Model of Circular Reinforced Concrete Members Subjected to Axial Load

Existing models that describe the behavior of reinforced concrete columns that are confined by transverse reinforcement have an empirical or semi-empirical basis. Many of these models refer to the uniaxial stress-strain relationship of the confined concrete, assuming that the transverse steel has yielded. This paper proposes a theoretical model of circular RC members confined by transverse reinforcement. The analysis is based on formulation of an equivalent, fully confined, concrete column. The equivalency is based on a preliminary elastic analysis of a cylinder confined by a sequence of equally spaced lateral elastic-perfectly plastic stiffening rings. Results of worked examples show that within a Reduced Cylinder Radius (RCR) there is a zone of uniformly distributed stresses in which the tangential stress is equal to the radial stress and the shear stresses are equal to zero. This finding leads to application of the concept of RCR that was observed in elastic solutions without leaving out the confinement influence near the cylinder surface. Thus, the discrete steel ties can be replaced by an equivalent tube and the equivalent confined cylinder can then be solved in the plastic range of the concrete material with the relatively simple boundary conditions at the concrete-tube interface. Using the Imran and Pantazopoulou (2001) concrete plasticity model, a computer program was written to solve this problem. Initial results of circular columns that were analyzed with this model show a very good agreement with published test results.