Optimization-Based Improved Softened-Membrane Model for Rectangular RC Members under Combined Shear and Torsion

Abstract

RC elements are often subjected to combined actions including torsion under seismic events. Understanding the behavior of RC members under combined actions including torsion is essential for safe design. Behavioral predictions of RC columns under combined loading can be improved by including the bidirectional stress effects. The objective of this work is to propose the improved combined actions softened-membrane model (CA-SMM) for predicting the behavior of RC elements under combined torsion (T) and shear loading (V). In this approach, the rectangular cross section is modeled as an assembly of four cracked shear panels. The applied external loads are distributed among these four shear panels. This assumption helps in reducing the complex stress state from combined loading to four different simple stress states on these panels. Additional equilibrium and compatibility conditions are imposed, and the system of nonlinear equations are solved by using an optimization technique called the gradient descent method. The developed improved model (CA-SMM) is validated with the experimental data available in the literature. After that, an interaction between the shear and torsion is developed to understand the behavior under various combinations of torsion and shear. A parametric study is carried out for understanding the effect of various sectional parameters such as longitudinal reinforcement ratio, transverse reinforcement ratio, and concrete strength. The predictions of the improved model had a close correlation with the test results.