混凝土框架考虑高强钢筋屈曲和应变局部集中的细化有限元模型及损伤特征研究

Buckling of longitudinal reinforcing steel bar is one of the important nonlinear behavior of reinforced concrete (RC) components, the buckling deformation and local strain concentration of high strength reinforcing bars are more serious than that of the normal strength reinforcing bars. The outward push effects of the lateral expansion of the core concrete will make the buckling of the high strength longitudinal bar further more serious, and the restraining effect on the buckling of the bar due to the cover concrete will disappear after the spalling. If finite element software ignore the buckling of reinforcing bars and the effects of concrete on buckling bars, thus calculation results of the finite element analyses always significantly overestimat the post-yield bearing capacity in the later softening stage and deformation ability of RC components. This research project will study the buckling behavior of 500MPa, 600MPa grade high strength steel bar systematically based on a series of experiment tests and finite element analyses, the effects of local strain concentration resulting from buckling on behavior and ultimate average tension strain of buckled steel bar under cyclic loading will be analyzed, The buckling deformations of high strength reinforcing bars and lateral deformations of core concrete and cover concrete in the experiments of low cyclic loaded RC components will be carefully measured. The refined finite element model of using solid element to model concrete and simulating the influence of concrete on buckling of reinforcing bars in plastic regions of component ends, and the modified constitutive model of regarding buckled high strength reinforcing bars as a uniaxial material will be developed respectively. Based on the two kinds of refined finite element model, the simulation results of the post-yield bearing capacity in the later softening stage and deformation ability of RC components could be effectively improved, and the influences of high strength reinforcing bars buckling and local strain concentration on bearing capacity degradation, failure characteristics and ultimate deformation ability of RC frames could be studied more carefully and more accurately. Improved design method will be proposed based on these analysis results.

纵筋屈曲是混凝土构件的主要非线性特征之一，高强钢筋的屈曲变形、局部应变集中现象比普通钢筋更显著，核心混凝土横向膨胀的外推作用会使高强纵筋的屈曲更严重，保护层混凝土剥落后对纵筋屈曲的约束作用将消失。如果有限元模型不考虑上述纵筋屈曲效应，其计算结果会更明显高估配置高强钢筋的混凝土构件软化受力后期的承载力、变形能力。项目将系统地对500MPa、600MPa高强钢筋的屈曲性能进行试验研究，分析屈曲导致的局部应变集中对钢筋循环受力性能、极限平均拉应变的影响。在混凝土构件的低周反复试验中详细测量高强纵筋屈曲变形、混凝土横向变形。分别建立在构件塑性铰区采用实体单元模拟混凝土对纵筋屈曲影响的精细有限元模型、将屈曲高强钢筋等效为单轴材料的修正本构模型。采用这两种细化有限元模型有效提高对构件后期受力性能的模拟精度，研究高强钢筋屈曲、局部应变集中对混凝土框架承载力退化、变形性能、破坏特征的影响，改进其设计方法。

采用高强钢筋有利于更高效地利用钢铁资源、降低用钢量。纵筋屈曲是混凝土构件受力后期的主要非线性特征之一，高强钢筋的屈曲变形、局部应变集中现象比普通钢筋更显著，其断裂问题也更突出；核心混凝土横向膨胀的外推作用会使高强纵筋的屈曲更严重，保护层混凝土剥落后对纵筋屈曲的约束作用将消失。如果有限元模型不考虑上述纵筋屈曲效应，其计算结果会更明显高估配置高强钢筋的混凝土构件软化受力后期的承载力、变形能力。.本项目以试验研究为基础，采用理论分析、统计回归和数值模拟相结合的方法，系统地从材料、构件、结构三个层次，对HRB600钢筋的屈曲受力特征及其对混凝土构件(梁柱)、框架结构抗震性能的影响进行了研究。.通过单根钢筋考虑屈曲的单调和循环加载试验和有限元模拟，项目研究了HRB600钢筋的单调受压性能、屈曲导致的局部塑性变形集中对低周疲劳性能的影响。基于混凝土柱、梁的低周反复加载试验，研究了混凝土柱、梁塑性铰区段HRB600纵筋的屈曲变形规律，以及核心混凝土横向膨胀变形和保护层混凝土约束效应对HRB600纵筋屈曲的影响，并采用基于实体有限元模型的细化数值模拟方法分析了混凝土对HRB600纵筋屈曲变形的影响。.项目提出了适用于HRB600钢筋、可考虑屈曲影响的改进材料本构模型、改进低周疲劳寿命模型，以及同时考虑低周疲劳损伤和初始极限拉应变影响的瞬时断裂平均应变计算方法，建议了基于HRB600纵筋屈曲特征的RC柱性能状态确定方法、HRB600柱纵筋断裂的实用评价方法。将上述方法用于混凝土框架结构典型算例之中，分析了强震作用下HRB600纵筋屈曲对柱纵筋断裂、构件损伤状态的影响规律。.项目取得的关于HRB600高强钢筋屈曲受力性能、断裂评估方法、材料本构模型等方面的研究成果具有较好的系统性，可提高配置高强钢筋混凝土构件受力后期的有限元模拟效果，项目建议的混凝土柱抗震设计方法改进建议可为相关规范的修订提供参考。