Research On The Application Of Variable Cross-section I-steel Launching Nose For Incremental Launching Construction Of Curved Bridges

With the development of incremental launching technology on the bridge construction,launching noses have become an indispensable equipment in the construction process.To reduce the internal forces of main girders during an incremental launching process,increase the launching span,enhance spanning ability of main girder,usually a temporary steel structure in the front of the main girder-launching nose is adopted.At present,the launching nose structure in practical project is mainly divided into the following three types,truss launching nose consisting of the bars(truss girder),the box girder composes of the wing plate and web plate(steel box girder),variable cross-section I-steel guide girder(steel plate girder)and the last one has great advantage in bearing capacity and convenience in construction,hence has been widely applied in many large and super-large bridge construction.Taking a practical project of incremental launching construction for a curved bridge as an example,this paper has investigated the variable cross-section I-steel launching nose on the following aspects:(1)A reasonable structure model was built by using the finite element software Midas/Civil,and the incremental launching construction process was simulated with “regression algorithm”.For the modeling it was assumed that the body position of girder remains unchanged,each block's position backwards in turn with the launching process,in order to simulate progressive process of the girder.(2)Through checking the dangerous section of launching nose during the launching process,the change rule of stress,stress check ratio and front deflection in variable cross-section I-steel launching nose has been found.It has been concluded that its strength and stiffness meet the construction requirements;and for the key incremental launching condition the deformation and displacement were analyzed respectively in addition to internal forces,the stresses and stress check ratios,concluding that the launching nose's deformation and mechanical characteristics in the same incremental launching condition.(3)Buckling analysis was carried out for the launching nose,it has revealed that under various loads,the launching nose has a large safety reserve,its stability meets the needs of the construction;gravity load is the controlling factor in the launching process,the safety margin of the structure can be measured by the buckling characteristic value under gravity(variable).Through the analysis of displacements,stresses and the overall stability of variable cross-section I-steel launching nose,it has been founded that the composition of launching nose is reasonable and is of certain practical guiding significance.At present,research on the application of variable cross-section I-steel launching nose in incremental launching construction of curved bridges is rare,this paper analyzes the load carrying capacity and stability,and provides useful scientific basis for the design and construction of variable cross-section I-steel launching nose.