Designing of concrete structures which are widely used need some information about load bearing capacity , ductility , failure mode and etc , that should be resulted from expansive and time consuming experiments , thus analytical investigation of nonlinear behavior of concrete structures is highly essential to be studied. Concrete is relatively unknown material – despite the incremental using – because of its brittle nature which leads to sudden failure under compressive stresses. Flexibility amount and energy absorption capacity in structures – especially under lateral load – are so important that new methods are needed to improve them. Strength and flexibility of concrete can be highly increased if lateral expansion be controlled by confining and enclosing. Using the rectangular stirrups is one of the most common method for confining the concrete members. In this paper in order to investigate the stirrups effect on increasing of strength and ductility of reinforced concrete columns, analysis is done by nonlinear layered finite element method , and concrete characteristics equation – in the form of Hypo elasticity theory - are considered. A stress-strain diagram is used for description of confining effect that Influence of confining on increasing of compressive stress and strain at the peak point is considered .The decreasing branch of compressive part of diagram is improved ( to the presented diagrams). Also influence of cross section on increasing or decreasing of stress and strain at the pick point is considered. At the end of the paper, the results of numerical analysis and experimental investigation are compared.

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