Recommended Resistance Factors for Load and Resistance Factor Design of Drilled Shafts in Iowa

Project Details



14-512, SPR RB03-12








Federal Highway Administration State Planning and Research Funding
Iowa Department of Transportation

Principal Investigator
Sri Sritharan

Faculty Affiliate

Co-Principal Investigator
Jeramy Ashlock

Faculty Affiliate

Student Researcher(s)
Philippe Kalmogo

About the research

The Federal Highway Administration (FHWA) mandated utilization of the Load and Resistance Factor Design (LRFD) approach for all new bridges initiated in the US after October 1, 2007. To achieve part of this goal, a database for Drilled SHAft Foundation Testing (DSHAFT) was developed and reported on by the researchers in 2012. DSHAFT is aimed at assimilating high-quality drilled shaft test data from Iowa and the surrounding regions. Using the available data in DSHAFT, preliminary resistance factors were calibrated and proposed by the researchers in a previous project. Compared to the American Association of State Highway and Transportation Officials (AASHTO) LRFD specifications, the preliminary values showed increased efficiency in some cases. DSHAFT is currently housed on a project website ( and has been expanded to include 51 drilled shaft tests from the previous number of 41.

As additional load test data became available, resistance factors were expected to be recalibrated; thus, the objective of this research was to utilize the expanded DSHAFT database to refine and recommend final resistance factor values for implementation. This was done by examining current design and construction practices used by the Iowa Department of Transportation (DOT) as well as recommendations given in the AASHTO LRFD Bridge Design Specifications and the FHWA drilled shaft design guidelines, and by reviewing calibration studies conducted by Iowa and other states.

Various static design methods were used to estimate side resistance and end bearing of drilled shafts in cohesive soil, cohesionless soil, intermediate geomaterial (IGM), and rock. The extrapolation procedures developed by the researchers and reported in 2014 were found to have significant limitations; therefore, a t-z analysis approach was adopted instead to obtain the measured resistances necessary for the calibration.

Using the estimated and measured resistances, regional resistance factors were calibrated at a target reliability of 3.0 following the AASHTO LRFD calibration framework and the modified first-order second-moment (FOSM) reliability method. Two different procedures (Approach I and Approach II) were used in the calibration of skin friction resistance factors. The calibration initially considered load tests performed in Iowa solely before including all usable load tests available in the database. Based on the calibration results, final resistance factors, which show improvement compared to preliminary values and AASHTO recommended values, are recommended for implementation.