【作者】 陈晖；

【导师】 易伟建；

【作者基本信息】 湖南大学 ， 土木工程， 2019， 博士

【摘要】 混凝土深梁广泛存在于桥梁、建筑与海洋平台结构中,作为典型的复杂受力构件,其剪切破坏受力机理仍有待进一步研究。此外,随着深梁尺寸的增大和新材料(如纤维增强复合材料)的应用,一些以往不被重视的问题(如剪切破坏尺寸效应、纵筋刚度对构件受剪性能的影响等)也凸显出来。因此,有必要为混凝土深梁的受剪机制与承载力建立更加合理的力学建模。本文的主要研究工作如下:建立了钢筋混凝土简支与连续深梁剪切试验数据库,据此评价了现行欧美混凝土设计规范中的压杆-拉杆模型(Strut-and-Tie Model,STM),结果表明这些模型应用于深梁抗剪设计时均存在不足之处,有必要进一步完善STM。为摆脱传统经验或半经验STM的局限性,认为应建立一个能反映各主要剪力传递机制对斜压杆承载力贡献的精细STM,从而将压杆有效系数和主要设计参数通过剪力传递机制联系起来,并为改进应用于工程设计的STM提供物理背景与理论指导。通过对钢筋混凝土简支与连续深梁试验结果的对比和非线性有限元分析,识别了两者典型的临界剪切裂缝分别为弯-剪斜裂缝与腹剪斜裂缝,并明确了不同类型临界剪切裂缝对压力传递路的影响。在此基础上,通过分别考虑弯-剪斜裂缝与腹剪斜裂缝对STM的影响,为钢筋混凝土简支与连续深梁建立了带裂缝的压杆-拉杆模型(Cracking STM,CSTM)。简支深梁CSTM中斜压杆被临界剪切裂缝(弯-剪斜裂缝)分为两部分,即未开裂区域与开裂区域。压杆未开裂区域的混凝土有效抗压强度同压杆轴压强度;开裂区域的有效抗压强度由临界剪切裂缝面上骨料咬合机制、腹筋受拉机制和纵筋销栓机制推导得出。而连续深梁CSTM中临界剪切裂缝(腹剪斜裂缝)则贯穿整个斜压杆(即不存在未开裂区域),压杆有效抗压强度由临界剪切裂缝面上各剪力传递机制导出。CSTM还考虑了临界剪切裂缝面上压应力对骨料咬合力的提高以及箍筋与受拉纵筋之间的相互作用。采用简支与连续深梁剪切破坏试验数据库对CSTM进行评价,并与其它STM进行了对比。结果表明,所提CSTM能够更加精确地预测深梁受剪承载力。结合基于单变量试验组的参数分析,表明CSTM能够准确反映各主要设计参数(包括剪跨比、纵筋配筋率、腹筋配筋率、混凝土抗压强度和截面有效高度)对受剪承载力的影响,并能够提供各主要剪力传递机制(即压杆未开裂部分受压机制、骨料咬合机制、箍筋受拉机制和纵筋销栓机制)所承担剪力,这为深入认识剪切破坏的本质提供了很大帮助。基于CSTM和非线性有限元方法,对钢筋混凝土深梁剪切破坏的尺寸效应进行了全面、深入的分析,表明导致深梁剪切破坏尺寸效应的可能原因是:1)垫板尺寸效应,指垫板(或柱)尺寸不随梁高增加而等比例变化导致的压杆相对宽度减小;2)梁高尺寸效应,指临界剪切裂缝面传递剪力的能力随着梁高(和裂缝宽度)增大而降低。将CSTM的应用拓展到了纤维增强复合材料(FRP)筋混凝土深梁中,其预测构件受剪承载力的精度优于现有模型,并且从机理层面解释了纵筋刚度对受剪承载力的影响。利用CSTM定量分析了各设计参数对压杆有效系数的影响,并产生一个无偏差的深梁受剪承载力数据库(包括简支与连续深梁数据各1024组),据此回归并简化得到了改进的压杆有效系数与相应的STM。改进STM能较好地预测简支与连续深梁受剪承载力,且能较准确反映各主要设计参数对承载力的影响,包括梁高尺寸效应。所提模型可以被用于工程设计,在没有增加计算量的条件下其表现优于现行欧美规范中的STM。最后,对我国混凝土设计规范GB 50010-2010中集中荷载作用下深受弯构件受剪承载力计算公式进行了评价,指出其不能准确反映剪跨比和腹筋配筋率对构件受剪承载力的影响,且没有考虑纵筋配筋率、截面高度和垫板(或柱)尺寸的影响。因此,建议我国规范采用本文提出的改进STM对深受弯构件进行设计。更多还原

