Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

PERFORMANCE OF TRIPLE-PENDULUM BEARINGS OBSERVED IN A FULL-SCALE

SHAKE-TABLE TEST PROGRAM

T. Okazaki1, E. Sato2, K. Ryan3, T. Sasaki4 and S. Mahin5

ABSTRACT A full-scale, steel moment-frame building, whose base was seismically isolated using nine triple-pendulum bearings (TPBs), was subjected to a number of horizontal and horizontal-plus-vertical ground motions using the E-Defense shake table. The tests demonstrated excellent performance of the base-isolation system against ground motions with different characteristics ranging from large high-frequency content, large vertical acceleration, to long duration. After experiencing 20 severe ground motions, the TPBs exhibited the same performance as they did in the first ground motion. Large variation in vertical forces, produced by overturning moment in the building and vertical ground acceleration, caused instantaneous disturbance to individual TPBs but had limited influence on the performance of the isolation system. Motions with extremely large vertical acceleration caused uplift in the TPBs of up to 20 mm. The uplift was followed by an impact that produced a large compression spike in the TPBs. The impact did not damage the TPBs and caused little influence on the hysteretic behavior of the TPBs. A height difference as large as 7 mm was recorded between the TPBs, but the difference reduced to below 3 mm when the horizontal displacement receded.

1Associate Professor, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan 2Senior Researcher, Hyogo Earthquake Engineering Research Center, National Research Institute for Earth Science and Disaster Prevention, Miki, Hyogo 673-0515, Japan 3Assistant Professor, Department of Civil and Environmental Engineering, University of Nevada, Reno, Reno, Nevada 89557, USA 4Researcher, Hyogo Earthquake Engineering Research Center, National Research Institute for Earth Science and Disaster Prevention, Miki, Hyogo 673-0515, Japan 5Professor, Department of Civil and Environmental Engineering, University of California, Berkeley, Berkeley, California 94720-1710, USA Okazaki T, Sato E, Ryan K, Sasaki T, Mahin S. Performance of Triple-Pendulum Bearings Observed in a Full-Scale Shake-Table Test Program. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

Performance of Triple-Pendulum Bearings Observed in a Full-Scale

Shake-Table Test Program

T. Okazaki1, E. Sato2, K. Ryan3, T. Sasaki4 and S. Mahin5

ABSTRACT A full-scale, steel moment-frame building, whose base was seismically isolated using nine triple-

pendulum bearings (TPBs), was subjected to a number of horizontal and horizontal-plus-vertical ground motions using the E-Defense shake table. The tests demonstrated excellent performance of the base-isolation system against ground motions with different characteristics ranging from large high-frequency content, large vertical acceleration, to long duration. After experiencing 20 severe ground motions, the TPBs exhibited the same performance as they did in the first ground motion. Large variation in vertical forces, produced by overturning moment in the building and vertical ground acceleration, caused instantaneous disturbance to individual TPBs but had limited influence on the performance of the isolation system. Motions with extremely large vertical acceleration caused uplift in the TPBs of up to 20 mm. The uplift was followed by an impact that produced a large compression spike in the TPBs. The impact did not damage the TPBs and caused little influence on the hysteretic behavior of the TPBs. A height difference as large as 7 mm was recorded between the TPBs, but the difference reduced to below 3 mm when the horizontal displacement receded.

Introduction

Triple-pendulum bearings (TPBs) are a seismic-isolation bearing that embodies an ability to address multiple performance objectives [1,2]. Because the restoring force of TPBs depends on the applied weight (compressive force), TPBs are suitable for light structures, extremely heavy structures, and structures with substantial mass eccentricity. A series of full-scale tests was conducted at E-Defense to examine the performance of a base-isolation system comprising TPBs. A steel moment-frame building, whose base was seismically isolated using nine TPBs, was subjected to a number of horizontal (XY) and horizontal-plus-vertical (XYZ) ground motions. The base-isolation system was designed to keep the building damage free under maximum 1Associate Professor, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan 2Senior Researcher, Hyogo Earthquake Engineering Research Center, National Research Institute for Earth Science and Disaster Prevention, Miki, Hyogo 673-0515, Japan 3Assistant Professor, Department of Civil and Environmental Engineering, University of Nevada, Reno, Reno, Nevada 89557, USA 4Researcher, Hyogo Earthquake Engineering Research Center, National Research Institute for Earth Science and Disaster Prevention, Miki, Hyogo 673-0515, Japan 5Professor, Department of Civil and Environmental Engineering, University of California, Berkeley, Berkeley, California 94720-1710, USA Okazaki T, Sato E, Ryan K, Sasaki T, Mahin S. Performance of Triple-Pendulum Bearings Observed in a Full-Scale Shake-Table Test Program. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

