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  • Key points of prestressed concrete continuous construction in curved rigid frame
  • Author / collation: YueShun chimney corrosion protection Co., Ltd. release time: April 30, 2020
  •  Key points of linear control in cantilever grouting construction of continuous rigid frame bridge with concrete curve under prestressed environment (Fig. 1)

    Key points of linear control in cantilever grouting construction of Concrete Curved Continuous Rigid Frame Bridge under prestressed environment

    In recent years, with the continuous development of social economy, China's scientific and technological level, engineering and technical level has been greatly improved, but also led to the development of highway and railway bridge construction, a variety of modern highway bridges, railway bridges, river crossing bridges continue to emerge. Limited by the linear requirements and terrain, many bridges have applied the curved steel structure form. The stress of curved bridge is more complex than that of straight line bridge, and its stress state will be greatly different in different construction stages. Therefore, it is necessary to further study the linear control method in the construction of curved continuous steel structure bridge.

    1. Characteristics of cantilever grouting construction of Curved Continuous Rigid Frame Bridge

    For the curved continuous steel structure bridge constructed by cantilever grouting method, the main features are as follows: first, in the construction process, the spatial position of the structure will change continuously, and the continuous extension of the cantilever will make the structural torsion deformation of the beam, the root torque will continue to increase, and the internal force will become more and more complex. In the design of this kind of bridge, it is necessary to consider the stress state of the early structure and the later structure at the same time; the second is that the temperature change, creep, concrete shrinkage and so on will have an impact on the structural deformation and internal force that can not be ignored, so it is difficult to accurately grasp the stress state of each part of the beam body in the construction process; the third is to correctly control the line shape of each section in the process of cantilever grouting construction The important premise of successful bridge construction.

    2. Project overview

    The main girder of Y highway curved bridge adopts single box and single chamber variable height and variable cross-section. The change of bridge curve is 1.7 times parabola, and the height changes from 6.0 m (pier Center) to 2.5 m (middle span). The piers are double thin-walled flexible piers. The cantilever length of flange plate is 2 m, and the width of box girder top is 12.2 M. the stress of box girder comes from three directions (vertical, horizontal and longitudinal). The plane of bridge body is located in straight line, circular curve and transition curve. The load design of the bridge is as follows: Trailer 120, vehicle over 20. Construction method: cantilever pouring construction is carried out for the upper structure of the bridge. The cast-in-place section is 9 m long and the zero block is 18 m long. The cantilever section has a total length of 65.0 m (11 × 4.0 + 7 × 3.0), the closure section is 2 m long, and the support construction beam section has No.1 and No.0. The longitudinal prestressed tendons are tensioned at both ends. The construction process of beam section is as follows: moving hanging basket erecting formwork binding steel bar pouring concrete tensioning prestressed steel strand moving hanging basket, cyclic operation. During the construction of closure section, the side span shall be closed first, and then the middle span shall be closed.

    3. Stress control and linear calculation of main girder

    3.1 establishment of simulation calculation model

    Under the coupling effect of bending and torsion, the beam section of curved bridge may appear torsion distortion and warpage. When the wall of prestressed box girder is thick and the ratio of beam width to span is small, the distortion and warpage effect can be ignored in the analysis process, so as to simplify the practical operation. According to the actual situation of the project, the following finite element models can be established

    (1) divide the construction stage

    In this project, the cantilever grouting method is used for construction, and temporary support is used in the construction process. In the calculation process, not only the permanent bearing but also the temporary vertical restraint to simulate the temporary support used in the construction should be designed. In Y bridge, there are 20 construction stages and 1 operation stage to correspond to the actual construction state. Firstly, the long span beam is tensioned to the No.18 long span side beam, and then the No.18 long span side beam is tensioned, and then the No, Close the side span; finally, pour the closure section of the middle span, and stretch the bottom plate bundle of the middle span with "long bundle first and short bundle later", then dismantle the hanging basket of the closure section of the mid span, construct the bridge deck system, and finally enter the operation stage.

    (2) construct calculation model

    In the simulation calculation of Y bridge, the structure is simplified firstly, and the main beam structure is divided into 237 beam elements and 237 nodes according to the construction stage. The beam and pier, pile foundation and pile cap are rigidly connected.

    3.2 construction control method

    Construction control mainly includes monitoring and monitoring. Monitoring refers to burying sensors and other components in the main beam and other parts to obtain relevant data; monitoring is based on monitoring, using software programs to analyze and process the data, so as to obtain the parameters of adjacent stages. In Y bridge project, the adaptive control method is adopted in the construction control, that is, by comparing and analyzing the elevation, internal force predicted value and measured value in the construction process, the cause of the error between them is found out, and then the parameters are adjusted and identified to determine the elevation of the next construction stage, so as to achieve the purpose of controlling displacement and internal force.

    3.3 linear control and determination of formwork elevation

    In the process of cantilever grouting construction of curved bridge, an elevation observation point should be set at the front end of each construction beam section to test the elevation change of main beam. According to the analysis results of construction control and monitoring data, the elevation of vertical formwork should be adjusted, so that the completed bridge alignment can meet the design requirements. An observation point (a steel bar with a length of 100 mm and a diameter of 15 mm) is set at the front end of each box girder to monitor the deflection. According to the design requirements, the observation points should be buried on the top of the bottom plate of the box girder and the cantilever end of the flange plate to ensure the stability and accuracy of the observation points.

    Fig. 1 layout of stress measuring points and deformation measuring points to monitor the vertical deformation of the main beam, the most commonly used method is to adjust the elevation of the vertical formwork. Due to the deviation between the actual elevation after completion and the elevation of formwork erection, it is necessary to set camber to offset the deformation in the construction, so the corresponding calculation is needed to obtain the deflection of each stage and determine the elevation. In the construction process, in order to ensure that the two cantilever ends to be closed in the same span are at the same horizontal line, the calculation can be carried out according to the following formula:

        Hn=h n-1+b +f n+h n

    Among them, HN: the elevation of formwork erection of beam section; HN: the design elevation of section; FN: the calculated deflection of section; B: the measured elastic deflection of hanging basket; the difference between the measured elevation of hn-1: n-1 section and (adjustment value + design elevation).

    4. Construction control effect

    The construction monitoring results show that the closure accuracy of the middle span of the main girder is 7 mm, and that of the side span is 4 mm and 6 mm, which are all less than the required closure accuracy (15 mm m). The alignment of the bridge after closure is consistent with the predicted alignment, and the monitoring effect is good. In the aspect of internal force, the error between the calculation result of monitoring program and the test result is less than 25%. The measured section stress is 10% ~ 20% larger than that of calculation except for a few measuring points. The whole section of all test sections is under compression. The internal force monitoring results show that with the construction progress until the whole bridge is closed smoothly, the internal force of the monitored section changes more evenly. During the whole construction stage, there are no abnormal and sudden changes in the internal force of the test section.


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