ABSTRACT
This chapter introduces a method of nonlinear analysis for membrane elements. In this model the constitutive equations are based on the actually observed stress-strain relationships of concrete and steel. The stress-strain curve of concrete must reflect two characteristics. The first is the nonlinear relationship between stress and strain. The second is the softening of concrete in compression caused by cracking due to tension in the perpendicular direction. Consequently, a softening coefficient is incorporated in the equation for the compressive stress-strain relationship of concrete. Because of the crucial importance of the softening effect on the biaxial constitutive laws of reinforced concrete, this model has been named the softened truss model. The truss model has been applied to treat shear and torsion of reinforced concrete since the turn of the 20th century. The discovery and the quantification of this softening phenomenon during the last two decades has provided the major breakthrough in understanding of the shear and torsion problem in reinforced concrete.
