COM624P-Based Drilled Shaft Torque and Lateral Load Analysis Method

Abstract

Large cantilever-type overhead freeway signs are often supported by a single drilled shaft foundation. Under high wind loads, the cantilever arm and sign panel develop significant torque loading that is distributed to the anchor bolt array cast into the top of the drilled shaft. This torque load is applied to the drilled shaft at approximately the same time as the large lateral load and moment developed by the wind loading. An analysis procedure was used to predict the torque capacity of drilled shaft foundations during the high lateral load condition. The analysis method starts with a typical COM624P p-y–type analysis for the design lateral shear and moment at the anchor bolts. The resulting lateral soil pressures estimated along the drilled shaft sidewalls present during the lateral loading are used to estimate whether voids will be present along the drilled shaft sidewall during torque loading. The estimated drilled shaft sidewall pressures and shear strengths for areas remaining in contact are used to estimate the torque capacity of the foundation. Some applications in different soil profiles are presented. This method was used as part of the development of a standard array of drilled shaft foundations for different soil conditions for standard cantilever-type freeway sign structures used by the Michigan Department of Transportation.