Improved softened truss model for RC circular columns under combined torsion and axial compression

The presence of torsional loading significantly alters the behaviour of reinforced concrete (RC) members. Although a number of studies have addressed flexure, shear and torsion independently, investigations on such loads in combination are scarce. The combination of torsion and axial load has not been investigated adequately, despite its frequent occurrence in bridge columns during earthquakes. Furthermore, the softened truss model (STM) was developed to predict the torsional response of square sections and it cannot be directly used for circular sections. In addition, the tensile capacity of concrete was ignored in the original STM, which resulted in less accurate predictions. The present study focuses on the development of improved analytical models for RC circular columns under combined torsion and axial compression. The developed model is validated with test data and a parametric study is carried out to investigate the effect of various transverse reinforcement ratios and levels of axial compression. The results indicate that the developed model is able to predict the global behaviour reasonably well. The effect of transverse reinforcement and axial compression is also in agreement with the experimental results.