Axial compression performance of CFST pier column under long-term load reinforced by encased UHPC
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摘要: 为研究长期荷载作用对墩柱加固后性能的影响,设计了20个不同长期荷载比和截面加固形式的轴压试验构件,采用预应力长期持荷加载装置对钢管混凝土(CFST)试件施加轴向压力,监测了试件应变变形发展情况;采用外包超高性能混凝土(UHPC)对试件进行扩大截面加固,形成组合墩柱构件,对加固后墩柱进行轴压极限加载,分析了长期荷载对加固后墩柱破坏特征和力学性能的影响;建立考虑核心混凝土收缩徐变的有限元模型,进一步开展了参数分析研究截面参数对加固效果的影响;基于试验和数值模拟结果,提出了考虑长期荷载影响的UHPC加固CFST墩柱轴压极限承载力预测公式。研究结果表明:低长期荷载比条件下加固后墩柱主要表现为加固层端部劈裂破坏,高长期荷载比作用下则表现为柱中截面加固层压溃破坏;长期荷载作用下因核心混凝土收缩徐变导致钢管承担更大荷载,柱脚发生局部变形并产生跨中附加挠度,降低变形协调性能,进而导致加固效果下降;加固层配筋率和长期荷载比对加固后墩柱承载力影响显著,低长期荷载比能够一定程度改善构件受力性能,当加固层配筋时能有效限制长期荷载的不利影响,使承载力折减系数降低到10%以内,建议实际应用中加固层配筋率达到1%以上。研究提出的极限承载力预测公式可为长期荷载作用下CFST构件的加固设计提供参考。Abstract: To investigate the influence of long-term loading on the post-reinforcement performance of pier columns, 20 axial compression specimens were designed with varying long-term load ratios and section reinforcement configurations. A prestressed long-term loading device was employed to apply sustained axial compression to concrete-filled steel tube (CFST) specimens, and the evolution of strain and deformation was monitored. The specimens were subsequently reinforced by encasing ultra-high performance concrete (UHPC) to form composite pier columns, which were then tested under ultimate axial compression. The effects of long-term loading on failure modes and mechanical behavior were systematically analyzed. A finite element model incorporating core concrete shrinkage and creep was established, and parametric studies were conducted to evaluate the influence of sectional parameters on reinforcement efficiency. Based on experimental and numerical results, a predictive formula for the ultimate axial bearing capacity of UHPC-reinforced CFST pier columns accounting for long-term loading effects was proposed. The results indicate that under low long-term load ratios, splitting failure occurs at the ends of the reinforced layer, whereas under high ratios, crushing failure predominates at the mid-height section. Long-term loading induces additional load transfer to the steel tube due to concrete shrinkage and creep, leading to local deformation at the column base and increased mid-span deflection, which impairs deformation compatibility and reduces reinforcement effectiveness. The reinforcement ratio of the encased layer and the long-term load ratio significantly influence the ultimate capacity. A moderate long-term load ratio can moderately improve the mechanical response, while the inclusion of reinforcement in the UHPC layer effectively mitigates the adverse effects of long-term loading, limiting the capacity reduction coefficient to within 10%. A reinforcement ratio exceeding 1% is recommended for practical applications. The proposed formula offers a reliable reference for the design of CFST member reinforcements under long-term loading.
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表 1 试件设计参数及测试结果
Table 1. Design parameters and test results of specimens
试件编号 加固前截面尺寸/mm 加固后截面边长/mm 长期荷载比 极限承载力/kN 轴压刚度/(kN·mm-1) 延性系数 承载力折减系数/% SCFST-Lr0.0 边长为120 0.0 911.6 387.5 3.2 0.0 SCFST-Lr0.3 边长为120 0.3 897.4 390.0 2.5 1.6 SCFST-Lr0.5 边长为120 0.5 836.4 388.7 2.8 8.2 SCFST-Lr0.7 边长为120 0.7 819.2 389.1 3.1 10.1 SUC-SCFST-Lr0.0 边长为120 160 0.0 2 195.2 590.8 1.2 0.0 SUC-SCFST-Lr0.3 边长为120 160 0.3 2 314.6 425.4 1.1 -5.4 SUC-SCFST-Lr0.5 边长为120 160 0.5 1 935.2 355.0 1.3 11.8 SUC-SCFST-Lr0.7 边长为120 160 0.7 981.8 304.0 1.8 55.3 SUCR-SCFST-Lr0.0 边长为120 160 0.0 2 305.2 582.3 1.3 0.0 SUCR-SCFST-Lr0.3 边长为120 160 0.3 2 341.6 467.9 1.2 -1.6 SUCR-SCFST-Lr0.5 边长为120 160 0.5 2 290.6 446.1 1.1 0.6 SUCR-SCFST-Lr0.7 边长为120 160 0.7 2 216.2 433.2 0.9 3.9 CCFST-Lr0.0 半径为120 0.0 867.2 188.1 1.8 0.0 CCFST-Lr0.3 半径为120 0.3 825.9 187.6 1.6 4.8 CCFST-Lr0.5 半径为120 0.5 789.3 185.6 1.6 9.0 CCFST-Lr0.7 半径为120 0.7 727.7 178.0 2.4 16.1 SUC-CCFST-Lr0.0 半径为120 160 0.0 2 360.2 629.9 1.1 0.0 SUC-CCFST-Lr0.3 半径为120 160 0.3 2 304.6 546.2 0.8 2.4 SUC-CCFST-Lr0.5 半径为120 160 0.5 1 646.8 475.2 1.1 30.2 SUC-CCFST-Lr0.7 半径为120 160 0.7 973.0 386.8 1.9 58.8 表 2 UHPC配合比
Table 2. Mix proportion of UHPC
kg·m-3 水泥 硅灰 石英砂 石英粉 减水剂 水 钢纤维 40~70目 20~40目 10~20目 400目 844.5 253.4 118.2 346.3 447.6 79.4 21.1 241.5 234 表 3 网格尺寸敏感性分析
Table 3. Sensitivity analysis of mesh size
钢管网格尺寸/mm 内填混凝土网格尺寸/mm 加固层网格尺寸/mm 单元数量/个 计算时间/min 极限承载力模拟值/kN 5.0 5.0 5.0 113 504 392 2 315.3 7.5 7.5 7.5 527 724 136 2 315.3 10.0 10.0 10.0 29 264 33 2 298.2 12.5 12.5 12.5 26 240 30 2 288.4 13.5 13.5 13.5 23 008 24 2 426.7 15.0 15.0 15.0 21 296 20 2 029.2 20.0 20.0 20.0 19 256 14 1 924.0 -
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