Strain gradient effect in RC elements subjected to torsion

Abstract

The strain gradient effect of two-dimensional (2D) constitutive models of softened concrete used in three-dimensional (3D) torsion problems has never been addressed. The latest 2D theoretical model for reinforced concrete (RC) membrane elements in shear, the softened membrane model (SMM), was recently applied to analyse the torsion of RC members to create a new theory called the softened membrane model for torsion (SMMT). The SMMT framework makes it possible to identify and quantify the strain gradient effect of concrete in RC torsional beams for the first time. Two amplification factors, λ and μ, for the pre-cracking stiffness and the strain at peak stress of the tensile stress–stress relationship of concrete, and a reduction factor R for Poisson's ratio (Hsu/Zhu ratio) v12 are proposed and used in the SMMT. This paper describes the quantification of the two amplification factors and the reduction factor, and discusses and interprets the physical meanings behind the amplification and reduction. This paper also uses the SMMT originally quantified for RC torsional beams to analyse an experiment on RC wall specimens under torsion, and compares the SMMT analytical results with experimental data to demonstrate the physical significance of the SMMT in detail.