Study On Key Technology Of System Transformation From Cable-stayed To Suspension For Self-anchored Suspension Bridge

The rapid development of engineering technology and the continuous progress of material science had led to the rapid transformation of bridge construction technology.The self-anchored suspension bridge has been applied more and more widely with its remarkable spanning capacity,good site adaptability and superior landscape effect.The self-anchored suspension bridge was especially favored by engineers.And a lot of the self-anchored suspension had been built at home and abroad,most of the main span were within 400 m and built by the scaffolding method.However,there was no successful case for those whose mainspan was over 600 m with cable-stayed method.Long-span self-anchored suspension bridge was a multi-statically indeterminate structure with complex mechanical properties and obvious geometric nonlinearity during the construction process.In this work,taking the Chongqing rail bridge whose mainspan was 600 m as project,using the finite element model established by Midas,the process of system transformation from cable-stayed to suspension were studied.The main work and results were as follows:(1)Research on modeling technology of system transformation from cable-stayed to suspensionThrough the function of "activation" "passivation" and multi-conditions of establishing,system transformation construction process was simulated.Based on the characteristics of Midas/civil cable element,empty cable was coupled with the temporary cable-stayed bridge to establish the initial tension model.By using the coupling of temperature effect and mechanical effect,a temperature bar element was used to simulate saddle pushing process.According to the characteristics of the equivalent and interchangeability of the internal force and the unstress length of the suspender during the tension process.The cable tensioning process was simulated by establishing different cable units and using "activation" and "passivation" functions.(2)Research on the target state of temporary cable-stayed bridge and the dismantling scheme of stayed cable.The cable-stayed bridge was a temporary structure,and its target linear did not affect the final result.In theory,the higher the linear shape of the main girder,the smaller the vertical height difference of the empty cable,the easier the suspender tensioning.If themainbeam was assembled according to raise linear,the unstressed state value of the components would differ with finished bridge design value,and the stress of cable and mainbeam was unreasonable in construction process,and finished bridge state was difficult to meet the design requirements.It was the opposite to establish with the line shape of the suspension bridge.Therefore,the temporary cable-stayed bridge was built according to the suspension bridge with the prearch line,and then the cable tension of mid-span was carried out after the mainbeam was rasied to or near line shape of the second phase.This method coluld be widely used in the construction of similar bridges because of saving the consumption of the connecting rod,shortening the duration of the project,and achieving remarkable economic benefits.Through the simulation of two primary election disassembly schemes,comparing the factors of safety,economy,operation and management,it was determined that the stay cables should be removed from the top to the bottom when the suspender tension was completed.The recommended scheme construction process of structural stress was clear and the control parameters were few,which could guarantee the construction safety without affecting the construction duration.(3)Research on the tensioning scheme of system transformation from cable-stayed to suspensionThe possible tensioning scheme was qualitatively analyzed to determine the idea of "starting from the main tower to the midspan,at the same time symmetrically tensioned and progressively advancing".Using Midas/civil,the proposed tensioning scheme was simulated.Through comparison,the better scheme was determined.The recommended scheme was studied in depth that the change of the main cable and girder displacement,the force of suspender,the length of connecting rod,the change of cable force,the stress characteristics of bridge tower,the deformation and stress of main components in whole construction process,etc.(4)Local stress analysis of steel box girder anchor points.The cable anchor point was a temporary force transmission component that transferred the cable tension to the main beam.The anchor point on the lower end of the cable was in the same cross section as the anchor point at the lower end of the suspension cable.In the steel box beam,the anchor box structure was specially designed for thesuspension bridge.However,the anchorage space was not reserved for the temporary cable.The stress state and stability of the anchorage point must be taken into consideration when the construction plan was formulated.For this reason,a local finite-element model was set up to analyze the local stress of the cable anchoring point structure.The representative stress points in the tensioning process of steel box girder were selected by qualitative analysis from macroscopic stress characteristics.The 3d solid model was established by Midas/FEA to analyze and calculate.The results showed that the local maximum stress at the bottom of the ear plate was 260.02 MPa under the condition of adverse load,while the stress level of the rest parts was low,which could meet strength requirement of structure.