Strictly based on experimental evidence, a non-linear material model is presented for concrete subjected to membrane stresses. The model assumes a smeared rotating crack approach, tension stiffening, stress degrading effect for concrete parallel to the crack direction, and shear retention of concrete on the crack surface. In addition, the concrete model considers the enhancement in stiffness and strength of concrete under bi-axial compression. A NLFEA computer code was developed by incorporating the model into standard finite element formulations. The code applies to the analysis of rc plates and membranes. Colnparison with experimental results indicates the efficiency of the material model in predicting the postcracking response of reinforced concrete even with lower-order type of elements.