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چکیده
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Strengthening reinforced concrete columns due to errors in the design and construction process, damage from natural factors, the end of the structure's useful life, changes in new design guidelines, and demands like increased ductility and bearing higher design loads are essential. Strengthening reinforced concrete columns with Carbon Fiber Reinforced Polymers (CFRP) is one of the latest methods to enhance structural performance. Moreover, using steel fibers often improves the structural behavior of the elements. Practically, reinforced concrete columns are often subjected to combined bending and axial loading due to factors like construction mistakes and damage from natural disasters such as earthquakes. Consequently, numerous laboratory investigations have been conducted to study the effect of Fiber Reinforced Polymer (FRP) strengthening on the performance of reinforced concrete columns under off-axis loads. However, numerical studies evaluating the effect of CFRP strengthening on RC columns with fiber concrete are rare. This study assessed the off-axis numerical behavior of 24 reinforced concrete column samples with height-to-width ratios (L/h) of 4.2 and 3.8, containing steel fibers and strengthened with CFRP sheets. The research method presents a new numerical model using existing confined and unconfined concrete stress-strain models, differentiating between core concrete properties and its cover and concrete and FRP damage parameters. Based on this method, parameters such as load-bearing capacity, initial stiffness, consumed energy, yield force, and its corresponding displacement were studied. The average discrepancy between the numerical model results and experimental findings for initial stiffness, load-bearing capacity, and consumed energy of the column was 6%, 2%, and 2%, respectively, confirming that the proposed model has acceptable accuracy in various parameters. The results indicated that using steel fibers effectively prevents early concrete damage at minor displacements, and CFRP materials significantly enhance the columns' final axial resistance. Compared to conventional concrete, the results of columns with L/h=2.4 made of fiber concrete showed that the consumed energy increased by 12 to 17 percent, and the axial resistance also increased by 4 to 10 percent. Also, using CFRP sheet coverings resulted in a 26 to 48 percent increase in consumed energy and a 4 to 14 percent increase in axial capacity. The improvement values for slender columns with L/h=8.3 were typically less. Finally, a parametric study was conducted after validating the proposed numerical model with the results of other studies.
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