Title. Author(s)KANATA, T.; MATSUMURA, M.; NAKANISHI, Y.; YAMAGUCHI, Issue Date Doc URL. Type. Note. File Information STRUCTURES

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1 Title FUNDAMENTAL STUDY ON DYNAMIC BEHAVIOR OF SLIDING BEA STRUCTURES Author(s)KANATA, T.; MATSUMURA, M.; NAKANISHI, Y.; YAMAGUCHI, Issue Date -9- Doc URL Type proceedings Note The Thirteenth East Asia-Pacific Conference on Struc,, Sapporo, Japan. File Information easec-b--6.pdf Instructions for use Hokkaido University Collection of Scholarly and Aca

2 FUNDAMENTAL STUDY ON DYNAMIC BEHAVIOR OF SLIDING BEARING WITH TRIGGER FOR BRIDGE STRUCTURES T. KANATA, M. MATSUMURA *, Y. NAKANISHI, and T. YAMAGUCHI 4 Department of Civil Engineering, Osaka City University, Japan Department of Civil Engineering, Osaka City University, Japan Engineering Department, Road Division, NEWJEC Inc., Japan 4 Department of Civil Engineering, Osaka City University, Japan ABSTRACT Isolation rubber bearings are widely used for damage mitigation of structures. However, isolation and damping effects tend to be optimized against the short-period earthquake grand motions defined by the Specifications for Highway Bridges in Japan. That is, as the isolation effects of the rubber bearings show strong frequency dependence, applicability against a long-period earthquake grand motion is not clear, for instance. Then the authors have focused on combination use of low friction and elastic sliding bearings, triggers and displacement restrainers for bridge bearings. Fundamental responses of the proposed bearings are checked through shaking table tests in this study and also discussed are analysis procedure of the proposed bearings with the trigger in the seismic response analysis. Application example of the proposed bearings to a viaduct is simulated through seismic response analysis. It is concluded that the analytical results show good agreement with the experimental results when considering the changes in the vibration modes and damping conditions before/after releasing the displacement of the superstructure by the triggers and the maximum displacement is approximately computed even when without considering them in the seismic response analysis. Keywords: sliding bearing, shaking table test, trigger, dynamic response, viaduct.. INTRODUCTION Recently, isolation rubber bearings are widely used to decrease seismic load as inertia force and to mitigate damages to substructures. However, isolation and damping effects tend to be optimized against short-period earthquake grand motions defined by the Specifications for Highway Bridges in Japan (hereafter called as JSHB) (Japan Road Association ). That is, the expected isolation and damping effects are not to be obtained against a long-period earthquake grand motion for instance, because the installation effects of the rubber bearing shows strong frequency dependence. * Corresponding author: m_matsu@civil.eng.osaka-cu.ac.jp Presenter: kanata@brdg.civil.eng.osaka-cu.ac.jp

3 On the other hand, the use of a sliding bearing of low friction coefficient and of less frequent dependence mitigates the damage to the substructures and provides a simple bearing system. Here, isolation effect is required only against a great seismic grand motion, not against a small and medium one. Then the authors also focus on the use of the triggers (K Ishihara et al. ), which break at a predetermined load and release the horizontal displacement of the superstructure after the trigger broke. Then the authors have focused on combination use of low frictional elastic sliding bearings, triggers and buffer as displacement restrainers for bridge bearings. Expected is that the sliding bearings provide less frequency dependence, the triggers restrict the displacement of the superstructure against a small and medium earthquake grand motion and release it against a strong earthquake grand motion and the buffers like energy absorbing rubber pads limit the maximum displacement of the superstructure. In this paper, fundamental dynamic responses of the proposed bearings are checked through shaking table tests. Also analysis procedure considering the changes in vibration mode and damping condition before/after the trigger break is verified through the comparison between the tested and analytical results. The application example of the proposed bearings to a viaduct is presented through seismic response analysis.. SHAKING TABLE TEST.. Overview of shaking table test As shown in Fig., vibration model having.77 kn of steel mass, which is supported by two supports; rollers assuming sliding bearing and vibration reduction rubber assuming rubber bearing, is used in the test. Multi-direction load cells are set at the both supports. A steel pin with 4 mm in the diameter at the slit is used as the trigger and is installed at the roller supported part. The break load of the trigger, P u is calculated by Equation (), which is proposed based on the tested results (M Matsumura et al. ). Material test of the steel pin indicates that yield stress is N/mm and tensile strength is 6.5 N/mm, respectively. Then the design break load, P u is calculated.57 kn. Seismic acceleration wave input in the shaking table test is the one measured at the JR Takatori station, defined as the Level Earthquake grand motion II-II- in the JSHB, and the acceleration amplitude is decreased to 8% and the time axis is shortened to 7% considering similarity rules. The prepared test cases are FREE without the triggers, FIX with a steel pin without the slit and KO with the trigger. In Case KO, the vibration mode and damping condition are to be changed from FIX to FREE by the break of the trigger. σ u Pu = τ u A= A () where τ u = ultimate shear strength of steel (N/mm ); A = cross section at the slit (mm ); and σ u = tensile strength of steel (N/mm ).

