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Bridge Seismic Isolation Study on a Full Scale Bridge Test

Bridge Seismic Isolation Study on a Full Scale Bridge Test. Myrto Anagnostopoulou SEESL Structural and Test Engineer Ricardo Ecker Lay Ph.D. Candidate Andre Filiatrault Professor, MCEER Director Dep. Of Civil, Structural and Environmental Engineering

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Bridge Seismic Isolation Study on a Full Scale Bridge Test

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  1. Bridge Seismic Isolation Study on a Full Scale Bridge Test MyrtoAnagnostopoulou SEESL Structural and Test Engineer Ricardo Ecker Lay Ph.D. Candidate Andre Filiatrault Professor, MCEER Director Dep. Of Civil, Structural and Environmental Engineering University at Buffalo – State University of New York

  2. Design of seismically isolated structures is based on the mechanical properties of newly-fabricated seismic isolation hardware • Environmental effects, history of loading, aging result in change in: properties of isolation hardware behavior of isolated structure • Collect field data on the aging characteristics and long-term service life of seismic isolation bearings

  3. Full-Scale Isolated Bridge • Calspan’s Ashford facility, Western NY – 50 miles from UB • Two 72-foot long adjacent single lane concrete girder bridges at a distance of 6 feet • 8 low-damping elastomeric bearings of two different elastomeric compounds • Free vibration testing will be repeated weekly for a period of 5 years starting end-October 2010 • Remotely controlled testing from SEESL/UB facilities

  4. Superstructure Geometry • 10 girder beams: AASHTO Box cross-section (BII-36), 70skew • 8 beams weight 26 tons and 2 beams weight 32 tons • longitudinal post-tensioning at bottom plate of girder beams • transverse post-tensioning of girder beams at the support sections • 9” of gravel fill equivalent to 7” concrete/asphalt deck

  5. Abutment • Isolation • Bearings • Girder • Beams • Gravel • Spreader • Beam

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  7. Elastomeric Isolators • Target period of isolated bridge: T=2 sec • Total weight per bearing: W=100 kips • Design deformation: D=4 in • 10 low-damping elastomeric bearings of circular cross-section with two different rubber compounds: • Group A -> G=120 psi -> k=2.7 kips/in • Group B -> G=70 psi -> k=1.6 kips/in • Groups A and B are assigned to each of the two adjacent bridges • Characterization testing of isolation bearings at SEESL/UB in order to acquire mechanical properties

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  9. Bearing Characterization at SEESL • Group A • kA=2.5 kips/in • ζA=5% • Group B • kB=1.5 kips/in • ζB=3%

  10. Full-Scale Bridge Testing • Actuator spans the gap between the two adjacent single span bridges • Slow extension rate of the actuator up to: • 16” to the reaction load cell • 4” design displacement of the bearings • Fast retraction rate of the actuator in order to subject the two bridges in free vibration • Actuator Load Cell

  11. Group A Group B F=24kips 2.4 in 4.0 in • Forcing System Properties: • Max actuator stroke: 24 in • Max actuator force: 50 kips F=24kips

  12. Group A • initial displacement: 2.4 in • damping: 5% • period: 2.0 sec • free vibration duration: 35 sec • number of cycles: 15 • Group B • initial displacement: 4 in • damping: 3% • period: 2.6 sec • free vibration duration: 70 sec • number of cycles: 25

  13. Testing Procedure • Collect the initial mechanical properties of the isolation bearings (stiffness, damping) • Run bridge free vibration set of tests remotely from SEESL/UB • Collect data/info from: • actuator, reaction load cell • accelerometers, load cells, string potentiometers, thermocouples (26 sensors in total) • cameras • weather station • Obtain post-testing mechanical properties of bearings and compare to pre-testing ones • Visit the bridge field station in order to check condition of bearings, actuator, instrumentation • Repeat the procedure weekly and for a period of five years

  14. System Property Modification Factors • Properties of seismic isolation bearings: • Characteristic strength, Qd • Effective stiffness, Keff • Post-yield stiffness, Kd • Damping ratio, ζ • Phenomena effecting isolator properties: • Temperature • Aging • Wear or Travel • History of loading • Which are the max and min probable values of the bearing properties within the structure’s lifetime? • Can all phenomena occur simultaneously?

  15. Pmax = λmax·Pn • Pmin = λmin·Pn • Pn • λmax= λmax,1·λmax,2·λmax,3··· • λmin= λmin,1·λmin,2·λmin,3··· • Bounding • Analysis • λ-factors: quantify the effect of a particular phenomenon on the nominal properties of an isolation bearing • Pmax controls the substructure and superstructure force response • Pmin controls the isolator displacement response • System Property Adjustment factors account for the probability that several events occur simultaneously, depend on the significance of the structure and their values are based on engineering judgment • According to AASHTO (1999): • “The λ-factors listed herein are based on the available limited data. In some cases the factors could not be established and need to be determined by test.”

  16. Temperatures for design: 700F to -220F (AASHTO, 1999) • Low temperatures cause increase in stiffness and strength • Duration of exposure is significant but usually neglected • Travel or Wear due to traffic and temperature changes: • For a cumulative movement of 1 mile 17 sets of free vibration tests should be conducted during one day of testing (AASHTO, 1999) • λ-factors depend significantly on the rubber compound of the bearing λ-factors for Elastomeric Bearings • λmax,t • λmax,tr • max: 800 to 1000F • min: -300 to 100F

  17. Conclusions • Better understanding of the effect of temperature, environmental conditions and ware on the mechanical properties of isolation bearings • Realistic determination of bounding values of isolator properties for analysis and design based on better estimated Property Modification Factors • Using different seismic isolation systems the bridge field station can provide an insight to the resilience of bridges due to naturally-occurring phenomena

  18. Acknowledgments • SEESL technical staff and students • Doug Stryker and Andrew Dailey from Calspan • H&K Services for constructing the bridge • Hubbell Galvanizing for donating the girder beams • Dynamic Isolation Systems for providing the bearings

  19. Thank you! Questions?

  20. Instrumentation/DAQ System • actuator displacement, load cell sensors • 26 sensors: • 10 accelerometers • 2 load cells (temperature range -10F – 100F) • 10 string potentiometers • 4 thermocouples • 7 digital cameras • 1 digital weather station • 32-channel portable DAQ System compatible with existing UB/NEES systems and software SEESL remote desktop controller internet Ashford hostPC/ Pump controller Actuator/Test SEESL remote desktop DAQ internet Ashford hostPC DAQ/Sensors

  21. Wear or Cumulative Travel • Temperature: • low temperatures -> increase in stiffness and strength • high temperatures -> degradation of the rubber • Coupling between wear and temperature • Duration of exposure and elastomeric compound control behavior • Lack of long-term in-situ performance data Effects on Elastomeric Bearings • max: 80 to 100F • min: -30 to 10F

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