Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
Jeramy C. Ashlock
The Federal Highway Administration (FHWA) mandated utilizing the Load and Resistance Factor Design (LRFD) for all new bridges initiated in the United States after October 1, 2007. To achieve part of this goal, a database for Drilled SHAft Foundation Testing (DSHAFT) was developed and reported on by Garder, Ng, Sritharan, and Roling in 2012. Using the available data in DSHAFT, preliminary resistance factors were calibrated and proposed by Ng et al. (2014). Compared to the American Association of State Highway and Transportation Officials (AASHTO) LRFD Specifications, the preliminary locally-developed factors showed the potential for improved design efficiency. As additional load test data become available, resistance factors are expected to be recalibrated, thus the objective of this research was to utilize an expanded version of DSHAFT to refine and recommend final resistance factor values for implementation. To achieve this goal, the research examined recommendations given in AASHTO LRFD Bridge Design Specifications and the FHWA drilled shaft design guidelines and reviewed calibration studies conducted by Iowa and other states. Several challenges in the calibration process were identified, and it was found that a regional calibration can lead to resistance factors lower than code recommended values contrary to expectations. To overcome the main challenges in the resistance factor calibration associated with the lack of good quality load test data, the use of load test on small-scale drilled shafts as a cost-effective approach to predict load-deformation behavior of larger diameter shafts was investigated. A total of five instrumented reduced-scale drilled shafts were constructed and load tested to investigate scale effects and develop appropriate scaling relations. Test data seem to indicate that skin friction decreases with increasing shaft diameter. Additional research is needed to confirm the findings and develop a suitable methodology to extrapolate test results on smaller shafts to larger diameter shafts. The research also investigated the accuracy of the finite element method to predict load-deformation response of drilled shafts so that displacement criteria can be integrated in design. Simulation results showed that the Mohr-Coulomb is simple to implement using CPT data, and it can provide adequate predictions. Resistance factors were calibrated in accordance with AASHTO LRFD framework for various drilled shaft design methods recommended by O’Neill and Reese (1999), Brown et al. (2010), and others. Two different procedures i.e., Approach I and Approach II were used in the calibration of skin friction resistance factors, with Approach II providing the highest resistance and efficiency factors.
Kalmogo, Philippe, "Improving efficiency of the load and resistance factor design of axially-loaded drilled shafts in bridge foundations" (2018). Graduate Theses and Dissertations. 17226.