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Recalibration of LRFR Permit Live Load Factors in the AASHTO Manual for Bridge Evaluation

Recalibration of LRFR Permit Live Load Factors in the AASHTO Manual for Bridge Evaluation. NCHRP 20-07/Task 285 Bala Sivakumar, HNTB Corp Michel Ghosn, City College, NY. Objectives of this study (March 2011 – June 2012).

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Recalibration of LRFR Permit Live Load Factors in the AASHTO Manual for Bridge Evaluation

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  1. Recalibration of LRFR Permit Live Load Factors in the AASHTO Manual for Bridge Evaluation NCHRP 20-07/Task 285 Bala Sivakumar, HNTB Corp Michel Ghosn, City College, NY

  2. Objectives of this study (March 2011 – June 2012) • Determine the appropriate target β for permit trucks aligned with current practice • Calibrate permit load factors for βTarget for use with LRFD distribution analysis and with refined analysis methods

  3. Research Team • Bala Sivakumar • HNTB Corp • Principal Investigator • Michel Ghosn • City University of New York • Co-Principal Investigator

  4. Permit Classifications • Routine Permits: unrestricted crossings; taken as up to 100 crossings a day of a bridge • Single-trip Special Permits: The permit truck is allowed to cross a bridge one time only but can mix with regular truck traffic; and • Escorted Single-trip Special Permits: permit truck crosses the bridge with no other vehicles allowed on the bridge. • Multiple-trip Special Permits: less than 100 crossings of a bridge within the permit validity period;

  5. LRFR Permits: Reliability Levels • Routine permits: •  = 2.5 • Special permits – single trip escorted: •  = 2.5 • Special permits – single & multiple trips allowed to mix with traffic: •  = 3.5

  6. LRFR load Factors & Permit Load Distribution • Routine Permits • Use LRFD two-lane distribution factors • Special Permits • Use LRFD one-lane distribution factor excluding multiple presence factor of 1.2

  7. LRFR Permits ROUTINE PERMIT ROUTINE PERMIT 2 Lane DF SPECIAL PERMIT Random Legal 1 Lane DF SPECIAL PERMIT 1 Lane DF 9-7

  8. LRFR Routine PermitLoad Factors

  9. LRFR Special PermitLoad Factors

  10. Reliability levels in Current LFR Ratings at the Operating Level

  11. Average β for legal Trucks at LFR Operating Level Using Recent National WIM Data from Interstate Sites with Heavy Trucks Average Operating Level Reliability = 1.35

  12. Average β for LFR Routine Permit Ratings LFR Operating Rating, Two Lane DF Average β = 2.94

  13. Composite Steel BridgesLFR Permit Ratings

  14. Average β for LFR Routine Permit Ratings LFR Operating Rating • Overall average reliability index for Routine Permits is on the order of 2.94. • considerably higher than β for legal load rating • Average reliability index for LFR legal load Operating ratings is less than 2.0. • LFR ratings may be imposing unnecessary conservatism on truck permits.

  15. β for Special Permits • For special permits, the reliability index values will be higher than those of the routine permits. • Special permits will have: a) no uncertainties in their weights and b) reduced probability of running alongside a heavy random truck.

  16. Recalibration of LRFR Permit Load Factors

  17. Criteria for Permit Recalibrations • The target reliability index set for the calibration is βtarget=2.5 • Reliability index values for all conditions that remain above a minimum βmin=1.50. • Live load factor: Min gL=1.10

  18. Recalibration Approach • Use recent Weigh-In-Motion data from a number of representative U.S. sites • Use representative samples of Routine and Special Permits from several states (NCHRP 12-78) • Use side-by-side probabilities from WIM data • Compare LRFD distribution factors to refined analysis / measured values, include bias and COV in reliability calculations

  19. Bridge Types • The analysis covered the bending and shear loading effects • Simple spans, 20 ft to 200 ft • T-beams, prestressed I-beams, composite and noncomposite steel I-beams.

  20. Traffic Data for Recalibration • Truck WIM data from Representative National Interstate Sites • Typical Permits configurations from national survey. • Special permit weights are taken to be accurately known. • Routine permit weights have some uncertainty in the GVW.

