Research On Shear Lag Effect In Construction Stage Of Wide Box Beam Short Tower Cable-stayed Bridge

With the continuous development of the low-rise cable-stayed bridge in China,some practical problems need to be solved urgently.The shear lag effect of the low-rise cable-stayed bridge is one of them.At present,the shear lag effect of the low-rise tower cable-stayed bridge is less studied at present,so it is particularly important to study the shear lag effect of the low-rise cable-stayed bridge.This paper takes the construction of Hengyang Dongzhou Xiangjiang Bridge as the background,and has completed the following contents:(1)The origin and development process of the low-rise cable-stayed bridge and the current situation at home and abroad are introduced.The characteristics of the low-rise cable-stayed bridge are briefly summarized.(2)The definition,research significance,research status and research methods of shear lag effect are introduced.(3)The finite element model of the construction stage was established by ANSYS,and the distribution law of shear lag was obtained through analysis and calculation.The effective width of the upper and lower flanges of the cantilever construction beam section is calculated according to the specification.The effective width of the upper and lower flanges of the cable-stayed axial component is calculated by ANSYS.Through the comparison of the measured data between the field measured data and the finite element software,the measured stress magnitude and regularity and the measured shear lag coefficient calculated by the measured stress data and the results obtained by the finite element method have a large deviation from the individual measurement points.The point is basically the same.The model data can generally reflect the actual stress state and the shear lag distribution law.(4)According to the finite element calculation results,the forces of the main beam and the shear lag effect under three different load conditions(gravity only,longitudinal prestress,gravity and longitudinal prestress)are analyzed.Through calculation and analysis,it can be seen that the stress distribution of the roof is not uniform under the three load conditions,and all of them have obvious shear lag effect,especially the vertical prestressing alone action and the combination of gravity and longitudinal prestress,roof and web.The shear lag coefficient at the junction has a large variation and exhibits a positive shear lag effect.Under the three load conditions,the stress distribution of the bottom plate is uniform,the shear lag coefficient changes little,and the shear lag effect is not obvious.(5)In the original construction drawing,there are 5.25 m flange plates on both sides of the box girder.The 5 girder sections of the lagging intermediate beam section are used for post-casting construction,but the post-casting section has many construction procedures and the process is complicated.In order to simplify the construction process,speed up the construction progress,and modify the plan for the overall one-time pouring.If the wholeconstruction of the whole section is carried out according to the original construction drawing,the finite element calculation will cause the tensile stress of 3MPa or so at the flange block of0# block and 1# block with the increase of the construction stage,which may cause the flange plate to crack.The main cause of this phenomenon is the shear lag effect,and the prestress at the web cannot be effectively transmitted to the end of the flange plate.In order to solve this problem,the most effective way is to increase the compressive stress reserve of the flange plate.Therefore,the new scheme increases the prestressed coarse steel bar on the flange plate from 8 to 12 and performs encryption arrangement through the finite element software.According to the calculation and analysis,the flange has no tensile stress during the construction process,and at least a compressive stress reserve of 1.5 MPa is guaranteed.Therefore,increasing the pre-stress of the flange can ensure that the flange of the box girder does not crack during the full-section construction of the main beam.