| In the ferroconcrete unitized beam construction,the steel-truss type unitized box beam not only is one up on no web plate cracking,but also takes into account the advantages of easy erection and quick construction,not only that,as a new type of composite structure,its weight is lighter and its design is economical.Many countries have studied and explored the mechanical characteristics of the constant section box beam so far,and some good conclusions have been obtained.However,there are few literatures studying the shear lag impact with variable profile.This article accord to actual bridge and the academical studying is combined with the contrast of finite element results.(1)On the basis of capacity variation principle,according to the principle of shear deformation equality,its design formula of shear stress is able to infer from replacing the web member with web plate.Through the calculation equation of shear stress,we can further work out the shear lag impact equation.Combined with the Shuitianbao overpass,two working conditions of concentrated load and uniform load are set,which are applied to the actual project for example analysis.The maximum shear lag coefficient is 1.37 under a concentrated load of 300 k N and 1.19 under a uniformly distributed load of Q = 10 k N /m.(2)Referring to the Shuinianbao Bridge,the finite element model is established by ANSYS.Through the actual mechanical loading,so as to select the cross-section of the 1/4and the mid location,then the rules of its shear stress here and shear lag is studied.Analysis shows that based on the theoretical equation of capacity variation principle,the error between the lengthwise stress value of the fin and the finite element value is small,and the stress value of the baseboard is close;There should be no shear lag when the pier is connected with the girder surface,but the additional axial force of the upper wing is generated by the beam force,which makes the shear stress distribution at the beam surface is not uniform.(3)By changing the width,height,etc.,the specific effects of the width-span or width-height ratio on the shear lag impact are further studied.Result display that the shear lag impact of is more affected by the width-span ratio under the concentrated force;When the width to height ratio changes from small to large,the influence of uniform load on it remains stable.;The roof shear lag impact reaches the minimum value when the suspension wing ratio is 1,that is,1 is the demarcation point,and then increases to both sides.The constraint of bearing has a great influence on the shear lag effect impact,and the influence degree should be considered seriously in the design of boundary conditions.(4)Then the calculation methods of "Valid width of plate distribution" in relevant countries are also analyzed.The results reflect that compared with other codes,the effective distribution width of highway and bridge regulations in China is much more comprehensive and detailed,and has a relatively perfect calculation method;In its top plate of the effective distribution width in the analysis,the model and specification for the highest precision,compared with China railway standard and Japanese standard error are smaller,so the two kinds of specification is applicable to variable cross-section in computing the effective width of the flange. |
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