Real-Time Hybrid Testing of Laminated Rubber Dampers for Seismic Retrofit of Bridges

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1 Real-Time Hybrid Testing of Laminated Rubber Dampers for Seismic Retrofit of Bridges Akira Igarashi, Fernando Sanchez, Kenta Fujii Kyoto University Hirokazu Iemura Kinki Polytechnic College and Akihiro Toyooka Railway Technical Research Institute

2 Background Seismic Retrofit & Upgrading of existing bridges Mainly focused on bridges designed before 1995 (Elastic design, Level-2 Earthquakes were not considered) Long-span bridges /w center span longer than 300m: more than 30 bridges One of the retrofit measures Application of Seismic Dampers Devices Energy Dissipation

3 Type 3 span continuous cable stayed bridge Type of Highway Group 2 Class 1 Width 13.5 x 2 decks Main Tower High 146.5m Main Girder Warren Truss (High 9m) Cables Harp type (12 parallel) Girder 14,100 Main tower 7,900 Weight Cables 1,300 Total 27,400 Length =885m Retrofit Project: Higashi Kobe Bridge Abutment 1,700 Others 2,400

4 Updated Intraplate Eq. spectrum Updated Interplate Eq. spectrum Design spectrum (orig.) Insufficient seismic performance against interplate earthquakes Longitudinal period

5 Background Issues of seismic dampers for long-span bridges Large desplecements of decks & girders Need of Large Displacement Stroke Capacity Massive Structure Need of High damping force capacity Cost requirements in manufacturing, installation work, maintenance

6 Conventional Dampers Mechanism> Load-Displ. Problems Friction type Oil Orific e Piston Reliability & stability of axial force and damping force Residual displ. Size Viscous type Chamber Chamber Cost, Maintenance Elastoplastic type Relatively small deformation Large size for large stroke Steel bar Residual Displ.

7 Laminated ubber Damper Displ. Shear Shear Displ. High Damping Rubber (laminated rubber assemblies) Use of energy dissipation capacity of High Damping Rubber (HDR) in shear deformation HDR: advantage in known performance via many test results, practical application to seismic isolators as laminated r. aseemblies Mechanism & principle: advantage in economy of manufacturing and maintenance compared with other conventional types of dampers Axial force not required: larger strain range and rubber thickness than the case of seismic isolators Large stroke capacity

8 Laminated Rubber Dampers & Implementation for a Cable Stayed Bridge Laminated Rubber Assembly Girder Damper cable Main Tower

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10 Verification Testing Program Trial Manufacturing of a scaled Laminated Rubber Damper model Verification of restoring force characteristics Experimental validation of performance as a seismic damper for bridges 1. Cyclic loading tests 2. Hybrid simulation tests 3. Real-time hybrid simulation tests

11 Test Setup <Plan> Elevation Strong wall Load cell Dynamic Actuator 2359 Specimen Rigid floor 1401 Reaction frame 2305 Actuator s capacity Stroke ± [mm] Load [ N]

12 Test System

13 Laminated Rubber Damper Model Laminated high damping rubber assemblies No. of Laminated Rubber blocks Rubber dimensions Rubber layer thickness Shear Modulus 2 150mm 150mm 7mm 5 layers 1.2 N/mm class Steel plate Direction of Loading

14 Cyclic Loading Test Objectives Energy dissipation performance of laminated rubber assemblies without axial loads Strain & strain rate dependence of equivalent stiffness and equivalent damping ratio Loading condition Unidirectional Sinusoidal displacement 11 cycles Frequency 0.1Hz Amplitude strain of 25%, 50% 75% 100% 150% 200%

15 Test Result Shear strain-load hysteresis loop L o a d (kn) % 200% Influence of repeated cycles of loading can be observed Displ. (mm)

16 Equivalent Stiffness & Damping K e q (kn/mm) Shear strain h e q Shear strain Equivalent Damping Although decrease of equivalent stiffness and equivalent damping ratio for larger shear strain levels can be seen, the test result indicates LR damper s stable behavior and efficient performance as a energy dissipation device.

