| The steel truss bridge combines the advantages of steel material and truss system,with features such as light weight,high stiffness,easy transportation and assembly,and high spanning capacity.The main methods of steel truss girder installation are gantry crane method,cantilever method,incremental launching method,floating and dragging method,etc.The use of floating and dragging method can effectively reduce the constraints of terrain and hydrological conditions.At present,the construction of large span steel girder by floating and dragging method has problems such as difficulty in controlling the alignment of steel girder assembly,complicated floating dragging process,relatively immature technology and poor research.Therefore,it is necessary to research on the assembling scheme and the influencing parameters of the floating and dragging process.In this paper,the main steel truss girder of the Yan He River Bridge,which spans 95 m and weighs 1450 t,The construction process of the floating and dragging method and its component systems were studied,the mechanical properties of the temporary construction structure are calculated and the effect of the system changeover on the steel truss truss girders was analysed.The finite element software Midas/civil was used to co MPare the overall and partial installation of the steel truss girder,while the dynamic analysis of the steel girder and the pontoon support elements was carried out during the floating stage.The reasons for the lateral deflection of the steel truss girder during towing was analysed,a mathematical model for deflection correction was theoretically derived and verified with the measured data.The main research contents and conclusions are as follows:(1)The construction process and composition system of the dragging method were studied,and the dragging process and application of key technologies for the main steel truss girder of the Yan He River Bridge were elaborated.The dragging process was divided into three stages: single-side bracket dragging on the north bank,coordinated floating dragging on land and water and bracket dragging on the north and south sides;Additionally,the mechanical properties of the temporary construction structure were calculated,the results showed that the ultimate bearing capacity and settlement value of the steel pipe pile,the calculated strength of the tension cable and ground anchor,and the strength of the anchor seat and pontoon bracket all met the requirements;The i MPact of the change of the simulated support system on the steel truss girder was analysed,the displacement and stress of the rod were greatly affected.(2)The finite element software Midas/civil was used to co MPare the steel truss overall overhanging and segmental installation.Two options were proposed for the steel truss overall overhanging from one end to the other(Option I)and symmetrical construction(Option II),considering two scenarios of applying and not applying adjustment force during the construction stage.Option I is recommended to be used for on-site assembly.The results show that the error is smaller when choosing the section-by-section lifting and the whole prefabricated solution.The stress monitoring of ten webs on the right side of the steel truss beam during the suspension process shows that the measured values basically match with the software calculated values.In addition,the stress changes of the webs were temperature dependent,the greater the temperature change,the greater the stress changes of the bars.(3)The causes of lateral deflection of the steel truss beam during dragging were investigated,including the difference in travel distance of the same group of jacks,the unbalanced elevation of the slide beam and the effect of water flow.Two mathematical models of hydraulic machine displacement difference correction and pontoon forward direction control were theoretically deduced.The actual measured data of hydraulic machine deflection and the calculated data of deflection model are co MPared,the maximum error of adjusting displacement difference is 17.5% and the maximum error of adjusting tension is 22.1%.So,the mathematical model can effectively guide the construction of deflection correction.(4)The front-end transverse offset,hydraulic press traction force and vertical elevation data were measured during the steel truss girder dragging process.The data of transverse offset shows that the front-end offset is mainly within-150 mm to 50 mm,mainly to the right;the linear regression of forward distance-time relationship shows that the sliding speed is 4.37m/h,3.37m/h and 2.84m/h in three stages respectively;the tension data shows that the tension value to maintain the steel truss forward in each stage fluctuates,and the magnitude of traction tension value matches with the theoretical value;the elevation data shows that the maximum difference is 74 mm during the floating stage.The correction threshold is discussed according to the construction conditions and lateral offset-distance relationship on site,it is suggested that the value should be controlled within 50 mm,and the maximum value should not exceed 200 mm.(5)Self-vibration characteristics and transient analysis under acceleration of the steel truss girder were carried out by using the finite element software Midas/civil.The three-way acceleration value was measured,and the longitudinal vibration acceleration value was obvious,the peak acceleration was about 1.0g.The transient analysis results show that the structural displacement near the front pontoon support is more obvious,and special attention should be paid to the stress state of the diagonal webs E4’A3’ and E2’A3’.During the towing process,the stress changes of steel truss webs,pontoon supports and some members of the temporary supports on the south shore were measured,the results showed that the stress value of webs was larger and more fluctuating than that in the stationary state,and the stress fluctuation in the floating towing stage was smaller than that in the unilateral support sliding stage;the stress of pontoon supports was also larger than that in the stationary state;the measured values of steel pipe piles of the south shore supports were basically consistent with the theoretical values. |
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