Torsional Load Transfer of Drilled Shaft Foundations.

Drilled shaft foundations commonly experience torsional loads, in addition to axial and lateral loading. Such cases include loads on mast arm traffic sign and signal poles, or seismically induced inertial loading of foundations supporting skewed or curved bridges. Despite the prevalence of drilled shafts, the understanding of the actual resistance to torsion provided by these deep foundation elements is not well established. To help address this gap in knowledge, two instrumented drilled shafts were constructed to evaluate the torsional capacity and load transfer at full scale. Both monotonic quasi-static and cyclic loading tests were performed. The imposed rotation and corresponding torque was monitored using string potentiometers and load cells, respectively. Strain gauges installed to measure shear strains facilitated computation of the torsional load transfer, which is described in detail. Design procedures for the calculation of the ultimate total and unit torsional resistances of drilled shafts are proposed and resulting estimates compared against the resistances observed in the testing program and other studies reported in the literature. The rational design methodology proposed herein overpredicts or underpredicts the torsional capacity, indicating the need for the development of improved methods for assessing the torsional resistance of drilled shaft foundations. [ABSTRACT FROM AUTHOR]