Study On Repair Design Of Bolted Steel Box Girder For Cable-stayed Bridges Considering The Effects Of Welding Deformation

Steel box girder has been widely used in long span Bridges at home and abroad because of its advantages of light dead weight and large bearing capacity.In the early design of steel box girder,the top plate,web plate and bottom plate are bolted together.After long-term operation,cracks and water seepage occur frequently at the bolt joints of the roof,leading to corrosion inside the steel box girder and bolt loosening,which seriously affects the normal use of the bridge.In practice,the replacement of bolts,sealing coating and other measures are often used to repair.These measures,while effective in the short term,are not a permanent solution to the roof seepage problem.If the top plate of the steel box girder can be welded instead of bolted,the diseases caused by water seepage on the bridge deck can be dealt with fundamentally.This paper takes a cable-stayed bridge as the engineering background and proposes a steel box girder repair design-the top plate of the steel box girder is changed from bolted to welded(referred to as "bolted to welded"),and the web and bottom plate are still bolted together.However,welding is an uneven heating and cooling process and can result in post-weld distortion.The fact that this solution is only applied to the top plate means that the weld deformation only occurs at the top of the box girder(on the neutral side of the section),which changes the local forces and overall alignment of the box girder,thus affecting the load bearing state of the cables and towers.Whether the repaired bridge can still meet the service requirements is then worthy of further study.In this regard,a feasibility study on the "bolt to weld solution" was carried out using numerical simulations and the main findings are as follows.(1)In order to obtain the size and distribution of welding deformation of steel box girder roof,two groups of butt joint specimens were made according to the actual welding process,and the welding deformation was tested.The results show that the 12 mm thick steel plate has an "inverse saddle shape" after butt welding,and the welding deformation is mainly transverse shrinkage,with the maximum shrinkage of about 4mm.The numerical simulation of welding was carried out by the thermal-elastic-plastic FEM,and the calculated results were compared with the measured data.The two results were in good agreement,which verified the reliability of welding simulation.Based on this,the influence of different welding process parameters on welding deformation was analyzed by numerical simulation,and the optimal welding process which can control transverse shrinkage deformation within 2mm was proposed.(2)Aiming at the maximum stress segments of the main beam,the "one-way cooling method" is adopted to consider the influence of welding deformation,and the influence of three plate replacement schemes(block by block replacement,from outside to inside,from inside to outside)on the stress and deformation of the beam segments is analyzed by numerical simulation.The results show that the tensile and compressive stresses of the roof change dramatically at the plate junction with these three schemes,and the maximum tensile stress exceeds 140 MPa and the maximum compressive stress exceeds 400 MPa.According to the uneven degree of stress in the girder section after reconstruction,it is suggested to adopt the scheme of "from inside to outside".(3)Based on the proposed "from inside to outside" plate replacement sequence,the influence of welding deformation and external load on the stress and deformation of the beam section after repair was studied.The results show that both of them have great influence.When the welding deformation increases from 1mm to 3mm,the peak compressive stress of the roof increases from-282 MPa to-402 MPa,increasing by about 43%.The peak value of tensile stress increases 196% from 73 MPa to 215 MPa.The displacement of beam end increases by20% from 101 mm to 122 mm.When the external load decreases from F to F/2,the maximum compressive stress decreases from 712 MPa to 540 MPa by 24%.The maximum tensile stress is basically unchanged.The vertical displacement of beam end decreases by 28% from 111 mm to 79 mm.(4)Midas/Civil was used to establish the whole bridge model,and more than 80 bolt belts of the whole bridge roof were repaired and reformed.The influence of welding shrinkage deformation was considered by applying gradient temperature load,and the changes of linearity and stress state of the whole bridge after the transformation were analyzed.The results show that the vertical displacement of the main beam increases obviously after the reconstruction,especially at the side span of the main beam.The displacement increases 247.6%from-10.5mm to-36.7mm.The stress on the lower edge of the main beam changes more significantly than that on the upper edge after reconstruction.The stress on the upper edge of the main beam varies from 0MPa to 39 MPa,and the stress on the lower edge varies from0 MPa to 95 MPa.After the reconstruction,the changes of cable force and pylon deflection are small,and their change rates are less than 10%.