Simulation of Torsionally Loaded Deep Foundations Considering State-Dependent Load Transfer

Abstract

Deep foundations may need to resist torsional loads, resulting from wind loading on traffic sign and signal pole structures, or seismic loading on curved or skewed bridges. Although design methods for deep foundations at the ultimate limit states are readily available, no significant effort exists to quantify the accuracy of existing load transfer–based torsion-rotation methods to predict the full-scale, in-service rotation performance that considers state-dependence of the soil. To facilitate the serviceability and ultimate limit state design of geometrically variable deep foundations constructed in multilayered soils, this paper presents a torsional load transfer method using a finite-difference model (FDM) framework. Simplified state-dependent load transfer models that relate the unit torsional resistance to the magnitude of relative displacement are developed considering soil-structure interface shear test results. The proposed FDM methodology is validated by comparison with existing analytical solutions and with physical model tests. Parametric studies are conducted to illustrate the role of various design parameters and demonstrate significant effects of nonlinear soil-structure response on the torsional behavior of deep foundations, including the effects of pressure-dependent softening at the soil-structure interface.