A Proposed Algorithm for Improvement of IDARC in Determination of Moment-Curvature Curve

The moment-curvature envelope describes the changes in the force capacity with deformation during a nonlinear analysis. Therefore، the moment-curvature envelopes for columns، beams and shear walls form an essential part of the analysis. The program IDARC now provides an option for users to input their own cross-section properties directly، and the moment-curvature is computed internally. Since IDARC is most practical and well-known software in damage analysis and the value of Park damage index depends strongly on the ultimate curvature، truth of IDARC''s results in section analysis is so highlighted. In IDARC، the moment-curvature analysis is carried out on the cross-section by dividing the concrete area into a number of strips or fibers. The section is subjected to increments of curvature and the strain distribution is obtained from compatibility and equilibrium considerations. In this study، the results of IDARC and OpenSees softwares in determination of moment-curvature curve and ultimate curvature of beams are evaluated. For assessment of the software''s responses، there have been used of the experimental results for 9 reinforced concrete beams with different section properties. The moment-curvature analyses of these beams have been done by use of IDARC، OpenSees and finally the code that has been written in this study. Comparison of the results shows significant differences between the IDARC''s results and other results. In order to control the accuracy of the analytical results two items have been checked، firstly، the strain of the last compressive concrete fiber in ultimate curvature and secondly، satisfying the equilibrium equations of section during the moment-curvature curve path. This study presents some cases that the axial equilibrium doesn''t satisfy in the moment-curvature obtained by IDARC. Also، IDARC doesn''t make attention to the ultimate strain of concrete when calculates the ultimate curvature. This result means that the damage index has been calculated by IDARC is not reliable when software accomplishes section analysis. Afterward، the used algorithm in IDARC for moment-curvature analysis has been revised and by proposing the modified algorithm a code has been written that can apply Chang&Mander and Kent&Park stress-strain concrete models. The effect of confinement on the concrete behavior has been applied based on Mander، Priestley and Park model and also based on Kent&Park model for each beam according to its transverse reinforcement. The results of the proposed algorithm conform exactly to OpenSees results. Although this results doesn''t match exactly with experimental results، but the general form of moment-curvature curves are similar to somewhat that has been obtained in experiments. The main reason of this difference relates to the value assigned to the ultimate compressive strain of concrete. Since the Park model for estimate of ultimate compressive strain of concrete has been applied، and the efficiency of that is not to the point of this study، the mentioned difference is acceptable