1. Home
  2. TRID
  3. View Record
https://www.nationalacademies.org/trb/events

Calibration of Resistance Factors Needed in the LRFD Design of Drilled Shafts

The load and resistance factor design (LRFD) has been used increasingly and has become mandatory for the design of all bridge projects funded by the Federal Highway Association (FHWA). Compared to the allowable stress design (ASD) method, LRFD can achieve a compatible reliability between the bridge superstructure and substructure. The uncertainties of load and resistance are quantified separately and reasonably incorporated into the design process. Therefore, this reliability-based design approach will generally produce a more efficient and consistent design than the traditional ASD factor of safety approach. To achieve these goals, many researchers have been working to develop a reasonable way to implement the LRFD method in bridge substructure design and to determine appropriate resistance factors for different regional soil conditions. Although the American Association of State Highway and Transportation Officials (AASHTO) LRFD specifications were approved for use in 1994, the implementation of these specifications for bridge design has been slow. The resistance factors ( ) proposed in the specifications were derived from ASD safety factors to maintain a consistent level of reliability with past practices. As a result, little improvement has been made toward a more efficient design. One outstanding problem with the resistance factor calibration is the lack of a good database. Even in the latest edition of the AASHTO specifications, a significant number of resistance factors in the foundation design were still selected based on the calibration with ASD. Several research efforts have been carried out to calibrate the resistance factors for drilled shafts from case histories available nationally. Currently, AASHTO specifications recommend using total resistance factors for single drilled shafts in an axial compression range from 0.40 to 0.60 at a reliability index (β) of 3.0 depending on different soil conditions. These factors were calibrated based on drilled shaft databases that were collected from various sites that do not necessarily reflect the local soil condition of individual states. As a result, the resistance factors recommended by the AASHTO LRFD design code should be verified and recalibrated to account for local soil conditions. The main objective of this study was to calibrate the resistance factors of axially loaded drilled shafts installed in Louisiana soils at strength I limit state based on the available drilled shaft load test databases collected from the Louisiana Department of Transportation and Development (LADOTD) and the Mississippi Department of Transportation (MDOT) as well as LADOTD’s design experience. The findings of this research effort will help Louisiana geotechnical engineers implement the LRFD design methodology for the design of all drilled shafts in future Louisiana projects as mandated by AASHTO.

Language

  • English

Media Info

  • Media Type: Web
  • Edition: Tech Summary
  • Pagination: 2p

Subject/Index Terms

Filing Info

  • Accession Number: 01343385
  • Record Type: Publication
  • Report/Paper Numbers: State Project No. 736-99-1408, LTRC Project No. 07-2GT
  • Files: NTL, TRIS, STATEDOT
  • Created Date: Jun 30 2011 1:42PM