Study On The Load Adjustment Of The Cable In The Concrete Pouring Process Of The Steel Tube Concrete Stiff Skeleton Arch Bridge

The steel tube concrete stiff skeleton arch bridge has become a competitive bridge type in mountainous area due to its advantages such as large stiffness,strong bearing capacity and coordination with topography.With the rapid development of highways and high-speed railways in mountainous areas in China,new requirements were put forward for the span of the steel tube concrete stiff skeleton arch bridge.Due to the limited bearing capacity of the stiff skeleton and the large amount of outsourcing concrete,the process stress exceeded the allowable value and the distribution of the permanent stress was uneven in the pouring process of outsourcing concrete.In this paper,taking the Nanpan River Bridge on the Yungui Railway and the Tian’e Longtan Bridge as examples,research was carried out on the theory of cable load adjustment during pouring outsourcing concrete,and the analysis and calculation of structural mechanical performance considering the effects of the entire construction process were carried out.The specific contents were as follows:（1）On the basis of analyzing the existing theory of cable load adjustment,in view of its limitations,the basis for determining the timing and position of the cable tensioning was proposed.Based on the response surface method（RSM）,the approximate function of the control stress and the cable force was fitted,and the engineering optimization problem was converted into a mathematical programming problem.Then,the cable force was solved by the NSGA-Ⅱ genetic algorithm.Therefore,the procedure to determine the cable load adjustment scheme was put forward.（2）In this paper,the finite element model of the arch rings of the two bridges was established based on the Nanpan River Bridge on the Yungui Railway and the Tian’e Longtan Bridge.Based on the measured data,the finite element model of the arch ring of the Nanpan River Bridge（416m）was verified,and it was found that the calculated results of the finite element model were in good agreement with the measured results.According to the theory of cable load adjustment proposed in this paper,the load adjustment scheme of cable was determined.Through one tension of the cable,the process stress of the string pipe was within the allowable range of the standard（the tensile stress of concrete in the pipeσ₂ was reduced from 3.25MPa to-0.94MPa）.The maximum permanent stress of the steel pipe and the concrete in the pipe were-308.14MPa and-23.83MPa respectively.Compared with the maximum permanent stress（-325.03MPa,-26.43 MPa）of the steel pipe and concrete in the pipe during actual construction,the reduction rate was 5.20%and 9.84%,respectively.The stress has been significantly improved.Moreover,the deflection curve of vault was smooth,and the stability coefficient of the structure in the most unfavorable stable state was 10.07,but its value was still greater than 4,which meets the engineering requirements.Based on the results,the correctness of the theory of cable load adjustment proposed in this paper was verified.Combined with the proposed cable load adjustment theory,the cable load adjustment scheme of the Tiane Longtan Bridge（600m）was further determined.After two times of active load adjustment of the cable,the process stress of the control section exceeding the standard was reduced to the allowable value（the tensile stress of concrete in the pipeσ₂ was reduced from 4.12MPa to 1.84MPa,and the compressive stress of steel pipeσ₃ was reduced from-354.23MPa to-319.42MPa）.Compared with the Nanpan River Bridge,the number of cross-sections with excessive stress was more,and the phenomenon of considering one and losing another was more obvious due to the mutual influence of stress adjustment,but the safety of the outsourcing of concrete construction was still assured.At the same time,the permanent stress was improved.It showed that the theory of cable load adjustment proposed in this paper has a more general applicability.（3）In two examples,the corresponding differences between the calculated values of the finite element model and the predicted values of the response surface model for each control stress were small,which indicates that the cable force scheme based on the response surface models were reliable.Moreover,the difference between the calculated values and predicted values of the control stress with poor ANOVA results was also large,indicating that the fitting of high-precision response surface models were necessary.The concrete filled steel tube stiff arch skeleton was the main bearing structure during pouring the outsourcing concrete.The cable load adjustment theory proposed in this paper can effectively control the process stress that exceeds the standard and the permanent stress of the concrete-filled steel tube rigid arch skeleton,which provides a way to make full use of existing materials to further increase the span of the steel tube concrete stiff skeleton arch bridge.