【作者】 刘斌；

【导师】 樊伟；

【作者基本信息】 湖南大学 ， 土木工程， 2019， 博士

【摘要】 圆形钢筋混凝土(RC)柱作为典型的桥墩墩柱形式之一,已被广泛地运用到实际工程中。在桥梁结构全寿命基准期内,这些墩柱除了承受上部结构的恒活载外,还可能遭受各种侧向冲击荷载,例如跨河桥墩遭受船舶的撞击,位于山区的桥墩遭受滚石的撞击以及城市跨线桥墩受到车辆的碰撞。过去几十年中,针对圆形RC墩柱的轴向承压能力和抗震性能已展开了大量的研究。然而,对于RC墩柱的侧向抗冲击性能的研究则鲜有报道。此外,墩柱主要作用是承受着轴向荷载,所以评估桥墩冲击后的剩余轴向承载能力和失效风险显得非常重要。因此,本文从冲击试验,冲击后轴压试验,精细化与简化分析方法等方面,对受压圆形RC墩柱的侧向抗冲击性能以及冲击后损伤评估开展了较为深入的研究。主要研究工作和结论如下:(1)开展了12根圆形RC柱落锤侧向冲击试验。通过试验获取了冲击力,RC柱的位移以及整个冲击过程的高速影像数据,分析研究了轴压比、配筋率、高径比对RC柱抗冲击性能的影响。试验结果表明:轴向压力对RC柱的冲击响应有着显著的影响。在相对较小的冲击位移时,轴向压力能够增强RC柱的侧向抗冲击能力。相反,轴力则将削弱柱的侧向抗冲击能力,体现在随着轴力的增加,RC柱的延性会减小。此外,轴力和剪跨比是导致RC柱破坏模式转变(如由弯曲损伤为主转换成剪切损伤为主)的关键因素。(2)建立了受压RC柱的接触-碰撞精细有限元模型。通过与试验对比表明:传统RC梁的模拟方法,在受压RC柱冲击动力响应存在明显不足。在此基础上,建立了受压RC柱遭受侧向冲击的改进模型。在改进模型中,通过修正混凝土材料模型合理考虑了螺旋箍筋对核心混凝土的约束效应,同时考虑了钢筋与混凝土之间的粘结-滑移行为以及混凝土裂缝闭合行为的影响。研究表明:改进的有限元模型在模拟精度上显著优于传统的模型,说明上述改进的必要性和有效性。考虑到目前混凝土本构在剪切分析中的内在不足,采用基于MCFT理论的膜单元建立了二维冲击分析模型,研究表明:建立的二维冲击分析模型不仅能较好地预测受压RC墩柱的冲击响应,而且能较好地识别出不同类型的冲击破坏模式。(3)对9根冲击损伤RC柱进行了静力轴向加载试验,以测量和评估不同配筋率和轴压比以及高径比的RC柱,在遭受不同冲击作用后的剩余轴向承载能力。试验结果表明:RC柱的剩余轴向承载能力随着侧向冲击引起的残余变形的增大而减小;冲击荷载导致的损伤模式对RC柱的剩余轴向承载能力有显著的影响;剪切损伤造成RC柱承载能力的降低程度明显高于弯曲损伤造成RC柱承载能力减低程度。(4)由于实际撞击事故中作用于墩柱上的冲击力和峰值位移是未知的,提出了一种基于RC柱冲击后状态(残余变形和损伤模式)近似的评估方法,用于预测冲击后墩柱的剩余轴向承载能力,通过试验验证了该方法的有效性。在此基础上,利用建立的方法进行了广泛的参数分析,阐明了墩柱配筋率和轴压比对圆形RC柱冲击后剩余轴向承载能力的影响,回归得到了经验公式,可用于简单估计侧向冲击后圆形RC墩柱的剩余轴向承载能力。(5)针对弯曲损伤为主的情况,考虑圆形RC墩柱的特点(如存在初始轴压和环形布筋),建立了适用于受压圆形RC柱的两自由度简化分析模型,给出了基于截面分析和基于纤维梁单元计算圆形RC墩柱抗力曲线的方法,探讨了合理确定抗力曲线中等效卸载刚度的方法。通过试验验证了建立的两自由度分析模型的有效性,并基于此广泛研究了冲击速度和质量对RC柱冲击损伤程度的影响。(6)提出了基于非线性纤维梁柱单元的高效分析模型,用于预测冲击作用下RC梁柱的弯曲和剪切行为。该高效分析模型主要包括:基于宏观单元的接触模型、用于模拟梁柱非线性行为的纤维梁柱单元和用于捕捉剪切行为的两类剪切弹簧。收集了近50个RC构件的冲击试验,并通过这些试验验证了提出的高效分析模型的有效性。相比现有的模型或方法,提出的高效分析方法具有计算效率高、易在实际问题分析中运用以及无需编程等优点。更多还原