  21. Recent Traffic Data from Six National WIM Sites • NY Site 9121 --- I-81 •  CA Site 0001 --- I -5 •  FL site 9926 --- I-75 •  IN Site 9512 --- I-74 •  MS Site 2606 --- I -55 •  TX Site 0526 --- I -20

  22. Truck Multiple Presence Statistics from WIM Data

  23. Permit RecalibrationDistribution Analysis • AASHTO LRFD Distribution Factors • Has limitations for checking permits • Two unequal trucks side-by-side • Non-standard gage widths • Refined Analysis Methods • LRFR permit load factors were not calibrated for use with refined analysis • MBE lacks guidance on load factors for permit and adjacent truck

  24. LRFR Permit Load Factors for use with Refined Methods of Analysis • Refined analysis use is more widespread • Refined analysis will lower the COV • LRFD DF has a conservative bias • Calibrated load factors are required for the permit load and for the alongside random truck for use with refined methods of analysis

  25. Comparison of AASHTO LRFD Distribution Factors to Refined Analysis

  26. Comparison of AASHTO LRFD Distribution Factors to Refined Analysis • Interior beams under two lanes of loading give the closest results compared to those of the refined analysis but are still about 11% more conservative with a COV =13%. • Shear of exterior beams under two lanes of loading, show that the AASHTO LRFD may be up to 44% more conservative than observed from the refined analysis with a COV that may reach up to 25%.

  27. Reliability Indices with Refined Analyses • NCHRP 592 (2007) found that the LRFD DF can be significantly more conservative when compared to those obtained from refined analysis and field measurements • The reliability indices when the Permit check is performed using refined analysis will be lower when compared to the values obtained if the Permit checking uses the AASHTO LRFD DF.

  28. Permit RecalibrationLoad Cases Recalibration reflects four load cases: Case I: Special permit vehicle alone Case II: Routine permit vehicle alongside another permit Case III: Routine permit vehicle alongside a random vehicle. Case IV: Special permit vehicle alongside a random vehicle

  29. Case I -- Special Permit AloneRefined Analysis & LRFD 1 Lane DF

  30. Case IV – Special Permit Mixed with Random Truck (1 Lane DF)

  31. Case IV -- Special Permit Alongside Random • Live Load Model: add effect of Permit to effect of random truck • Limited number of crossings (100 total) • Conservatively assume that a random truck always crosses alongside Special Permit • Use gL =1 .4 with D.F. from LRFD Tables. • Use gL =1 .0 for Permit and gL =1.1 for alongside AASHTO Legal with refined analysis.

  32. Recommended LRFR Load FactorsSpecial Permits For escorted Special Permits at crawl speed, use IM=1.05 with refined analysis to maintain a β min≥ 1.50

  33. LRFR Special Permit load FactorsOld vs New

  34. Routine Permits Load Cases • Case II= Two permits side-by-side does not govern because of low probability of side-by-side permits • Case III = Routine permit alongside Random Truck governs bmin=1.5 governs

  35. Case III – Routine Permit Mixed with Random Truck (2 Lane DF)

  36. Recommended LRFR Load FactorsRoutine Permits GVW = Gross Vehicle Weight AL = Front axle to rear axle length Use only axles on the bridge

  37. RecommendationsRoutine Permits Live load factors for routine permits can be reduced for the cases where the Permit truck’s Gross vehicle weight is high Reflects the lower probability of having a random truck of equal or higher weight crossing alongside the Permit truck.

  38. LRFR Routine Permit load FactorsOld vs. New

  39. MBE Revisions Deliverables • Recommended revisions to LRFR permit rating specifications and commentary suitable for inclusion in the AASHTO MBE. • New table of LRFR permit load factors

  40. Table 6A.4.5.4.2a-1—Permit Load Factors: γL

  41. Closing • The recalibration performed in this study is based: • National WIM data for random trucks that may cross alongside Permits • Used representative permit configurations • Reliability levels better aligned with LFR ratings • Accounts for conservative bias in LRFD distribution factors compared to FEM • Calibration criteria: average btarget=2.5, bmin=1.5, min gL=1.1

  42. Closing • Avoids unnecessary conservatism in permit ratings as MP events are low • Promotes greater use of refined analysis for permit evaluations • Will be beneficial for LRFR permit ratings • T-18 should consider for adoption in 2012

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