Performance Estimates in Seismically Isolated Bridge Structures

1
Performance Estimates in Seismically Isolated
Bridge Structures

• Gordon Warn
• Young Researchers Symposium
• Tokyo, Japan
• June 2003

2
Personal information

• Graduate research assistant
• Department of Civil, Structural, and
  Environmental Engineering
• Primary research interests
• Seismic Isolation, Passive Energy Dissipation
  Systems, Bridge Engineering
• Academic advisor Professor Andrew Whittaker

3
Technical approach

• Perform numerical simulation
• Simple bridge model
• Varied isolator parameters
• Performance measures
• Maximum isolator displacement
• Review accuracy of static analysis procedure
• Determine increase in displacement due to
  bidirectional seismic excitation
• Energy demands
• Develop prototype testing protocol

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Seismic isolation systems
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Seismic isolation hardware
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Performance measures

• Displacement estimate influences all aspects of
  analysis, design and construction
• Superstructure and substructure forces
• Design and full-scale testing of seismic
  isolators
• Stability and strain demands for elastomeric
  bearings
• Plan dimensions of sliding systems
• Energy demand on seismic isolators
• Design and full-scale testing of seismic
  isolators
• Bearing acceptance criteria

7
Modeling seismic isolators

• Lead-Rubber (LR) bearings
• Coupled plasticity model
• Bouc-Wen model
• Friction Pendulum (FP) bearings
• Coupled plasticity model
• Account for variations in axial load
• Bouc-Wen model

8
Effect of bidirectional shaking
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Displacement estimates

• Displacement estimate
• Based upon work in the 1980s
• Constant velocity region of the spectrum
• Unidirectional response
• Results of nonlinear response analysis
• Benchmarked to static equation (buildings)

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Displacement estimates
Bin Description Moment Magnitude Dist. to Fault (km) Site Class Classification
1 NF 6.7 7.6 lt 10 D NEHRP
2M LMSD1,3 6.5 7.3 10 30 A, C USGS
3 LMLD1,3 6.5 7.3 30 60 A, C USGS
4 SMSD1,3 5.2 6.5 10 30 A, C USGS
5 SMLD1,3 5.2 6.5 30 60 A, C USGS
6 NF SS 6.0 7.32 lt 362 E, F NEHRP
7 LM SS 6.9 8.1 2.6 385 E, F NEHRP

1. Bin description adapted from that developed by
   Krawinkler
2. Magnitude and distance-to-fault based on
   mainshock
3. Ground motions extracted from the PEER and SAC
   databases

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Displacement estimates

• Spectral demands for NF (Bin 1)
• 5 critical damping

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Displacement estimates

• Response-history analysis
• Unidirectional (URHA)
• Bidirectional (BRHA)
• Simple isolated bridge model
• Rigid super- and substructures
• Bilinear isolation systems considered

Td (seconds) Td (seconds) Td (seconds) Td (seconds) Td (seconds)
1.5 2.0 2.5 3.0 4.0
Qd/W 0.03 A11 A12 A13 A14 A15
Qd/W 0.06 A21 A22 A23 A24 A25
Qd/W 0.09 A31 A32 A33 A34 A35
Qd/W 0.12 A41 A42 A43 A44 A45
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Displacement estimates

• Bidirectional excitation

• Unidirectional excitation

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Displacement estimates

• Update displacement equation
• Option a
• Unidirectional displacement multiplier
• Based on results of URHA and BRHA
• Orthogonal component
• Coupled behavior of LR and FP bearings

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Displacement estimates

• Displacement multiplier

Bin 2M LMSD
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Energy dissipation demands

• Interpretation of isolator performance

• AASHTO (Seismic Test)
• 3 fully reversed cycles at 0.25dt,....1.25dt
• 10 to 25 fully reversed cycles at 1.0d
• 3 fully reversed cycles at dt

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Energy dissipated demands
Normalized Energy Dissipated (NED )
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NED for NF ground motions
Bidirectional response-history analysis
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Normalized energy dissipated
Td 2.0 seconds
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Energy dissipated demands
Rate-of-energy dissipated
Isolator properties Qd0.06 Td 4.0
sec. Ground motion component RIO360
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Energy dissipated demands
Equivalent harmonic frequency Results of URHA
using ground motion record RIO360
Qd / W Td feq 1/T
(sec.) (Hz) (Hz)
0.06 2.0 0.58 0.69
0.09 2.5 0.56 0.74
0.12 3.0 0.86 0.80
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Conclusions

• Update displacement equation
• Orthogonal component
• Coupled behavior
• Preliminary estimates of suggest
  1.5-1.75
• AASHTO testing protocols
• Overly demanding (NED )
• Performed statically (no specified frequency)
• Replace with 4 fully reversed cycles at T

23
Acknowledgements

• Professor Andrew Whittaker
• Professor Kawashima
• MCEER / FHWA
• Natural Hazard Mitigation in Japan Program
• National Science Foundation
• Japan Society for the Promotion of Science