The Bar Simulation Method Of Shear Lag Effect Of Box Girder

At present,with the continuous increase of urban traffic volume,the width of box girder of viaducts and overpasses on urban main roads,expressways,and ring roads is constantly increasing.On the other hand,with the application of three-way prestressed system,the cantilever length and web spacing of the top plate of the box girder gradually increase as the beam body width increases,so as to reduce the self-weight of the structure and adapt to the bridge structure with larger span,which will lead to more and more significant shear lag effect of the box girder wing plate.In view of this,based on the shear lag effect of box girder under dead weight load as the research object,such as originally only adapted to the beam shear lag analysis of analogy method to expand into the variable cross-section beam,and discusses the variable cross-section cantilever beam and the shear lag effect of continuous beam along the span long distribution characteristics,analyzes the bracket cast-in-situ,pusher is used for the continuous girder bridge construction,the cantilever,simply supported continuous construction methods,such as control section of the evolution process of the shear lag coefficient along with the construction progress.The main research work includes:（1）As to the inability of traditional comparison method to solve the shear lag effect of variable-section box girder,the shear lag control equation and the calculation formula of web shear force for variable-section box girder are re-derived,the equivalent beam model is improved.The equivalent method of the stiffening plate in the middle is extended to the variable-section box girder,which is only applicable to the solution of the shear lag of the equal-section box girder.The results of solid finite element analysis verify the rationality of the equivalent method,and it is found that the additional shear flow in the web of the box girder with variable section is opposite to the self-weight shear flow,which slows down the change of the shear flow along the span of the bridge and weakens the shear lag effect of the box girder with variable section.（2）The influence of structural parameters such as box girder height and web thickness on the shear lag effect of box girder is analyzed by using the comparison method of shear lag effect of variable section box girder.The variable section cantilever beam and continuous beam are discussed.The shear lag effect along the span length gives the factors affecting the distribution of the positive and negative shear lag of the cantilever box girder.It is found that the variation of the box girder height and the web thickness will reduce the shear lag effect.However,the influence of the thickness of the web is relatively weak,and the shear lag effect of the continuous box girder span is more significant than the norm,and should be given attention when designing.（3）By using the superposition principle,the shear lag effect of continuous box girder constructed in stages is analyzed,and it is found that the construction method has a significant impact on the shear lag effect of continuous beam,and the shear lag effect at the same location will change greatly in different construction stages.For the shear lag coefficient at the middle support,the continuous construction with simple support has the maximum shear lag coefficient and the minimum cantilever casting.For the shear lag coefficient of mid-span section,it is the largest in cantilever casting and the smallest in simply supported continuous construction.（4）The reasons for the influence of the length of the positive and negative moment zone on the shear lag effect are analyzed.It is found that the boundary condition at the bearing affects the distribution of the shear force flow of the wing,which makes the shear lag effect in the boundary affected zone accumulate faster.The shear lag effect away from the boundary affected zone accumulates slowly.At the same time,the length of the boundary affected zone is relatively fixed under different construction modes,resulting in the longer the length in the positive and negative moment zone,The greater the integral ratio of the inter-rod shear flow q_2,1 to the web shear flow q_E,the smaller the shear lag effect.（5）Single box office box girder are analyzed on the influence of shear lag effect of box number in,it is found that in the wide span ratio given conditions,a single box double room to improve evident in the cross section of maximum shear lag coefficient,single box three rooms box girder to the maximum shear lag coefficient improvement of cross section is not obvious,but the shear lag coefficient of the edge web is improved obviously,and the shear lag coefficient of the middle web is larger than that of the web.The design should be paid more attention.（6）Through the physical finite element analysis of a low-rise tower cable-stayed bridge,it is found that the cantilever plate post-casting construction under the self-heavy load will reduce the longitudinal tensile stress of the cantilever plate post-casting belt,but the longitudinal normal stress of the middle beam body will increase,as a result,the shear lag coefficient of the box girder roof increases,and the shear lag coefficient of the roof increases with the increase of the width of the post-casting strip.