تحليل عددي نقش كمانش ميلگرد در رفتار غيرخطي ستونهاي بتن مسلح

Numerical Analysis of Bar’s Buckling Role in the Nonlinear Behavior of RC Columns

كمانش ميلگردهاي فشاري باعث تغيير رفتارغيرخطي معمول مقطع بتن مسلح مي گردد. در تحقيق حاضر به دليل بهره گيري از روش تفكيك درجات آزادي بتن و ميلگرد در المان هاي طولي بتن مسلح، امكان مدلسازي مناسب اثر كمانش در كنار ديگر اثرات اندركنشي بين بتن و ميلگرد از جمله اثر پيوستگي- لغزش فراهم شده است. بر اين مبنا به تجزيه و تحليل اثر كمانش ميلگرد تحت فشار در رفتار غيرخطي بار افزون و چرخه اي ستون بتن مسلح پرداخته شده است. ضمن اعتبارسنجي روش مورد استفاده، تحليل هاي متنوع عددي براي بررسي اثر كمانش ميلگرد در رفتار غيرخطي ستونهاي بتن مسلح شامل ظرفيت و شكل پذيري صورت گرفته و نتايج حاصله مورد بحث قرار گرفته است. نتايج نشان مي دهد ميزان تاثير كاهشي كمانش ميلگرد و تشديد آن با افزايش فاصله ميلگرد عرضي روي رفتار غيرخطي مقطع قابل توجه است، ولي مقدار كاهش به حدي نيست كه ظرفيت برآورد شده حاصل از تحليل عددي و متاثر از كمانش از مقدار ظرفيت نهايي به دست آمده از آيين نامه كمتر شود. اما شكل-پذيري و رفتار پس از تسليم مقطع به مقدار قابل توجهي وابسته به اثر كمانش بوده و با تشديد اثر كمانش، تردشكني و نرخ افت مقاومتي مقطع افزايش مي يابد.

In this research, for the purpose of nonlinear analyzing of RC columns and investigating the bar’s buckling, separate degrees of freedom are used for the reinforcing bars and concrete to allow for the difference in displacement between the reinforcing bars and the surrounding concrete. The capacity of the reinforced concrete (RC) sections was formed based on the interaction between reinforcing bars and the surrounding concrete. Contact between concrete and the longitudinal bars is provided by a bond force around the bars. A computer program is created in MATLAB software. For modeling a column element, a length segment of an RC element is considered as the combination of a length segment of a 2-node concrete element and a number of steel bar elements. Contact between concrete and the longitudinal bars is provided by a bond force around the bars. The buckling effect is considered in the numerical modeling of the bars. Buckling of longitudinal reinforcement has more important role in the behavior of RC members by increasing the stirrup spacing. The weak form of displacement based finite element formulation is determined and the ability and reliability of the numerical method has been assessed through the verification of numerical and experimental results. The buckling effect of longitudinal bars on the nonlinear pushover and cyclic behavior of the RC columns is investigated. Also, in the presence of buckling effect, a comparison is carried out between numerical capacity calculated from analysis and ultimate capacity calculated from ACI criteria. Several numerical analyses were carried out and the capacity of the stocky columns is investigated. The results show, that in pure axial compressive force condition, the numerical capacity with buckling is only about 4% lower than that obtained without buckling. Based on ACI criteria, the nominal capacity (calculated capacity without strength reduction factor ?) is conservative and less than numerical capacity. The ultimate capacity of the section (calculated capacity after applying reduction factor ?), which represents the ultimate design capacity, is more conservative. In pure bending condition, the numerical capacity with buckling is only about 2% lower than that obtained without buckling. Based on ACI criteria, the ultimate capacity is conservative and less than numerical capacity. In the presence of axial force with eccentricity, the numerical capacity with buckling is about 10% lower than that obtained without buckling. Based on ACI criteria, the ultimate capacity is conservative and ensures less than numerical capacity. The results show that, barʹs buckling has considerable effect on the capacity. But, the fact that the beneficial effect of stirrups confinement on the concrete compressive strength is neglected in the ACI criteria for capacity estimation, and also the use of the reduction factor ?, does not make any modification necessary for considering buckling effect on the ultimate capacity of RC section calculated by ACI criteria, since the results are conservative anyway. However, buckling of reinforcing bars has considerable effect on the ductility and strength reduction rate in the post-yielding state. This effect will be more by increasing the bar’s percentage of the RC sections. Keywords: Buckling; Reinforced concrete column; Stirrup; Cyclic analysis