| The jacking method is a common construction method for bridges crossing mountains,rivers,lakes and railway lines,while the jacking span directly determines the safety and economy of the jacking method.Zhengzhou Zhengbei bridge is 43 meters wide and spans Zhengzhou north station,the largest railway marshalling station in Asia.It is constructed by walking type jacking method.The maximum jacking span is 92 meters,which is the largest jacking span of the wide bridge in China.At present,there are relatively few researches on large-span jacking construction of wide-span bridges.In this paper,based on the engineering background of Zhengbei bridge,the key technologies used in the construction process of Zhengbei bridge for crossing the railway marshalling station are studied,such as the auxiliary jacking of temporary cable pylon,the combination of step and monitoring for offset and elevation control,wind load and sunshine impact control.Based on the idea of displacement analysis and stress analysis,the displacement changes and stress conditions of the beam and temporary auxiliary facilities in each stage of the jacking process are simulated and analyzed.In addtion,the most unfavorable conditions are selected for wind load and temperature effect analysis.Through on-site monitoring,the monitoring results are compared with the calculation results to verify the feasibility of the analysis and provide reference for the construction of the same type of bridge in the future.This paper mainly completes the following work.(1)Taking the jacking construction of Zhengzhou Zhengbei bridge as an example,the influencing factors,construction scheme,control points of elevation,alignment,wind load,temperature,etc.and key technologies of elevation and alignment control,wind load and sunshine influence control,etc.are proposed.(2)In the ANSYS finite element software,the simulation analysis of the launching process in each stage of the bridge is carried out.It is concluded that the maximum displacement of the beam occurs when the guide beam is in the maximum cantilever state in each stage,and the maximum stress of the beam body is at the bottom of the beam above the most front temporary pier.Besides the deflection andmaximum stress of the beam increase with the increasing of the cantilever length of the beam.When the beam is in the maximum cantilever state(condition 2),the reaction force of temporary pier support reaction force and the deflection of the beam reach the maximum value,which is the most unfavorable condition in the jacking process.(3)Determine the maximum deflection of beam control.When forced displacement is applied to the beam,the maximum stress of the beam and the reaction force of the temporary pier on the offset side increase linearly with the increasing of the beam offset.When the transverse displacement of the front end of the guide beam reaches 30 mm,the maximum stress of the beam and the axial stress of the front temporary pier column exceed the allowable stress.Considering the special properties of Zhengbei bridge crossing the railway marshalling station,the deviation of the front end of the guide beam should be controlled within 20 mm during the construction.(4)Through the simulation analysis,the maximum span of 92 m can be reduced from 453.76MPa(the beam has been damaged)to 170.23 MPa,and the deflection of guide beam can be reduced from 2176.12mm(the structure has been damaged)to610.47 mm,which can greatly improve the stress of the beam and reduce the deflection of the guide beam.When any cable is damaged,it has little influence on the stress of the beam,and when the front cable is damaged,it has great influence on the deflection of the front end of the guide beam,which affects the normal jacking of the project.(5)Under the most unfavorable conditions,the local maximum wind speed is selected,and the fluctuating wind speed is simulated in MATLAB software.Using ANSYS software to analyze the local maximum wind load,the transverse deviation of the beam increases with the increasing of the cantilever length of the beam,and the maximum deviation is 36.71mm(exceeding the warning value).The maximum stress of the beam increases from 170.36 MPa under no wind condition to 334.50 MPa(exceeding the allowable stress).During the jacking construction of Zhengbei bridge,gale weather shall be avoided;the wind load shall be controlled by increasing the rigidity of temporary piers or adding adjustable limit devices.(6)The atmospheric temperature,solar intensity and wind speed at each time of the construction day under the most unfavorable working condition are selected to convert sunlight and atmospheric temperature into comprehensive temperature,which is applied to the beam surface by means of convection load.The analysis shows that the vertical temperature difference of the beam starts to increase at 6:00 a.m.andreaches the maximum at 13:00 p.m.,which is 13.1 ℃,then gradually decreases,and the temperature difference reaches a stable state at 19:00 p.m.Under the maximum vertical temperature difference,the deflection of the cantilever section of the beam increases linearly with the increasing of the cantilever length,and the maximum deformation of the front end of the guide beam is 113.7mm,which affects the normal pier of the guide beam,resulting in the maximum stress of 23.79 MPa and the temporary reaction force of 2503.8kN.Therefore,during the construction of long-span jacking,the time when the temperature changes more evenly should be selected to avoid the high temperature weather or the measures such as calculating,checking and adjusting the jacking force of the jack and the elevation of the beam in advance should be taken to control the temperature.(7)The traditional monitoring of total station and GNSS intelligent wireless automatic real-time monitoring are used to monitor the alignment and displacement of each structure during the jacking process of Zhengbei bridge.The elevation and deviation in the process of beam stepping shall be forewarned in time to ensure that the beam alignment is within the control requirements.The monitoring results are consistent with the finite element calculation results as a whole,the beam offset is controlled within the allowable range,and the overall alignment meets the requirements of construction monitoring,which verifies the accuracy and applicability of the calculation and analysis.Through the extraction of the step displacement,offset and time of each cycle,it is found that the step displacement and offset of each cycle obey the normal random distribution,and the jacking efficiency is the highest when the step speed is controlled at 1.5m/h. |
