Torsional Safety of Highway Traffic Signal and Signage Support

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 methods for designing deep foundations at the ultimate limit states are readily available, and their accuracy has been quantified for selected loading test data, no significant effort to quantify the accuracy of existing load transfer-based torsion-rotation serviceability limit state methods to predict full-scale, in-service rotation performance has been reported in the literature. To facilitate the serviceability design of geometrically variable deep foundations constructed in multi-layered soils, a torsional load transfer method was developed using a finite difference model (FDM) framework. Simplified pressure- and state-dependent spring models, relating the unit torsional resistance to the magnitude of relative displacement, were developed and validated by using available interface shear tests and load transfer data from full-scale torsional loading tests. The proposed FDM methodology was validated by comparing the torsional responses of deep foundations designed with the FDM with results from previous analytical solutions and with the centrifuge and full-scale torsional loading tests. Parametric studies were conducted to illustrate the role of various design parameters and to demonstrate significant effects of nonlinear soil and structural response on the torsional behavior of deep foundations.