Improved softened truss model for reinforced concrete members under combined loading including torsion

Reinforced concrete (RC) members are often subjected to combinations of axial, shear, and bending loads. Torsion develops in addition to compression, bending, and shearing forces in the case of curved bridges, beams, and columns of bridges with outriggers. This study focuses on developing an improved model for predicting the behavior of RC members under combined actions. A softened truss model (STM) for combined actions (CA-STM) is used as a base and improved in this study. The RC element made of concrete and steel is modeled as a membrane element. Membrane element is a basic unit of negligible dimensions subjected only to in-plane stresses. The cross section of the member is modeled as an assembly of four cracked shear panels. The model includes various assumptions for modeling the geometry to satisfy the equilibrium and compatibility conditions. The model gives a complete solution satisfying the Navier's principles of mechanics. The developed improved model is capable of predicting the load-deformation response, torque-twist response, strains, and curvatures of RC members from cracking up to the peak loads. The predictions of the developed model are validated with test data of different RC members subjected to various combinations of torsion, flexure, and shear. The major load combinations included in the validations are pure torsion (T), torsion combined with shear (T + V), and torsion combined with shear and bending (T + V+M). Comparison of results shows that the CA-STM model predicted the behavior of RC members reasonably well.