considered earthquakes in high seismic regions in Japan and the US. As described in earlier reports [3], the tests demonstrated excellent performance of the system against ground motions with various characteristics ranging from large high-frequency content, large vertical acceleration, to long duration. The peak horizontal floor acceleration in the building was 30 to 40% of the peak ground acceleration. This paper describes the performance of individual TPBs observed in these tests. Due to the light weight of the building, overturning moment associated with horizontal vibration, and vertical table motion, the compressive force in the TPBs varied substantially during the tests. Motions with large vertical acceleration caused uplift (disengagement of components) in some of the TPBs. Consequently, the test produced a rich set of data on the performance of full-scale TPBs subjected to dynamic loading conditions.

Test Plan Steel Building and Isolation System Fig. 1(a) shows the elevation and beam layout of the building along the Cartesian coordinate system. The two-by-two bay, five-story building was 10 by 12 m wide and 15.8 m tall. The total mass was 543 metric tons including a 54-ton mass on the roof (see Figure (b)), which produced mass eccentricity to the building, and rigid base beams. The aspect ratio of the building, taken as the height of the center of mass divided by the floor plan dimension, was 0.82 and 0.68, respectively, in the X and Y-directions. The light weight, mass eccentricity, and large aspect

Figure 1. Base-isolated building specimen: (a) elevation and beam layout of building; (b) roof

with steel plates; and (c) layout of TPBs.

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ratio of the building produced a challenging condition for seismic isolation. From white noise excitation under the base-fixed configuration, the fundamental vibration frequency of the superstructure was determined as 0.7 s in the X and Y-directions [3]. Fig. 1(c) shows the location of TPB bearings with respect to the shake table and the building envelope. One TPB was placed below each of the nine columns. The bearings are identified by their relative locations, for example, as S for South edge, C for center, and NE for North-East corner. The eccentricity at the isolation layer, computed as the distance between the center of mass of the entire superstructure and center of rigidity divided by the floor plan dimension, was 1% and 2.8%, respectively, in the X and Y-directions. Load measurements taken prior to the tests indicated that bearings SE, SW, C, and W were subjected to only half of the load placed on bearing E. Considering that the influence of overturning moments is greater in the corner bearings, bearings SE and SW were expected to be more susceptible to uplift during severe motions, while Bearing E was less likely to uplift. TPB Bearings The horizontal reaction of TPBs is proportional to the vertical force [1,2]. Fig. 2 shows the target hysteresis of the TPBs used in the tests, plotting the horizontal force Fx divided by the vertical force Fz against a unidirectional displacement u. The parameters were chosen so that the TPBs act in two stages rather than three stages. The TPBs were designed to act rigidly (stage 0) until the friction coefficient 0.02 of the inner pendulum is overcome. The inner pendulum is activated (stage 1) until the friction coefficient 0.08 of the outer pendulum is overcome to activate the outer pendulum (stage 2). When the displacement of 1.08 m is exceeded, the inner pendulum is once again activated (stage 3). At a displacement of 1.13 m, both the inner and outer pendulums bear against their respective circular rim. Upon load reversal, the response stages are activated in the same order as stage 0, 1, 2, and finally 3. Instrumentation Six displacement transducers were placed as indicated in Fig. 3(a) to measure the relative motion between the rigid base and the shake table. The geometric relationship between translation and rotation of the rigid base and change in the measured distances was used to compute the bidirectional displacement at each bearing. Additional displacement transducers were placed to measure the change in height of bearings NE, SE, and NW. An assembly of seven to nine tri-directional load cells was placed under each bearing to measure the reactions. Fig. 3(b) and (c) show how nine load cells were placed underneath bearing C using two

Figure 2. Target force-displacement response of

the tested TPBs.

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cell assembly; and (c) plan view of load cells and steel plates.

steel plates. Accelerometers were placed on the upper steel plate. The force components were evaluated at level A-A’ indicated in the Figure (b) by subtracting the inertia and weight of the upper steel plate from the load cell measurements. Loading Program The base-isolation system was subjected to a total of twenty-one motions. This paper primarily discusses the response obtained from (a) the Rinaldi Receiving Station motion from the 1994 Northridge earthquake, reproduced in 88% scale in XY and XYZ, (b) the JR Takatori Station motion from the 1995 Kobe earthquake, reproduced in 100% in XYZ, and (c) the K-NET Iwanuma motion from the 2011 Tohoku earthquake, reproduced in 100% in XY. Fig. 4 shows the target acceleration history of these motions. The Rinaldi record is characterized by very large vertical acceleration exceeding 1 g. The JR Takatori record is a near-fault motion. The Iwanuma

Figure 4. Target ground motions: (a) JR Takatori; (b) Rinaldi; and (c) Iwanuma.

