A Comparison of AASHTO Bridge Load Rating Methods

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A Comparison of AASHTO Bridge Load Rating Methods
Cristopher D. Moen, Ph.D., P.E. May 2,
2009 Leo A. Fernandez, P.E. Dedicated to
Dr. Greg Patron
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History of Load Rating

• Collapse of the Silver Bridge in West Virginia in
  1967

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History of Load Rating

• National Bridge Inspection Standards (NBIS)
• 1970 - AASHTO Manual for Maintenance Inspection
  of Bridges
• 1989 - AASHTO Guide Specifications for Strength
  Evaluation of Existing Steel and Concrete
  Bridges
• 1994 - AASHTO Manual for Condition Evaluation of
  Bridges - MCEB
• (Allowable Stress Load Factor Rating)
• 2003 - AASHTO Manual for Condition Evaluation and
  Load Resistance and Factor Rating of Highway
  Bridges - LRFR

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Load Rating Methods

• Allowable Stress Rating (ASR)
• Load Factor Rating (LFR)
• Load and Resistance Factor Rating (LRFR)

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Load Rating Factor
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Load Rating Levels

• Inventory Rating - safe load capacity for
  indefinite number of crossings over a bridge.
• Operating Rating - maximum permissible live load
  for the structure, suitable for one-time
  or limited number of crossings over a
  bridge.

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Allowable Stress Rating Method (ASR)

• Capacity is calculated with a working stress
  approach (e.g. 0.55Fy)
• Dead Load and Live Load are unfactored, i.e. A1
  and A2 1.0
• Variability in the demand loads is neglected.

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Load Factor Rating Method (LFR)

• Capacity is evaluated at the ultimate limit state
  (e.g. Mu and Vu)
• Dead Load Factor, A1 1.3
• Live Load Factor, A1 1.3 (inventory), A2 2.17
  (operating)
• Quantifies the potential for failure of a bridge.

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Load and Resistance Factor Rating Method (LRFR)

• Capacity is evaluated at the ultimate limit state
  with the condition and redundancy of the system
  incorporated.
• Wearing surface dead load is separated.
• ADTT is incorporated into the live load factor.

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Design Live Load ASR LFR

• HS-20 live load is used as a notional design live
  load for ASR and LFR.

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Design Live Load - LRFR

• HL-93 live load is used as a notional design live
  load for LRFR.

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AASHTO Legal Loads

• ASR, LFR and LRFR use AASHTO Legal Loads for
  Legal Load Rating

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Evolution of the Load Rating Factor
(ASR)
(LFR)
(LRFR)
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LFR vs. LRFR
(LFR)
(LRFR)
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LFR vs. LRFR
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LFR vs. LRFR
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LFR vs. LRFR
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LFR vs. LRFR
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LFR vs. LRFR
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Conclusion

• Interior Steel Composite Girder Multi Girder
  Bridge
• LRFR rating factor up to 40 lower than LFR
  operating level
• LRFR rating factor increases as span length and
  girder spacing increases due to LRFD live load
  distribution factor.
• AASHTO LRFR Load Rating Method
• Uniform reliability across all existing highway
  bridges
• Volume of traffic
• Redundancy of the superstructure
• Structural condition

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