LRFD Drilled-Shaft Foundations
5.5.1 LRFD Drilled Shafts – Overview
Geotechnical LRFD analyses of drilled shafts are basically analyses of drilled shafts for ultimate load capacity and settlement (Brown et al., 2010). The differences between what geotechnical engineers did for decades (Bowles, 1982 and 1996; Reese and O’Neil, 1999; Chen and Kulhawy, 2002) and LRFD design and analysis requirements for drilled shafts is partly terminology and partly the analysis methods used. As we discussed above in Section 5.2, three primary loading conditions need to be analyzed: service loadings, strength loadings, and extreme loadings. Generally drilled shafts are designed for strength loadings and checked for service and extreme loading cases.
During the design process, geotechnical and structural/bridge engineers work together to assure that both geotechnical and structural drilled-shaft design concerns are resolved. The geotechnical engineer first develops a drilled shaft axial resistance chart for use by the structural engineer in design of foundations. The structural engineer needs to check factored structural strength limit state loadings to be sure that they are equal to or less than factored foundation resistances. This first geotechnical chart is a plot of factored drilled-shaft axial strength resistance versus depth of embedment for a range of shaft diameters that will likely cover the range of anticipated structural factored loadings.