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record is distinguished by its very long duration of five minutes.

Behavior of the Isolation System Force Distribution Fig. 5 shows the horizontal displacement paths recorded for the nine bearings during the 100% JR Takatori-XYZ motion. The figure shows that the nine bearings underwent very similar displacement and the isolation layer developed little twist. The largest twist angle measured during this test was 0.005 rad. The restoring force is indicated by a vector on the displacement paths at two time instants: the square and circle mark the instant when bearing C reached the largest positive and negative X-displacement, respectively. The value placed next to the marks is the vertical force (negative indicates compression) at that instant. When the system displaced in the positive X, bearings NW, N, and NE developed large compression in reaction to overturning moment. Likewise, when the system displaced in the negative X, bearings SW, S, and SE developed large compression. Because the restoring force is proportional to compression, the horizontal forces varied substantially in magnitude between the nine bearings. Fig. 6 plots the location of the center of mass, evaluated from the vertical reactions at the nine bearings, and the location of the center of torsion, evaluated from the horizontal reactions.

Figure 5. Horizontal displacement paths from 100% JR Takatori-XYZ.

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Figures (a) and (b) are constructed from the 100% JR Takatori-XYZ and 100% Iwanuma-XY motions, respectively. For both motions, the center of torsion followed the center of mass to control twist of the system. The feature to match the center of action of the restoring force to the center of mass makes TPBs beneficial to control twisting. Effect of Vertical Motion Fig. 7 shows the Y-direction response of the isolation layer obtained from the two 88% Rinaldi motions. The Y-reaction is estimated as the sum of forces acting in the nine bearings. Figures (a) and (b) show the response to the XY and XYZ motions, respectively. The numbers in Figure (b) indicate key time instants. The dotted lines indicate the target response when the outer pendulum activates and assuming that the net compressive force acting in the bearings is constant at 5320 kN. The measured hysteresis was close to the target during the XY motion, but differed substantially from the target during the XYZ motion. The very low restoring force at instants 1 and 2 and large restoring force at instants 3 and 4 is caused by the variation in compression. Uplift was detected in six bearings at instant 1 and seven bearings at instant 2. At instants 3 and 4,

Figure 6. Location of center of mass and center of torsion measured during: (a) 100% JR

Takatori-XY; and (b) 100% Iwanuma-XY.

Figure 7. Response of isolation system to 88% Rinaldi motions: (a) XY motion; and (b) XYZ

motion.

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impact following uplift of the bearings produced a large compression spike. While the figure shows how uplift in the bearings can affect the cyclic response of the isolation system, the figure also shows that, for the system and ground motions reported in this study, the effect is instantaneous and does not alter the properties of the isolation system. Durability Figure 8 shows the X-direction response of the isolation layer obtained from two motions. Figure (a) is from the sinusoidal motion, which was the second of the 21 motions, while Figure (b) is from the Michoacan motion, which was the twentieth motion introduced to the specimen. The horizontal restoring force is normalized by compressive force. The dotted lines indicate the target response when the outer pendulum activates. While both responses suggest that the actual friction coefficient was greater than the target value, little difference is seen between the two responses in the friction coefficient or in the shape of the hysteretic loop. Seventeen strong motions were introduced to the specimen between these two motions. Therefore, even after experiencing an unrealistically large number of strong ground motions, the base isolation system comprising TPBs showed little sign of degradation. Height Change Fig. 9 compares the horizontal and vertical displacements measured in three bearings, NE, SE, and NW, during the 100% JR Takatori-XYZ motion. Figure (a) shows that the three bearings underwent very similar horizontal displacements. It is cautioned that the displacement history does not uniquely determine the relative motion between the four sliding surfaces, and thereby, the same displacement history can result in different height. Figure (b) shows the time history of the vertical displacement with respect to the original height. At instant A, when the largest twist angle of 0.005 rad. was measured, the difference in vertical displacement was 6 mm. The largest height difference from all tests in this program, 7 mm, was measured at instant B. In these tests, the differential height did not remain for a prolonged time, and reduced to below 3 mm as the horizontal displacement receded.

Figure 8. Response of isolation system: (a) sinusoidal motion; and (b) Michoakan motion.

