isolation bearings are designed for alleviating horizontal vibration, and only a few vertical bearing systems are available now [3]. Observation on recent major earthquakes, such as the 1995 Great Hanshin earthquake, revealed that the vertical component of earthquake vibration can have considerable magnitude comparing to the horizontal component. In response to the stated phenomenon, some design standards such as ASCE 4-98[4] and ASCE 43-05[5] includes requirements to account for the vertical vibration effects.

In this paper, the shaking table test result to verify the vertical vibration isolation performance of a newly developed bearing system [1] is presented. The bearing system or the isolator is developed based on friction damping and spring-based restoring mechanism. The shaking table test

Manuscript received July 9, 2013; revised September 4, 2013. Youin Lee, Yong-Su Ji and Woojin Han are with ESCO RTS Co. Ltd.,

Seoul Korea (e-mail:[email protected]). Sanghyun Choi is with the Dept. of Railroad Facility Engineering, Korea

National University of transportation, Korea Sung Gook Cho is with INNOSETECH Co. Ltd., Inchon Korea.

was conducted by exerting various vibration waves to a 120kN concrete structure supported by the developed isolators. The basic concepts and analytical model of the bearing system as well as the shaking test result are summarized along with the test results in this paper.

II. VERTICAL ISOLATION SYSTEM The vertical seismic isolation system shown in Fig. 1

consists of four wedge-shape steel parts which provide friction surface at their interfaces, a centre and two side springs, and a lateral bar which functions as a guide to side springs. The centre and side springs designed to support a superstructure and to supply restoring force, respectively, can be made of various materials including steel, polyurethane, etc. At the interfaces of wedge parts, materials such as engineering plastic, PTFE (Poly Tetra Fluoro Ethylene) can be used to provide friction damping mechanism. The mechanical characteristics of the isolator can be modulated by changing the spring constants, the slope of the interfaces, and the friction coefficient of the material installed at the interfaces.

Fig. 1. Vertical isolation system

The theoretical model is derived from the force

equilibrium relationship as shown in Fig. 2 (a). From the figure, the vertical force P can be obtained as follows:

( ) zSS

SkkP sc ⎥

⎦

⎤⎢⎣

⎡ ±+=μ

μm2

)1( (1)

where ck and sk represent the stiffness of the centre spring

and the side springs, respectively; S is the slope of the interfaces; μ is the friction coefficient; and z is the vertical displacement of the isolation system. In the equation, the positive and negative signs mean upward and downward movements, respectively. The analytical equation in Eq. (1) was verified via a series of prototype tests as well as numerical analyses [6]. Fig. 2 (b) shows the numerical model for the isolator and Fig. 2 (c) depicts the prototype specimen used in the tests. The correspondence between the analysis and the test was well established as indicated in Fig. 3 which

Shaking Table Test of Vertical Isolation Device

Youin Lee, Yong-Su Ji, Woojin Han, Sanghyun Choi, and Sung Gook Cho

International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014

5

Index Terms—Vertical isolation device, friction damper,

shaking table test.

DOI: 10.7763/IJESD.2014.V5.440

shows the comparison of load-displacement relationship obtained from the analysis and the test. Note that, to reduce uncertainties in the behaviour of the isolator, Belleville springs which can provide constant stiffness were utilized for centre and lateral springs in the test specimen.

(a) Theoretical model

(b) Numerical model

(c) Prototype specimen

Fig. 2. Analytical models and prototype specimen of the isolator

III. SHAKING TABLE TEST The performance of the isolator was further verified via a

series of shaking table tests. The hexahedron-shaped concrete superstructure supported by four isolators was utilized in the verification test as shown in Fig. 4. The superstructure designed to induce 40mm deflection for a 30KN vertical load weighs 120kN. The test setup includes five accelerometers (ARF-50A manufactured by Tokyo Sokki), four installed to the superstructure and one to the shaking table, an LVDT to measure the relative displacement between the isolators and the superstructure, and a data logger. The shaking table capable of 300kN has six degree of freedom. The tests were conducted with three types of vibration waves, i.e., sine

waves, an artificial seismic wave, and seismic waves. Two sine waves with the peak amplitude of 0.6g and 1.0g were applied in the test while one 0.5g wave for the artificial wave. The seismic waves included the El Centro and the Bonds Corner El Centro earthquakes with amplified magnitude by 240% and 153%, respectively.

