Construction control; Mistakes; Influencing factors; Structural parameters; Pier stiffness
1. Influence of prestress loss on deflection, deformation and internal force
After prestress stretching, considerable losses will occur, which are mainly caused by anchorage deformation, friction between prestressed reinforcement and pipeline wall, prestress retraction, stress loss caused by joint compression, stress loss caused by elastic compression, stress caused by prestress relaxation, concrete shrinkage and creep, etc. From the calculation and analysis, it can be seen that the prestress loss will cause great changes in the alignment and internal force of the main span, which is unfavorable to the completed bridge state. In construction, the effective prestress should be determined by pipeline friction test to ensure the effectiveness of prestress from the aspects of technology, personnel, machinery and management.
The second is the influence of self-weight on deflection and internal force.
In the process of cantilever box girder construction, the self-weight of main girder and temporary construction load will produce great deflection and bending moment for cantilever structure. The longer the cantilever length of the main girder, the greater the bending moment of the cantilever root. Therefore, it is very important to accurately estimate the self-weight of the main girder and the temporary load during construction, and strictly control it during construction. Through calculation and analysis, it can be seen that changing the beam weight has great influence on the internal force and deflection of the main beam, so the sensitivity analysis of the beam weight is more important. An increase of 5% in self-weight will cause great linear and internal force changes, which is inconsistent with the design.
Third, the bridge structure stiffness error analysis
Cantilever hanging basket is usually used in the construction of long-span beam bridges. According to the 5% increase of E value in the code, the influence on structural deflection and stress is analyzed and compared. From the calculation and analysis, it can be seen that the increase of elastic modulus by 5% will cause a large linear change, which can not be ignored, and its change is basically consistent with the influence of prestress and self-weight. The analysis shows that although the elastic modulus has little influence on the internal force, its influence on the alignment can not be ignored. In the process of construction, this parameter should be measured and the model should be constantly revised to simulate the construction state more truly.
Fourth, the influence of shrinkage and creep calculation model
There are many calculation methods of shrinkage and creep, and the calculation mode recommended by CEB-FIP Association has been adopted in two consecutive bridge codes in China. By comparing CEB-FIP 1978 suggested in the original bridge code (JTJ023-85) with CEB-FIP 1990 suggested in the new code (JTGD62-2O04), it is found that CEB-FIP 1978 does not directly reflect the influence of cement varieties on concrete.
Influence of verb (abbreviation of verb) on pier stiffness
Generally speaking, the superstructure of a continuous rigid frame bridge or continuous beam bridge constructed by cantilever method is symmetrical around the pier. Therefore, in theory, it is considered that in the cantilever construction process, the pier does not bear the bending moment, and the stiffness of the pier only affects the deformation and stress state in the construction closing stage and the later stage of the bridge. The accuracy of the longitudinal thrust stiffness simulation of the pier not only affects the deformation in the closing stage, but also affects the grasp of the mechanical behavior of the cantilever construction stage.
Influence of temperature load of intransitive verbs
The bridge structure is in a certain external environment, and the change of external environment will often cause the change of internal force and alignment of the structure. The closing time of the main span box girder should ensure that the atmospheric temperature of the bridge site and the closing temperature of the main span can only be basically the same at night. Measures to control the influence of temperature during construction: (1) When the cantilever construction reaches a certain length, the time for measuring the vertical contact elevation should be arranged before sunrise in the morning to avoid the influence of temperature as much as possible; (2) If you want to get a more realistic elevation value in the stages of formwork erection, concrete pouring and prestressed tensioning, you should arrange the relevant measurement before sunrise in the morning to minimize the temperature influence of the measurement; (3) If the influence of temperature is inevitable, the influence of temperature can be corrected by measuring the temperature field; (4) At the stage that is not particularly important or the temperature influence is small, the temperature influence during the construction period can be estimated by measuring the day-night deflection curve.
Through the sensitivity analysis of several parameters, it can be known that the prestress loss, the weight of the main girder, the shrinkage and creep of concrete and the temperature have great influence on the deflection and stress of the main girder of the bridge, while the structural stiffness has relatively little influence on the internal force, so the influence on the alignment cannot be ignored. In the process of construction, the parameters that have great influence should be strictly controlled, and the theoretical calculation should be adjusted according to the measured parameters.
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