Study Of Shear Mechanisms And Strength Of Concrete Deep Beams Based On Strut-and-tie Model

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.