Multi-span Prestressed Concrete Dwarf Tower Cable-stayed Bridge Construction Control

With the ceaseless development of bridge industry, more and more bridge builders are fond of long-span extradosed cable-stayed bridge because of its attractive novelty and beauty. This kind of bridge is usually a high order hyperstatic structure and takes long construction period. The influencing factors in bridge construction are commonly a lot. Although the structure is constructed strictly according to designed drawing sheets, it is not sure for the inner force and line shape of the structure to reach their expected results when the bridge is constructed completely. Because adopted designed parameters such as the Young's modulus of material, the component weight, the modulus of both shrinkage and creep of concrete, the temperature fluctuation and the temporary load condition are not absolutely consistent with their real values in bridge construction. Therefore, we must get the whole construction process of extradosed cable-stayed bridge under control to make sure the bridge is secure in construction. In addition, we can also insure the inner force and line shape of the structure accord with designed standards when the bridge is constructed completely.The main bridge of the 2nd Yellow River bridge in Kaifeng is a long-span prestressed concrete bridge, which is also a kind of multi-span extradosed cable-stayed bridge. Piecewise cantilever construction method is adopted to pour the main body of the bridge. Seven uniform T rigid frames are constructed simultaneously when the main girder is established. The bridge is closed symmetrically from both north side and south side when closure sections are constructed from abutment span to mid-span. This dissertation takes the main bridge of the 2nd Yellow River bridge in Kaifeng as the project example to analyse and research the construction control of multi-span prestressed concrete extradosed cable-stayed bridge. A finite element calculation model is established for simulating the whole construction process of the bridge by using software MIDAS/Civil. The author uses retrogressive analysis method to artificially simulate and calculate the construction stage. The method of least squares is adopted to correct the errors of designed parameters in bridge construction. The main contents and productions of this dissertation are:1. Considering the actual mechanics characteristic of the 2nd Yellow River bridge in Kaifeng, we must give first place to the line shape control of the main girder and give second place to the section stress control and the cable tension control in bridge construction control. The retrogressive analysis method is used to calculate the construction stage of the bridge. The progressive analysis and retrogressive analysis are combined to deal with the problem of both shrinkage and creep of concrete.2. The contents and targets of the bridge construction control should be calculated before construction, which include the line shape control of main girder, the section stress control of both main girder and bridge tower, the cable tension control, the displacement control of bridge tower and the construction stability control of the bridge. In the line shape control of the main girder, the author calculates its displacements under first period dead load, second period dead load and live load respectively, and combines them according to certain modulus principle to educe the construction camber of the bridge. Consequently we can get the design elevation of the main girder when the bridge is constructed completely by adding construction camber to deck vertical curved elevation. When the displacement of the main girder corresponding to each construction stage is added to the design elevation in retrogressive order, we can elicit the structural ideal deflection curves of the bridge in construction. As a result, the ultimate line shape of the main girder will be conformed to the design line shape when the structure is constructed strictly according to these deflection curves.3. In the section stress control of both main girder and bridge tower, the author draws their timing graphs of root section stress by retrogressive analysis. The computed results indicate that the total cross-sections bear compression stress in the whole process of bridge construction. The root section stresses are maximal in the two components, but they are smaller than standard values all the same. The bridge still has biggish stress safety storage in construction. When the structure system is transformed and closure sections are unloaded the root section stresses change less, but they decrease obviously when cantilever beam fragments are unburdened gradually.4. In the cable tension control of both edge and middle tower, the author draws their tension timing graphs of the furthest inside and outside cables by retrogressive analysis. The cable tensions have almost no change when closure sections are unloaded and during the period of system transformation, while they diminish appreciably when cantilever beam fragments are unburdened gradually.5. In the displacement control of the bridge tower, the author draws the displacement timing graphs of both edge and middle tower top by retrogressive analysis. When closure sections are unloaded the displacement of edge tower top is large, but the displacement of middle tower top is tiny. When cantilever beam fragments are unburdened both of them are tiny.6. The construction stability analysis of the bridge indicates that the 1st step destabilization characteristic corresponding to each construction stage is out-plane antisymmetry destabilization of the bridge tower. The smallest stability safety coefficient is greatly bigger than the prescriptive value in current criterion, which demonstrates that the whole stability of the structure is well. It is not easy for the bridge to be destroyed because of the whole destabilization of the structure in construction, but it is necessary for us to pay attention to the destabilization of local components of the bridge.7. During the period of designed parameters' sensitivity analysis, the author works out their sensitivities firstly through increasing them by 10 percent, and then calculates their sensitivities corresponding to construction control objectives. The primary and secondary designed parameters are determined synthetically on the basis of the above two calculation results. The primary designed parameters include the bulk density of reinforced concrete, the sectional dimension of main girder, the initial tension of stayed cables, the prestress of main girder and the construction loads. On the contrary, the Young's modulus of reinforced concrete and the Young's modulus of stayed cables are the secondary designed parameters.8. The method of least squares is adopted to amend the errors of primary designed parameters according to the deviation of construction control targets. New construction control targets are calculated again by retrogressive analysis when correctional parameters are inputted into the calculation model. The targets include the structural ideal deflection curve of the main girder corresponding to each construction stage, the root section stress timing graphs of both main girder and bridge tower, the tension timing graphs of both furthest inside and outside cables of both edge and middle tower, the displacement timing graphs of both edge and middle tower top.9. Some softwares related to bridge construction control is exploited by using mathematics software MATLAB. These softwares are based upon the principle of self-adapt construction control, which include the designed parameters identification software and the construction errors adjustment software. It may be convenient for the bridge construction control to use them.The above research results can provide some reference for the design, construction and construction control of homogeneous bridge. The dissertation has prodigious engineering practical value.