【Abstract】 Concrete deep beams are widely used in bridge,building and offshore structures.As a typical component with a complex stress state,the shear mechanism of deep beams still needs further study.In addition,with the increase of the beam height and the application of new materials(e.g.fiber reinforced polymer),some problems(e.g.the shear size effect and the influence of longitudinal reinforcement stiff ness on the shear strength)that were not paid attention to in the past have become obvious.Therefore,it is necessary to develop more rational mechanical models for the shear mechanisms and strength of concrete deep beams.The main works of this dissertation are as follows.A shear test database of reinforced concrete(RC)simple and continuous deep beams was created to evaluate strut-and-tie models(STMs)in current design codes for structural concrete.The evaluation results showed theses models still h ave limitations when applied to the shear design of RC deep beams,so it is necessary to improve the STM.In order to get rid of the limitations of the traditional STM,a refined STM considering contributions of shear transfer mechanisms to the strut capacity should be developed,so that the strut efficiency coefficient and main design parameters could be linked with the shear transfer mechanisms,also physical backgrounds and theoretical guidance for improving the traditional STMs could be provided.By comparing experimental and numerical results of RC simple and continuous deep beams failed in shear,it was identified that the typical critical shear cracks of simple and continuous deep beams were the flexural-shear crack and the web-shear crack,respectively,as well as their different effects on shear transfer mechanisms.On this basis,by considering the influence of the flexural-and web-shear cracks on the STM,a cracking strut-and-tie models(CSTM)was developed for RC simple and continuous deep beams.The diagonal strut in the CSTM of simple deep beams was divided into an uncracked part and a cracked part by the critical shear crack(flexural-shear crack).The effective compressive strength of the uncracked part was defined as the ultimate strength of concrete struts under axial compression;the effective compressive strength of the cracked part was derived from the forces transferred by the aggregate interlock,web reinforcement,and dowel action of longitudinal bars on the critical shear crack surface.On the other hand,the diagonal strut in the CSTM of continuous deep beams was entirely penetrated by the critical shear crack(web-shear crack),which means there was no uncracked part in the strut.And the strut effective compressive strength was derived from the forces transferred by the mechanisms on the critical shear crack surface.In addition,the CSTM also considered the improvement of aggregate interlock action under compression and the interaction between stirrups and longitudinal bars.The CSTM was evaluated by the shear test databases of simple and continuous deep beams and compared with other STMs,showing the proposed models better predicted the shear strength.Combined with the parameter analysis based on univariate test groups,it was shown the CSTM accurately reflected the effects of the main design parameters(including shear span-to-depth ratio,longitudinal reinforcement ratio,stirrup ratio,concrete compressive strength,and effective depth)on the shear strength,and could also provide the shear forces carried by the shear mechanisms(including aggregate interlock,dowel action of longitudinal bars,and stirrups),which is of great help to understand the nature of the shear failure of RC deep beams.Based on the CSTM and the nonlinear finite element model,the shear size effect of RC deep beams was analyzed in-depth,and possible explanations were provided: 1)bearing plate size effect,that is the reduced relative strut width caused by the disproportionately varied bearing plate(or column)size with the beam height;and 2)beam depth effect,which refers to the deterioration of the shear transfer strength by aggregate interlock of the critical shear crack due to the increase of the beam depth.The application of the CSTM was extended to concrete deep beams reinforced with fiber reinforced polymer(FRP)bars.The proposed model well predicted the shear strength of FRP-reinforced deep beams,and well reflected the effects of the main design parameters on the shear strength.Also,the model explained that the shear force carried by aggregate interlock is the critical mechanism that leads to the effect of the reinforcement stiffness on the shear strength.The effects of the main design parameters on the strut efficiency coefficient was quantitatively analyzed using the CSTM,and a shear strength database of deep beams(including 1024 simple deep beams and 1024 continuous deep beams)without bias was generated.Based on the database,a modified strut efficiency coefficient was obtained by fitting.Then a modified strut-and-tie model for the purpose of practical design was proposed,and it better predicted the shear strength of deep beams and the influences of the main parameters on the shear strength compared to the STMs in current codes.At the end of the dissertation,the shear capacity equations of deep flexural members under point load in the Chinese code for design of concrete structures GB 50010-2010 was evaluated,showing the equations inaccurately reflected the influences of shear span-to-depth ratio and web reinforcement ratio on the shear strength,and did not consider the influences of longitudinal bars,section depth and bearing plate(or column)size.Therefore,it was suggested the Chinese code adopt the proposed modified strut-and-tie model for the shear design of deep flexural members.更多还原