Torsional Resistance of Drilled Shaft Foundations

Abstract

Drilled shaft foundations are commonly selected by public transportation agencies to support mast-arm traffic sign and signal pole structures. Those inverted L-shaped structures are subjected to lateral and torsional loading simultaneously during wind. Although drilled shafts are widely used, there is no standard to help engineers to design for torsional loading. Furthermore, the resistance mechanism is not well understood, and the existing design methods have been validated against limited experimental data. This research presents an up-to-date literature review regarding torsional resistance of drilled shafts. Several methods for calculating the torsional resistance were compared in a statistical manner against available test data. Cohesive, cohesionless, and layered soils were included in the analysis. It is not clear which is the best method to predict the torsional capacity of drilled shafts. a and b methods are recommended for ALDOT. The response under an eccentrically applied lateral load of a mast-arm assembly supported by a short drilled shaft was simulated using ABAQUS/Standard for FEM analysis. Two loading conditions and five eccentricities were considered. The results of the analyses suggest that the torsional resistance is enhanced when combined loading is applied. The eccentricity of the lateral load decreases the overturning resistance in sand. No significant reduction in the overturning resistance was observed for the clay soil considered. The interaction between torque and overturning resistance is not considered in the reliability analysis. Based on the reliability analysis performed in this research, a factor of safety of 1.10 for cohesionless soils and a factor of safety of 1.60 for cohesive soils are recommended. These factors of safety are large enough to obtain a reliability index within the range 1.5 to 2.0, which is considered an adequate range for this kind of foundation and its consequences of failure. Recommendations for layered soils are also made.