Research for AASHTO Standing Committee on Highways. Task 285. Recalibaration of the LRFR Load Factors in the AASHTO Manual for Bridge Evaluation

For single and multiple-trip special permits that are allowed to mix with traffic (no restrictions on other traffic), the LRFR live load factors were explicitly derived to provide a higher level of reliability consistent with AASHTO inventory ratings and LRFD-design level reliability. The higher target reliability was prescribed considering the increased risk of structural damage and associated benefit/cost considerations for very heavy special permit vehicles than for other classes of trucks. This higher level reliability index though justifiably based on structural safety has caused operational difficulties for bridge owners whose past permitting practices have allowed permits to operate at a lower level reliability. The target reliability level for routine permit crossings is currently established at either beta index of 2.5 or 3.5. This reliability index needs to be verified by comparing with reliability indices used in current routine permit practices and adjustments need to be made as appropriate for meeting operational needs. The live load distribution for Special permits is based on the tabulated LRFD one-lane distribution factors with the built-in multiple presence factor (such as a value of 1.2) divided out. The live load distribution analysis for routine permits is done using LRFD two-lane distribution factors which assume the simultaneous side-by-side presence of non-permit heavy trucks on the bridge. Thus, the load factors are higher for spans with higher ADTTs and lower for heavier permits. The current LRFR permit load factor calibration for Routine and Special permits is tied to the LRFD distribution analysis method and does not provide guidance to States that want to use refined methods of analysis for heavy permits and for permits with non-standard gage widths. Live load factors and analysis guidance that are appropriate for analysis methods other than the use of LRFD distribution formulas need to be derived.


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  • Status: Active
  • Contract Numbers:

    Project 20-07, Task

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Dekelbab, Waseem

  • Performing Organizations:

    HNTB Corporation

    11414 W. Park Place - Ste 300
    Milwaukee, WI  United States  53224
  • Principal Investigators:

    Sivakumar, Bala

  • Start Date: 20100324
  • Expected Completion Date: 0
  • Actual Completion Date: 20110527
  • Source Data: RiP Project 24976

Subject/Index Terms

Filing Info

  • Accession Number: 01464020
  • Record Type: Research project
  • Source Agency: National Cooperative Highway Research Program
  • Contract Numbers: Project 20-07, Task
  • Files: TRB, RiP, USDOT
  • Created Date: Jan 3 2013 2:34PM