Study On Shear Lag Effect In Flat Steel Box Girder Of A Cable-stayed Bridge

Streamlined flat steel box girder is becoming the most popular in the long spancable-stayed bridges recently,for its light weight,good performance of bending,torsion andalso wind resistance.As this type of structure has thinner slabs and smaller height to widthratio,the shear lag effect in it is more notable.On the basis of previous researches,the shearlag effect in the flat steel box girder of a cable-stayed bridge is systematic studied in thisessay.(1)The governing differential equations for analyzing the shear lag effect of the singlerectangular section with three cells are set up,separately according to the energy variationmethod proposed by Reissner and analogous bar method.Then the analogous bar method isimproved to conclude the influence of diaphragms and the lateral loading position.The flatbox section is simplified to a standardized form,so that the shear lag effect in indeterminatemain girder of a cable-stayed bridge bearing compression and flexural moment can beanalyzed by superposition or dismemberment principle.The comparisons with FEA resultsshow that stresses of the flat steel box girder of a cable-stayed bridge can be approximatelycalculated by the above method.(2)The parametric analysis of flat steel box girder is carried out by the shell elementmodel using FEM,concluding the dimension parameters,the transverse slope of the sectionand the transverse position of load acting.Results show that the influence of width to spanratio is the most remarkable on the shear lag coefficient of the girder,influence of thethickness and spacing of inside longitudinal partition is relatively small,and others' influencesare negligible.The transverse slope influences the normal stress of the section but not theshear lag effect.The influence of transverse position of load acting is obvious near the actingregion,but is negligible far away.According to above regularities,the practical calculatingformulas of the shear lag coefficient in compression-flexure flat steel box girders is derived,on the basis of mathematical statistics and using the method of multiple regression analysis.The FEA results confirm that practical formulas are reliable and can be easily used inengineering calculations.The proposal of formulas is one of innovations in this thesis. (3)Using the mixed FEM containing both beam element and shell element,the NorthChannel cable-stayed bridge of Hang Zhou Bay Sea-crossing Bridge and cable-stayed bridgeof Ji Nan Huang River are simulated,considering different construction stages.Thelongitudinal distribution rules of shear lag effect in main girder are concluded.Then the maingirder is suggested to be divided into three regions,Regionâ… ,â…¡,â…¢,to make detailed analysisby designers.(4)In order to know the shear lag effect in the ultimate limit state,the elasto-plasticperformance of the flat steel box girder is analyzed by FEM models,considering differentloading conditions.Results show that different parts of the girder reached yielding state bythe order of inside longitudinal partition,the side longitudinal partition and the middle ofthe section.The procedure relationship of the upper slab and bottom slab is determined bythe proportion of the compression load and flexural moment.Under the condition of limitplastic deformation,residual stress is induced in the structure after unloading.In thissituation,there is a state like negative shear lag effect in the plates undergoing the plasticdeformation,while the positive shear lag effect still in the elastic plates.