【Abstract】 As one of the typical forms of bridge columns,circular reinforced concrete(RC)column structures have been extensively used as bridge piers in engineering practices.During the service life of the bridge structure,besides these static and dynamic loads,the RC column may also suffer from various lateral impact loads.For example,bridge piers located close to a navigable waterway may be vulnerable to vessel collisions,bridge piers in mountain areas may be hit by falling rocks and the viaduct piers may be impacted by a vehicular.In the past few decades,the static compression strengths and seismic performances of circular RC columns have been widely investigated.However,few reseaches have been performed with an emphasis on clarifying the dynamic behavior of the axially-loaded RC column subjected to lateral impact loading.On the other hand,because that these columns are all mainly designed to support the axial loads,it is of great importance to quantify the residual axial capacity for predicting the damage extent and collapse risk of the impact-damaged RC column.To this end,low-velocity impact tests,compression after impact tests,develop the detailed FE modeling method and simplified methods on axially-loaded circular RC columns are designed and conducted to evaluate the impact responses of axially-loaded circular RC columns and the residual strengths of impact-damaged RC columns in this paper.The main contents of this study are summarized as follows:(1)Twelve circular RC column specimens are tested under impact loading through the drop hammer test system.Through the test,the impact force,the displacement of the RC column and the high-speed image data of the whole impact process were obtained.The effects of axial compression ratio,reinforcement ratio and height-diameter ratio on the impact reponse of the RC column were discussed.It is found that the applied axial load have a great effect on the impact responses of the circular RC columns.Generally,the axial load plays a positive effect when the deformations of the column specimen are relatively small.On the contrary,the presence of the axial load could have catastrophic effects,as the ductility of the RC column decreases as the axial force increases.In addition,the axial force and the span-depth ratio are the key factors cause the transformation of the RC column failure mode(such as from the flexural failures to the shear failures).(2)The corresponding finite element(FE)model is established for the impact simulation of axially-loaded RC columns.Comparison with the test date shows that the conventional modeling used in RC beams is shown to have some disadvantages in the predictions of the impact-induced responses of the axially-loaded columns.On this basis,an improved FE modeling are proposed for the simulation of the axially-loaded RC columns subjected to impact loading.In the improved FE model,the concrete material model is modified to accurately consider the confinement effect of the spiral stirrup.In addition,the influences of the bond-slip behavior and the behavior of crack opening and closing on the residual deformation after impact are considered in the improved FE models.According to the research,it can be concluded that the improved FE model are far superior to the conventional model in simulation accuracy,indicating the necessity and effectiveness of the above improvement.Considering calculation accuracy cannot be guaranteed in shear analysis because there are still some limitations for concrete material models.The two-dimensional reinforced concrete membrane code that based on the Modified Compression Field Theory(MCFT)was employed to estimate impact analysis model.The research shows that the established two-dimensional impact analysis model not only can better predict the impact response of axially-loaded RC column,but also can better identify different types of impact failure modes.(3)A total of 9 impact-damaged circular RC column specimens with different axial compression ratio and reinforcement ratio as well as height-diameter ratio are tested under quasi-static axial loading to evaluate their residual compressive strengths after they suffered from different drop-hammer impact tests(or events).The test results shows that the residual axial strengths of these RC columns usually decreased with increasing the residual deformations;Damage modes caused by impact loadings have an significant impact on the residual axial strength.The specimen with shear-dominated failure mode exhibit lower residual capacity than that with flexural-dominated failure mode.(4)Since the maximum load imposed during a collision is usually unknown,an assessment method based on post-impact state(deformation and damage mode)is proposed and demonstrated to be capable of predicting post-impact residual axial capacity.The results obtained from the developed high-resolution FE models are in good agreement with the CAI test data.On this base,an extensive parametric study was conducted by using the validated FE models to investigate the influences of column aspect ratio,reinforcement ratio and axial load ratio on the post-impact strengths of circular RC column.An empirical formula was derived by a multiple regression fit to the FE results for providing an easy estimation of residual strength of circular RC columns after lateral impact.(5)For circular RC columns with flexural-dominated failure modes,considering the characteristics of circular RC columns(such as the initial axial load and annular reinforcement),a two-degree-of-freedom(TDOF)simplified analysis model suitable for axially-loaded circular RC columns is established.The methods based on section analysis and fiber beam element model to calculate the resistance curve of circular RC column were proposed.The method for reasonably determining the equivalent unloading stiffness of the resistance curve is discussed.The validity of the established TDOF analysis model was verified by experiments.Based on this,the effects of impact velocity and mass on the impact damage of RC columns were extensively studied.(6)An efficient modeling method based on fiber beam element is proposed to capture both flexural and shear behaviors of RC beams and columns under impact loading.The method includes a macroelement-based contact model,fiber beam element for simulating nonlinear behavior and two types of shear springs for capturing shear behavior.Nearly fifty impact tests on RC beams and columns reported in the literature were employed to validate the proposed modeling method.Compared with the existing models or methods,the proposed efficient modeling method has the advantages of high computational efficiency,easy application in practical problem analysis,and can be readily implemented without coding in any FE software as long as traditional fiber-section elements,and discrete macroelements are available.更多还原