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Figure 9. Height difference measured during 100% JR Takatori-XYZ motion: (a) horizontal

displacement paths; and (b) time history of vertical displacement. Residual Displacement Fig. 10 plots the residual displacement at the end of each test, evaluated for bearing C. The displacement is taken with respect to the original position. Some of the motions left a residual displacement as large as 100 mm. The circles in the figure indicate 8 motions that produced a displacement near or exceeding 500 mm. Five of these motions left a residual displacement exceeding 50 mm while one motion left no residual displacement. The cause of large residual displacement warrants further examination. However, the residual displacement always vanished during the subsequent motion and did not accumulate towards any given direction. After the last motion, the residual displacement was 18 and 34 mm, in the X and Y-directions, respectively.

Uplift in TPBs Uplift and Impact Fig. 11 examines the vertical forces measured in the NE bearing and NE column during the 88% Rinaldi motions. Figures (a) and (b) compare the vertical table acceleration and vertical force in the NE bearing, respectively, measured during the XY and XYZ motions. Figure (c) shows the axial force in the NE column, evaluated by four strain gauges per section, from the XYZ motion. During the XYZ motion, the table introduced an acceleration spike that reached a peak positive value of 8 m/s2, and subsequently changed direction. The large upward spike in vertical table acceleration was followed by uplift of the NE bearing, as indicated by the duration when the bearing force was zero. In contrast, the XY motion did not include an acceleration spike. During the XY motion, the variation in bearing force was much smaller and bearing uplift did not occur.

Figure 10. Residual displacement at

the end of each test.

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Figure 11. Response from 88% Rinaldi motions: (a) vertical table acceleration; (b) vertical force

in bearing NE; and (c) axial force in NE column. The reengagement of the bearing, following the uplift, caused an instantaneous increase in vertical force in the NE column. The measured increase was 2740 kN in the bearing and 1800 kN in the bottom of the first-story column. The impact factor, computed by dividing the peak increase by the static load, was 2.8. The maximum compressive force was 30% of the squash load of the column. Such large increase in column axial force can be detrimental to the structural behavior and serviceability of the building. The instantaneous increase diminished along the height of the column and nearly vanished before reaching the fifth story. Vertical Displacement Fig. 12 shows the displacement response measured for the NE bearing during the 88% Rinaldi- XYZ motion (the same motion studied in Fig. 11). Figure (a) plots the relationship between the horizontal-radial displacement and the height change, both measured with respect to the original position. Figure (b) shows the horizontal displacement path. In both figures, the dotted lines indicate the duration when zero vertical force acted in the bearing [see Fig. 11(b)]. The same time instants 1 and 2 indicated in Fig. 11 are also indicated in Fig. 12. An uplift displacement of 5 mm was measured at instant 1, and a much larger, 20 mm was measured at instant 2. The bearing uplift at instant 1 might be attributed solely to the upward spike in vertical acceleration as described above. The more substantial bearing uplift at instant 2 might be attributed to the combination of the spike in acceleration and the overturning moment producing tension at the NE bearing as the isolation system travelled in the diagonal direction.

Figure 12. Displacement measured from 88% Rinaldi-XYZ motion: (a) radial versus vertical

displacement; (b) horizontal displacement path.

Conclusions A full-scale steel moment-frame building base isolated with triple-friction-pendulum bearings (TPBs) was subjected to a number of bidirectional (XY) and bidirectional-plus-vertical (XYZ) ground motions. The following observations were made from the test results: The TPB isolation system adjusts the horizontal restoring force according to the center of

mass, and thereby, controls twisting efficiently. The TPBs showed little sign of degradation after experiencing 20 strong ground motions. The TPB isolation system can leave residual displacements after a major event. A large vertical spike in ground acceleration can cause uplift in TPBs. Uplift and the subsequent reengagement of the TPB will produce a rapid and large change in

compression, and thereby, affect the behavior of the TPB instantaneously.

Acknowledgments Funding for the research project was provided by the National Institute of Earth Science and Disaster Prevention of Japan and the U.S. National Science Foundation. The isolation devices and design service for the triple-pendulum bearings was donated by Earthquake Protection Systems. The views described in this paper are those of the authors and do not necessarily represent the organizations mentioned herein.

References

1. Fenz DM, Constaninou MC. Spherical sliding isolation bearings with adaptive behavior: Theory. Earthquake

Engineering and Structural Dynamics 2008; 37: 163-183.

2. Morgan TA, Mahin SA. The use of base isolation systems to achieve complex seismic performance objectives. PEER Report 2011/06, Pacific Earthquake Engineering Research Center, College of Engineering, University of California, Berkeley, Berkeley, California, USA, 2011.

3. Sasaki T, Sato E, Ryan KL, Okazaki T, Mahin, S., and Kajiwara, K. NEES/E-Defense base-isolation tests: effectiveness of friction pendulum and lead-rubber bearings systems. Proceedings of the 15th World Conference on Earthquake Engineering, Lisbon, Portugal, September 24-28, 2012.

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