4 .. Test results Time histories of mass displacement at the sliding part and at the elastic supporting part and relationship between load and mass displacement of all the tested cases are plotted in Figs. -4. In Case FREE, the mass vibrates periodically depending horizontal elasticity of the rubber and the maximum displacement of the mass takes -.9 mm at the time of. sec. A very much small horizontal force is observed at the sliding part showing low friction. The load-displacement relationships show elastic responses and the coefficients of horizontal elasticity at the sliding part and at the elastic supported part are. N/mm and 95 N/mm, respectively. In Case FIX, the mass displacement is kept small during the test and the repeated loading of high frequency is observed at the sliding part. On the other hand, in Case KO, the mass displacement was restricted small like Case FIX and the elastic coefficient of the trigger is,49 N/mm until the trigger broke at the time of.68 sec. The dynamic response before the trigger break is very much similar to Case FIX and the one after the trigger broke is to Case FREE. The maximum displacement of the mass in Case KO is -4.4 mm at the time of. sec, which is a bit larger than the one in Case FREE. Accelerometer Weight displacement transducer multi-direction load cell roller trigger rubber sliding part elastic supported part Accelerometer X Figure : Specimen set up elastic supported part sliding part Figure : Case FREE elastic supported part sliding part Figure : Case FIX elastic supported part sliding part Figure 4: Case KO.

5 . ANALYSIS PROCEDURE.. Modeling and Analysis procedure To analyze the changes in the vibration mode brought by the trigger s break during the vibration with high accuracy, the specimen used in the shaking table test is modeled into a single degree of freedom (SDF) model. The elastic rubber bearing, the sliding bearing and the trigger are modeled by the spring elements as shown in Fig. 5. All the spring constants are determined by referring to the tested results mentioned in the above. The damping of the SDF is considered stiffness-proportional damping. The inputting damping coefficient of Cases FREE and FIX are set.9 and.5 by a trial-and-error method. The seismic accelerations wave measured in the shaking table is inputted. Here, the changes in the vibration mode and the damping condition before/after the trigger break are taken into account in the analysis procedure by changing the spring constants and the damping coefficient. That is, in the analysis of Case KO, the spring constants and the damping coefficient in Case FIX are inputted before the trigger breaks and they are re-inputted into those in Case FREE after the trigger broke. Hereafter this re-inputted case is called RST as opposed to norst without the re-input. k=95n/mm k=.n/mm k=49n/mm Experiment Modeling Experiment Modeling Experiment Modeling - (a)elastic supported part (b)sliding part (c)trigger Figure 5: Modeling of each component... Results of Analysis The time histories of mass displacement measured in the test and calculated in the analysis are plotted in Fig. 6. The trigger breaks at.6 sec and.66 sec in Cases norst and RST, respectively. The displacement response in Case norst takes larger than the experimented response at the time of.6~. sec, just after the trigger broke. On the other hand, the displacement response in Case RST is close to experimented one. The maximum responses after the trigger broke among all the cases take almost the same value and the difference between Cases norst and RST is.% in the maximum displacement. After sec, the calculated dynamic responses take greater than the experimented one, but this will be caused by the modeling not by the re-input. 4