17 Influence of Loading Rate Decrease of equivalent damping ratio for higher loading rate

18 Hybrid Simulation Test Objective Validation of LR damper s response reduction performance as a energy dissipation device applied to a real bridge structure Loading rate Real time vs. conventional loading rate Similitude: considered to evaluate the performance of prototype structure based on the bahavior of the scaled model specimen

19 Concept of Hybrid Simulation Test System Host PC Input seismic ground acceleration Recording DSP system AD Test Control DA Test Damper load Filter Displ. Displ. Command signal measurement Control signal Servo Controller

20 DSP board system PCI bus 64 bit floating point DSP system Chip: TMS320C6701 (167MHz) Type ADM16-4 DAM16-4N In 4ch Out 4ch Resolution 16bit 16bit Conversion Time 10 µ sec 1 µ sec Input/Output range Input/Output Interface AD/DA ±1v and ±2.5v ±5v and ±10v ±5v and ±10v

21 Issues on Real-time Hybrid Simlation Test In the actual test due to the inherent delay in the response of the actuator and the delay in the data transfer between the computational hardware, the control signal is not properly achieved in real-time (Need of delay compensation). Most of the displacement-controlled approaches for delay compensation conventionally used are based on the extrapolation and interpolation of the actuator displacements

22 Real-time Implementation in this study The velocity-based loading developed by the authors was adopted for simple coding framework d Δt Δt d(i) v(i) v(i-1) v(i+1) Time-slicing in a single step into substeps for computation (time integration) and continuous actuator movements. i-1 10@0.001=0.01 sec. Δt =0.01 i i+1 time time t Calculated command signal Sent command signal Calculation and sending of target displacement to the actuator Actuator motion Corrections and calculation of the final displacement, velocity and acceleration vectors

23 Evaluation of Actuator Response Delay Actuator delay can be estimated as δ=0.030 sec

24 Assumed Structure Cable Stayed Bridge Vane damper Application of HDR dampers Natural period: approx. 4 sec.

25 Natural Modes 1 st mode T1= sec Transverse motion 2nd mode T2=2.4148sec Lateral motion

26 Assumed Damper Installation Dampers are effective in longitudinal modes, not hindered by the lateral & transverse modes

27 Reduced 3-DOF Model for Hybrid Simulation Tests x 2 m 2 m 1 (tower 1) m 2 (tower 2) m 3 (girder) k 3 m 1 k 2 x 1 m 3 x 3 Vane damper HDR damper k 1 Experimental substructure

28 Scaling of LR damper specimen and prototype structure 6mm X 5 layers 25mm X 8 layers Specimen Prototype Requirements Max shear strain 250% max damper loads 2000kN Similitude Sdispl Sspec height Sload=Sarea No. of LR assemblies S displ =5.7 S load =28.1

29 Displacement (m) Test result Time (sec) JMA Kobe Hyogoken Nanbu Earthquake w/o damper /w damper Load (kn) Displacement (cm) Relative displ. (Girder- Tower) LR Damper Hysteresis Loop

30 Test result Maximum Response Without damper With Damper Relative Displ. Girder- Tower (m) Relative Vel. (m/s) Girder acceleration (m/ s 2 ) Factor of response reduction Approx. 30.6% relative disp.

31 Real-time vs. Conventional loading Quasi-Static Hybrid Sim. Real-Time Hybrid Sim. (a) Displacement response (b) Specimen hysteresis loop Difference is considered to be due to the effect of the loading rates. For conventional hybrid simulation tests, the restoring force characteristics can be measured to be lower than the actual performance, which is the issue that can be avoided by the use of real-time tests.

32 Bending Moment Requirement Check 66-DOF numerical model Push-over of the bridge girder to simulate maximum loading to the main towers 1,080,000(kN m Allowable

33 Conclusions (1) LR damper has been proposed as a new seismic energy dissipation device Laminated rubber assemblies without axial loads show considerably good energy dissipation performance In order to evaluate the behavior of LR dampers that can show strain rate dependence, a real-time hybrid loading test system was developed. The real-time hybrid experimental system was implemented using the concept of velocity-based loading control.

34 Conclusions (2) Real-time and conventional hybrid loading tests of LR dampers, simulating dynamic response of a long-span steel cable stayed bridge with the LR damper were conducted. Comparison shows the difference that reflects the loading rate effect on the LR damper specimen, suggesting the necessity of real-time testing in the evaluation of the performance of LR dampers. For the case of cable stayed bridge, the factor of response reduction by the application of LR damper is shown to be as much as 30.6%