The paper discusses a nonlinear analysis of 2-D reinforced concrete frames using the Macroscopic Finite Element Model (Mac.FEM). In the elastic analysis, the reference axis for calculation of the flexural rigidity (EI) is the centroid of the cross section. This axis cannot be used for the nonlinear analysis because of variation of the modulus of elasticity over the depth of the cross section. The paper proposes a rational reference axis for the flexural rigidity (i.e., rigidity center) using the equilibrium equation and finite element theories. The proposed reference axis is used in the nonlinear analysis to predi_t the structural behavior of reinforced concrete frames. In the analysis, a nonlinear stress versus strain relationship for the confined concrete in compression is used to account for the effect of transverse reinforcement on the strength and ductility of structural frames. Comparison of the test results with the proposed analysis shows that the Mac.FEM can accurately predict the strength and behavior of reinforced concrete frames if the confining effect of transverse reinforcement and the rigidity center are taken into account.
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