Fig. 3. Comparison of load-displacement behaviour of the isolator.

Fig. 4. Shaking table test setup

Fig. 5. Comparison of input and output accelerations for sine wave tests

Two sine waves, peak amplitude of 0.6g and 1.0g, were

applied at 5Hz frequency. Fig. 5 shows the results of the sine wave test. In the figure, it can be seen that the amplitude of the output accelerations are much smaller than those of the inputs. The results indicate that the isolator can actually reduce the vertical vibration. The result of the artificial seismic wave test is depicted in Fig. 6. Note that the artificial

International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014

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seismic wave was applied with the frequency range of 0.5~50Hz. It can be also seen that the amplitude of the vertical vibration was reduced significantly.

Fig. 6. Artificial seismic wave result

Fig. 7. Seismic wave test results

The test results of the seismic waves were demonstrated in

Fig. 7. For the El Centro wave, the peak amplitude of the acceleration was measured as 0.735g at the table while 0.398g at the superstructure. For the Bonds Corner El Centro wave, the peak accelerations were observed as 0.620g at the table and 0.391g at the superstructure. Note that all the test results are summarized in Table I.

TABLE I: RESULTS OF EXCITATION SEISMIC WAVE TEST Waves

form [input peak

amplitude]

Frequency (Hz)

Table acceleration

(g)

Superstructure acceleration

(g)

Percent reduction

(%)

Sine Wave [0.6g] 5Hz 0.680g 0.360g 47%

Sine Wave [1.0g] 5Hz 1.044g 0.477g 54%

Artificial [0.5g] 0.5Hz~50Hz 1.100g 0.460g 58%

El Centro [0.5g] - 0.735g 0.398g 45%

Bonds Corner El

Centro [0.5g]

- 0.620g 0.391g 37%

IV. CONCLUSION In this paper, the performance of a recently developed

vertical vibration isolator was verified analytically and experimentally. The tests include the basic performance test to identify load-displacement relationship and the shaking table test to verify actual isolation capacity. From the analytical and experimental studies, the following conclusions can be drawn: 1) The theoretical formulation for the basic behaviour of

the isolator was verified via numerical analyses and prototype tests. The analysis and test results showed that the theoretically predicted load-displacement relationship matched perfectly with the analysis and test results.

2) The shaking table test results showed that the measured vertical acceleration responses were reduced in the range of 37% to 58%. From the test results, it can be concluded that the isolator can reduce the vertical vibration effectively.

ACKNOWLEDGEMENTS This research was supported by the Power Generation and

Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (Contract Number 2011151010010D, 2011151010010A).

REFERENCES [1] S. Choi, J. Baek, Y. Lee, and I. Bang. “Development of a sliding

bearing system for seismic isolation in vertical direction,” in Proc. 13th East Asia-Pacific Conference on Structural Engineering and Construction. Sapporo, 2013.

[2] F. Naeim and J. M. Kelly, Design of Seismic Isolated Structures, John Wiley and Sons, 1999.

[3] S. Choi, J. Baek, and Y. Lee. “Vertical vibration isolator for reducing structural vibration,” Journal of Korean Society of Disaster Information, vol. 8, no. 2, pp.198-204, 2012.

[4] Seismic Analysis of Safety-Related Nuclear Structures and Commentary, ASCE Standard 4, 1998.

[5] Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities, ASCE Standard 43, 2005.

[6] Y. Lee, G.-S. Lim, Y.-S. Ji, and J. Baek. “A study for verifying equation of the vertical isolation device,” in Proc. of Computational Structural Engineering Institute of Korea, Pyungchang, 2012.

International Journal of Environmental Science and Development, Vol. 5, No. 1, February 2014

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Youin Lee was born in Seoul in April 18, 1978. He

graduated from Seoul National University of Science

and Technology in 2005.08. His major is Civil

Engineering.

He was discharged the military service as a

sergeant. He works in ESCORTS Co.,Ltd. Research

and Development Center as a senior researcher from

2005.04 ~ present. He studies about bridge bearing in

company.

Mr. Lee is member of APCBEES other than the IAENG.