FRP布加固混凝土框架子结构抗连续倒塌的精细有限元分析

A DETAILED NUMERICAL ANALYSIS FOR THE PROGRESSIVE COLLAPSE OF CONCRETE FRAME SUBSTRUCTURES STRENGTHENED WITH FRP STRIPS

外贴FRP布加固是一种有效提高既有建筑抗连续倒塌性能的手段,但现有FRP布加固方式存在降低结构抗震性能、加固施工不便等缺点。该文采用数值模拟方法分析了FRP布加固方式对现浇和装配式混凝土框架子结构抗连续倒塌与抗震性能的影响,并开展了优化方案研究。基于通用有限元软件LS-DYNA建立了FRP布加固混凝土框架子结构的连续倒塌精细数值模型,其中混凝土、钢筋与FRP布分别采用实体、梁与壳单元进行模拟,考虑了FRP布和钢筋的滑移、新旧混凝土界面的粘结失效和机械套筒处的钢筋截面损失。试验验证表明该方法可准确模拟试验试件的破坏模式和承载力发展。分析试验试件的不同粘贴方案结果发现:对现浇混凝土子结构,梁底与梁侧中性轴粘贴纵向FRP布并在梁端塑性铰区粘贴U形横向FRP布后,小变形下的结构倒塌抗力提升有限(最大仅2.6%)、基本不影响结构抗震性能,而对大变形下的结构倒塌抗力提升幅度可达49.5%;对于装配式混凝土子结构,在梁底、梁顶与梁侧底部外贴纵向布并在梁端塑性铰区粘贴U形横向FRP布可将小变形和大变形下的结构抗力最大提升24.2%和48.1%,使得装配式子结构在小变形下受力等同现浇结构,提升了原装配式子结构的抗震性能。对上述最优方案进一步的分析表明:保持FRP布用量不变而将塑性铰区内U形横向FRP布的分布范围和条数增加可提高大变形下的结构倒塌抗力,而不影响小变形下的加固效果。

Externally bonded FRP strips can effectively improve the progressive collapse-resisting performance of existing structures,but the seismic performance of structures and the ease of construction were not taken into consideration in existing FRP strengthening schemes.Numerical simulation was performed to study the influences of strengthening schemes using FRP strips on the seismic and progressive collapse-resisting behavior of cast-in-site and precast concrete frame substructures,by which the strengthening schemes were consequently optimized.The detailed numerical models of concrete frame substructures strengthened with FRP strips were established using the general finite element(FE)software LS-DYNA,in which the concrete,steel reinforcement and FRP strips were simulated by solid,beam and shell elements,respectively.The bond-slip of steel bars and FRP strips,the bond failure between precast and cast-in-site concrete and the loss of cross-sectional areas of bars at the mechanical sleeves were considered in the numerical models.The numerical models were validated by experimental results,which showed that the failure modes and the strengths of the substructures in the experiment were well captured by the numerical models.The results of the different strengthening schemes suggested that the bonding of the longitudinal FRP strips at the beam bottoms and the neutral axes of beam sides and U-shaped transverse FRP strips in the plastic hinge regions of the beams hardly improved the structural collapse resistances under small deformations for cast-in-site concrete substructures(the maximum percentage increase was only 2.6%).Such a strengthening scheme had almost no effect on the seismic performance of the substructures,while the progressive collapse resistance under large deformations was increased by at most 49.5%.For precast concrete substructures,applying longitudinal FRP strips at the beam tops,beam bottoms and bottoms of the beam sides and U-shaped transverse FRP strips in the plastic hinge regions would improve their maximum resistance under small and large deformations by at most 24.2%and 48.1%,respectively.Under small deformations,their collapse resistance was increased to the same level as cast-in-place ones,which was advantageous for improving the seismic performance of precast concrete substructures.A further analysis of the aforementioned optimal schemes shows that keeping the amount of FRP unchanged and at the same time increasing the covering length and the number of U-shaped transverse FRP strips applied in the plastic hinge regions of the beams could improve the structural collapse resistance under large deformation,while the effect of FRP strengthening on the collapse resistances under small deformation remained unchanged.