The study reported herein aims at investigating the behavior of medium-scale circular reinforced concrete columns wrapped with fiber reinforced polymer (FRP) sheets under concentric and eccentric axial loads. The experimental program was devised to assess the effects of loading conditions, absence/presence of an FRP jacket as well as the FRP wrapping system. To achieve the study objectives, four column groups were tested under axial compression at 0, 25, 50 and 65 mm loading eccentricities corresponding to eccentricity-to-diameter ratios of 0, 0.13, 0.26 and 0.34, respectively. Specimens in a fifth group were tested in pure bending simulating axial compression at infinite loading eccentricities. Three column subcategories were tested under each of the 5 loading eccentricities: unwrapped; wrapped with one ply of hoop FRP sheets; and wrapped with two FRP plies with fibers oriented at 0 and 90° to the longitudinal column axis thereby providing externally-bonded longitudinal reinforcement and hoop confinement, respectively. Tests confirmed that FRP confinement enhances the axial–flexural column resistance even at large eccentricities that exceed the balanced state of unconfined columns. Although axial column resistance decreased with increasing bending moments, relative enhancements (25–35%) in axial resistance provided by FRP confinement were found to be more significant under eccentric loading than in pure compression. Compared to hoop FRP-confined columns, using additional longitudinal sheets resulted in minor (7–9%) but stable enhancements in axial resistance that were unaffected by the increase in loading eccentricity. The FRP hoop wraps had a minor effect on the flexural resistance of specimens tested in pure bending but managed to double their resistance when combined with the externally-bonded longitudinal FRP sheets. Finally, three stress–strain models of FRP-confined concrete were used in conventional section analysis to assess the axial–flexural interaction in the FRP-jacketed columns. Strength predictions made using the stress–strain model proposed in ACI 440.2R-17 design guidelines did not agree with the test results of the eccentrically-loaded columns and underestimated the moment resistance at a given axial force even when considering higher confinement ratios than those permitted by the guidelines. Strength predictions made using eccentricity-dependent stress–strain models showed better results especially when accounting for the increase in ultimate axial strains under eccentric loading.

中文翻译：

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FRP 包裹 RC 柱中的轴向-弯曲相互作用

本文报告的研究旨在研究用纤维增强聚合物 (FRP) 板包裹的中等规模圆形钢筋混凝土柱在同心和偏心轴向载荷下的行为。设计实验程序以评估加载条件、FRP 护套的缺失/存在以及 FRP 包裹系统的影响。为实现研究目标，四个柱组分别在 0、25、50 和 65 毫米的加载偏心下进行轴向压缩测试，偏心直径比分别为 0、0.13、0.26 和 0.34。第五组中的试样在模拟无限载荷偏心下的轴向压缩的纯弯曲中进行了测试。在 5 种加载偏心的每一种下测试了三个柱子类别：展开；用一层箍 FRP 板包裹；并用两个 FRP 层包裹，纤维与纵向柱轴成 0°和 90°，从而分别提供外部粘合的纵向钢筋和箍约束。试验证实，即使在超过无约束柱平衡状态的大偏心时，FRP 约束也能增强轴向弯曲柱的阻力。尽管轴向柱阻力随着弯矩的增加而降低，但发现 FRP 约束提供的轴向阻力的相对增强（25-35%）在偏心载荷下比在纯压缩下更为显着。与箍 FRP 约束柱相比，使用额外的纵向板会导致轴向阻力较小（7-9%）但稳定的增强，而不受载荷偏心增加的影响。FRP 环箍对纯弯曲测试试样的抗弯强度影响较小，但与外部粘合的纵向 FRP 板结合时，其抗弯强度增加了一倍。最后，在常规截面分析中使用了 FRP 约束混凝土的三个应力 - 应变模型来评估 FRP 夹套柱中的轴 - 弯曲相互作用。使用 ACI 440.2R-17 设计指南中提出的应力 - 应变模型进行的强度预测与偏载柱的测试结果不一致，并且即使考虑比允许的更高的约束比，也低估了给定轴力下的力矩阻力根据指南。