SEISMIC PERFORMANCE OF BRIDGES AFFECTED BY LATERAL SPREADING

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SEISMIC PERFORMANCE OF BRIDGES AFFECTED BY
LATERAL SPREADING

• Jonathan D. Bray, Ph.D., P.E. Christian
  Ledezma, Ph.D.
• University of California at Berkeley
• Sponsored by the Pacific Earthquake Engineering
  Research Center CALTRANS

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PROBLEM
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Probabilistic Simplified Approach Ledezma and
Bray 2008 PEER Report

• Post-earthquake bridge condition can be related
  to lateral seismic displacement
• Estimate lateral seismic displacement for ground
  motion hazard levels

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Example Bridge
Right Abutment
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Overall Probability of Displacement Exceeding a
Threshold at Selected Ground Motion Hazard Levels
TR 2475 years P(D gt0.5 m)?? 30
0 0.5 1.0 1.5
2.0 Residual Lateral Deformation, d (m)
0 0.5 1.0 1.5
2.0 Residual Lateral Deformation, d (m)
LEFT ABUTMENT
RIGHT ABUTMENT
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Probable Damage State
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Expected Repair Cost Ratios Downtimes (low
uncertainty)
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Simplified Coupled Soil-Foundation-Bridge Model
Based on Mackie and Stojadinovic Model and Data
File
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Contribution to Repair Cost
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• EVALUATE RELATIVE IMPORTANCE
• OF KEY FACTORS IN SEISMIC DISPLACEMENT
  ESTIMATE
• Potentially important factors
• Spectral acceleration at degraded period,
  Sa(1.5Ts)
• Fundamental period of sliding mass, Ts
• Thickness of liquefiable material, H
• Distance to point of pile fixity, a
• Passive reaction against abutment backwall, Pa
• Error term in estimating Newmark-type
  displacement, e
• Residual undrained shear strength, Sur

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Relative Importance of Random Variables at
different hazard and displacement levels Right
Abutment
0 1.25
2.50 Lateral Displacement (m)
0 1.25
2.50 Lateral Displacement (m)

• Most important factors
• Sa
• e
• Sur

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Outcomes

• Probabilistic procedure for evaluating the
  seismic performance of bridges affected by
  lateral spreading has been developed
• Calibrated against available case histories and
  model studies
• Uncertainty in key parameters has been
  incorporated
• Can be used to estimate impact on seismic
  performance in terms of death, dollars, and
  downtime
• Simplified user guide provided (10 pp)

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Uses

• Methodology can be used to identify likely damage
  state and repairs required
• Methodology can be used to evaluate the relative
  importance of key parameters
• Intensity of the ground motion, Sa(1.5Ts)
• Error term in estimating lateral displacement, e
• Residual undrained shear strength of liquefied
  material, Sur
• Identify what matters most to impact on death,
  dollars, and downtime.

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Limitations Opportunities

• Key limitation in PEER methodology is relating
  death and downtime to damage and then to seismic
  displacement.
• Opportunity to capitalize on this effort and
  develop a simplified deterministic procedure to
  use for standard designs.