Study On Shear Performance Of Webs Of Concrete And UHPC Box Girder

Due to its excellent flexural and torsional performance and good construction performance,the box girder has been widely used in the large-span prestressed concrete girder bridge.However,according to some survey and studies,it is found that two types of damage—numerous cracks in the main girder and excessive mid-span deflections—are commonly observed in conventional long-span prestressed concrete box-girder bridges in service,especially diagonal cracks on the webs of concrete box girder.Aiming to solve the problem of cracking on the webs of box girder,the shear performance of webs of concrete box girder vertically prestressed by double-tensioned prestressing steel strands anchorage system(DTPSSAS)and webs of UHPC box girder with densely distributed diaphragms in the new bridge structure is studied.Combined with the numerical analysis,shear tests and analytical analysis,the shear cracking behavior,shear stability and ultimate shear resistance of the web of UHPC box girder are mainly explored in present thesis.The main research work are as follows:(1)The time-dependent prestress loss of DTPSSAS was predicted using the step-by-step method,which can consider the stress redistribution effect caused by stirrups into account.This method was used to calculate the time-dependent prestress loss in the rectangular plate and the real box girder bridge.Results indicate that the predicting results agree well with the test results.After obtaining the time-dependent prestress loss,the cracking resistance of webs of box girder provided by the vertical prestress of DTPSSAS is identified in the thesis.It illustrates that the cracking shear stress of the web can be increased by up to 10.4%.(2)According to the structural characteristics of the new UHPC continuous box girder bridge,namely,thin-walled box girder,densely distributed diaphragms,only longitudinal prestressing,partial prestressing system,the UHPC box girder model was design and fabricated in the laboratory to experimentally verify the feasibility of this new bridge structure.Results indicate that,the shear cracking load on the four-diaphragm side is 8%greater than that on the three-diaphragm side.Moreover,the diaphragms can improve the post-cracking performance of the web of UHPC box girder,including its rigidity and ultimate shear resistance.In addition,the cracking-resistance safety margin is approximately 184.4%for the continuous RPC box-girder bridge with a main span of 400 m according to the test result and numerical analysis.Hence,it is feasible to remove the vertical prestressing tendons in the new box-girder bridge structure.The first-cracking loads of the joints are 34%?65%greater than that on the webs.Therefore it is reasonable to use precast segmental construction in the UHPC continuous box girder bridge.(3)Based on the design scheme of UHPC continuous box girder bridge with the main span of 400m,a 3:20 reduced-scale model of the stiffened web of UHPC box girder at the supports of the main span was designed to experimentally study the shear-cracking behavior and ultimate shear resistance of UHPC box girder bridge with the main span of 400m,as well as to verify its safety margin of shear stability.The load-deflection curves,concrete strain profiles,strains of stirrups,cracking behavior and shear failure mode of the web experimental model are studied in the shear test.The finite element analysis of the experimental model is carried out to identify the influence of the main parameters on the ultimate shear resistance of the stiffened web of UHPC box girder.By using the finite element model verified by the test data,the safety margin of the shear stability of UHPC web was evaluated and its parametric analysis was performed.Results show that the width of diaphragm has little effect on the elastic performance of UHPC stiffened web,as well as on its shear-cracking behavior.On contrary,the diaphragm can improve the post-cracking behavior and ultimate shear resistance of the UHPC box girder web.The basic mechanism is that the diagonal cracks extend into the diaphragm after the UHPC box girder web reaches its shear-cracking load,and then the ribbed structure consisting of the diaphragm and web came into effect.Thus,it slows the diagonal crack to get cross the diaphragm and limits the increase of the width and length of diagonal cracks,so that the post-cracking stiffness and ultimate shear capacity of UHPC box girder web are improved.(4)Combined with the experimental results and finite element analysis of UHPC stiffened web,two basic shear failure modes of UHPC ribbed webs are proposed,including the overall shear buckling and web local shear buckling.Based on the simplified mechanical model of two types of shear failure modes and buckling criterion of plate under shear-bending load,the theoretical formulas of shear stability for the UHPC ribbed web with simplified boundary is deduced by Rayleigh-Leeds method and Galerkin method.The theoretical formulas for predicting the shear buckling behavior are verified by the finite element buckling eigenvalue analysis to be correct and reasonable.It is assumed that UHPC always stays in its elastic stage.Thus,the analytical and design formulas of shear-cracking load of web of UHPC box girder are deduced by using Mohr's stress circle.In terms of ultimate shear resistance,the shear load transfer mechanism of UHPC stiffened webs is explored according to limit equilibrium method.The theoretical model for predicting the ultimate shear capacity of the stiffened web structure of UHPC box girder is deduced based on the theory of truss model.According to the experimental results,it is found that the theoretical value of the shear capacity of the model shows close agreement with the experimental data,and the prediction errors are less than 15%.Based on the theoretical model for predicting the ultimate shear resistance of the ribbed webs,simplified formulas are proposed combined the AFGC specification,Japanese JSCE specification and Swiss SIA specification.