6 Based on the above observations, the re-inputs of the spring constants and the damping coefficient in order to consider the changes in the vibration mode and the damping condition before/after the trigger break are very much effective to simulate the tested results with the trigger but do not greatly influence on the maximum displacement after the trigger broke. - Experiment norst RST (a) ~7sec - Experiment(.68sec) norst(.6sec) RST(.66sec) (b) ~.5sec Figure 6: Time history of displacement of Case KO (Experiment, norst& RST). 4. DYNAMIC RESPONSE ANALYSIS OF VIADUCT 4.. Analytical model of viaduct Dynamic response analysis of a viaduct with continuous 5 spans subjected to the seismic loading was carried out to verify the application effect of the combination use of the sliding bearings, isolation rubber bearings and the triggers. The superstructure of, tf is supported by 6 bridge piers. The bridge piers are modeled by beam-column element, the superstructure is modeled by the rigid beam element, the isolation bearing and the triggers are modeled by the spring element. The seismic acceleration waves of Level Earthquake as a small and medium earthquake and of Level Earthquake as a strong earthquake both defined in the JSHB are input in the transverse direction of the viaduct. analytical cases with different bearing conditions; HDR, HDR, and PTFE, are prepared in the analysis as shown in Table. Case HDR supposes a typical use of the isolation bearings to obtain the isolated effects only in the bridge direction not in the transverse direction. So the displacement in the transverse direction is restricted. Case HDR allows the transverse move in addition to Case HDR. Case PTFE adopts the sliding bearings, the trigger and the rubber buffer as displacement restrainers. The analysis software UC-WIN FRAMED is used in the analysis. 4.. Analysis results Among the bridge piers, P4 is picked up for the discussion below. The time histories of horizontal displacement of the superstructure and the top of the bridge pier, P4 are plotted in Fig. 7. The relationship between the load and displacement of the bearing and the time histories of reaction force of the triggers are plotted in Fig. 8. The maximum responses are summarized in Fig. 9. In Case HDR, which restricts the transverse move of the superstructure, the displacement and vibrating period of the superstructure and the pier are coincident and the reaction force and the 5

7 displacement of the pier become larger. That means some triggers, which allow the transverse move of the superstructure at an intended timing, will be effective in decreasing the seismic load delivered to the bridge pier. On the other hand, in Case PTFE, the triggers restrict the horizontal move of the superstructure in the beginning, then the superstructure moves larger onto the sliding bearings after the trigger broke and finally the rubber buffers prevent an excess displacement of the superstructure. The reaction force of the bearing and the displacement of the pier in Case PTFE take smaller than those in Case HDR. That is, the proposed bearing system, consisting of sliding bearing, trigger and buffer, can decrease damages to the substructures compared with the conventional use of the isolation rubber bearings. But attentions have to be paid for that the position of the superstructure after the earthquake is different from the original position. Table : Bearing conditions and modeling Bridge axial direction high-damping rubber bearing Horizontal force 水平力 P P y Bridge transverse direction K K Fix 橋脚 Pier K K Py (kn) (kn/m) (kn/m) P,P6 7,56,67 6 P~P5 8,4,64 79 high-damping rubber bearing Horizontal force 水平力 P P y K K 橋脚 Pier K K P y (kn) (kn/m) (kn/m) P,P6 7,56,67 6 P~P5 8,4,64 79 Sliding bearing + trigger + buffer Friction force 摩擦力 P P y K K Horizontal force 水平力 Pu K P cr Horizontal force 水平力 P K 遊間 u K Expansion gap Sliding Bearing Trigger Buffer Pier K K R d P y (μ=.) P u K u K K (kn/m) (kn/m) (kn) (kn) (kn) (kn/m) (mm) (kn/m) (kn/m) P,P6, ,, P,P5,., ,57, P,P4,., ,857, 6

8 4 x (mm) - -4 Pier max=-mm Supperstructure max=-45mm 4 Figure 7: Time history of displacement at the top of P4 (Case PTFE) sliding bearing Pmax=84kN trigger Pmax=494kN buffer Pmax=567kN trigger Pmax=494kN baffer Pmax=567kN 4 Figure 8: Relationship between load and displacement (Case PTFE). Maximum displacement of the pier(mm) Superstructure Superstructure Superstructure Maximum displacement of superstructure(mm) (a) of pier and superstructure Maximum load (kn) trigger buffer (b) Reaction force of bearing Bending moment (kn m) (c) Moment at the base of pier Figure 9: Maximum responses of P4. 7

9 5. CONCLUSIONS In this study, fundamental responses of bearings, consisting of sliding bearings, triggers and buffers, are checked through shaking table tests and analytical procedure accounting for the changes in vibration modes and damping conditions before/after the trigger break is verified. The application example of the proposed bearings to a viaduct is simulated through seismic response analysis. The analytical results show good agreement with the experimental results when considering the changes in the vibration modes and damping conditions before/after releasing the displacement of the superstructure by the triggers. Also it is revealed that the maximum displacement is approximately computed even when without considering them in the seismic response analysis. Based on the seismic response analysis of a viaduct, the application of the bearing system consisting of sliding bearings, triggers and buffers, can decrease damages to the substructures compared with the conventional use of the isolation rubber bearings. REFERENCES Japan Road Association (). Specifications for Highway Bridges. (in Japanese) K Ishihara, M Matsumura, M Yoshida, and M Sakaida (). Knock-off Effect of Steel Side Block as Restrainers on Dynamic Response of Isolated Bridge Structure. EASEC, China, pp M Matsumura and M Yoshida (). Dynamic response of isolated viaduct considering knocking-off effects of displacement restrainers. IABMAS, pp