Since the founding of New China, especially since the reform and opening up, due to the progress of science and technology and the improvement of industrial level, the construction of bridges in China has increased significantly, and the types of bridges have changed with each passing day. Cantilever construction technology has been widely used in bridge construction since it was invented in the late 1950s. The completion of long-span and difficult bridges has written a new chapter in the history of bridge construction.
Cantilever construction technology has been widely used in bridge construction. With the diversification of bridge design styles, this technology will surely exert greater potential. This paper discusses the cantilever construction technology of prestressed box girder in the upper box girder construction of Zhangpigou Bridge in the eighth contract section of Qilin Expressway.
I. Introduction of the Project
Zhangpigou Bridge is located in Yanjialing Village, Lingshi County, Shanxi Province, and spans Zhangpigou. It is a typical V-shaped valley with a valley bottom of about 45 meters wide and a large topographic relief. The upper part of the main bridge is a 458,045-meter continuous rigid frame box girder with variable cross section. The root beam of box girder is 4.2m high, the height-span ratio119.05, the height of mid-span beam 1.8m, the height-span ratio 1/44.44, the top plate of box girder is 25cm thick, and the bottom plate is a quadratic parabola.
Second, the cantilever casting construction technology
When the cantilever construction method is used to build a prestressed concrete beam bridge, it is not necessary to erect a bracket, but directly extend from the top of the existing pier to the span direction, and prestress each extension to connect it with the existing part as a whole. According to the manufacturing method of beam, cantilever construction method can be divided into cantilever assembly and cantilever pouring. This paper briefly introduces the cantilever pouring construction technology.
Cantilever pouring construction uses suspended mobile scaffolding (referred to as hanging basket) to pour beam concrete symmetrically and evenly on both sides of pier column (generally 3-8m). After each section is poured, after the concrete reaches the design strength, the longitudinal prestressed steel strand is tensioned, and then the hanging basket is moved forward for the next section.
The cantilever casting of the upper box girder of Zhangpigou Bridge is divided into four parts, namely the first part of 0 block construction; The second part is the cantilever casting part of 2~8 beam segments; The third part is the construction of side span cast-in-place section; The fourth part is the construction of closure section, including the construction of side span closure section and the construction of middle span closure section.
1.0 block construction
Block 0 is the tallest and heaviest of all beams, and the hanging basket is not used for construction at the top of pier column. Due to the large volume and many internal chamfers of No.0 Expressway, it will be poured in two times. For the first time, there will be longitudinal prestressed steel strands and horizontal and vertical finish rolled steel bars in the area where the web height exceeds1.5m.. In addition, all longitudinal prestressed steel strand pipes on the top plate of block 0 shall pass through. Therefore, the coordinates and quality of the pipeline should be carefully checked before pouring, so as to avoid too much error in block 0.
2. Cantilever casting part construction
Cantilever casting part * * * has 2-8 pieces, and each bridge has 28 pieces in total. Hanging basket, as the main construction equipment for cantilever casting, needs steel bar binding, hole installation, concrete pouring, steel strand tensioning and other operations. Therefore, in addition to being economical in design, there must be enough safety and flexibility to facilitate safe operation in the construction process. The main truss of the hanging basket of Zhangpigou Bridge is designed with domestic Bailey pieces, each piece is 3m long, 1.5m high and weighs 270kg ... The upper and lower beams are welded with 240 I-beams. The external mold adopts fixed template, and the internal mold adopts combined template. See hanging basket assembly drawing.
After the tensioning of the vertical finishing rolled steel bar of block 0 is completed, the hanging basket can be assembled. Before the hanging basket is officially used, it should be pre-pressed to test its actual bearing capacity and safety and reliability, and obtain elastic deformation data to eliminate inelastic deformation of the hanging basket, which provides a basis for construction control. 1 block is the first stage of cantilever casting, which accurately controls the elevation and plane position of the hanging basket and provides guarantee for the control of the whole bridge. The following factors should be considered in the elevation of hanging basket: the design elevation of bridge deck; Hanging basket deformation under different loads, concrete deformation, including temperature stress deformation and late creep deformation. Cantilever pouring part of the concrete should be completed at one time to avoid vertical cracks in the middle of the beam. 1 block after the completion of pouring, tensioning longitudinal prestressed steel strand. After tensioning is completed, the back anchor can be loosened, and the hanging basket moves forward to proceed to the next section of construction until cantilever casting is completed.
3. Cast-in-place section construction
The transition pier of Zhangpigou Bridge is 33 meters high. If it is difficult to construct the cast-in-place section of the support, we use the hanging basket as the hanger, and the front fulcrum of the main truss of the hanging basket is at the top of the transition pier. The construction of cast-in-place section shall be carried out simultaneously with the construction of Block 5, so as to ensure that the span after the construction of Block 8 can be closed in time.
4. Construction of closure section
The construction of the closure section is the process of system transformation, and the bridge can complete the system transformation through the construction of the closure section. Close the side span first and the middle span last. Side span closure adopts hanger construction. After the cast-in-place section is completed and the concrete strength reaches the design strength, the side span closure section can be constructed. The rigid skeleton of the mid-span closure section can only be closed after the concrete pouring of the side span is completed and the hanging basket of the side span is dismantled. Concrete pouring of the mid-span closure section shall be carried out at the lowest temperature in a day. It should be pointed out that the closure procedure should be strictly controlled, otherwise there will be great errors in the design closure, the internal force of the structure will also change under dead load, and the redistribution of internal force caused by creep during system transformation is also different. Therefore, different closure procedures will produce different final internal forces in the structure, which will have a great influence on the use of the whole structure after completion.
Third, the construction control
1. Deflection control
Deflection calculation in the construction process is not only related to the choice of mechanical calculation mode, but also related to many factors affecting deflection. These main factors include: the first stage dead load in the construction stage, that is, the self-weight and prestress of the beam; Temporary construction load: hanging basket and formwork equipment are heavy; Crowd load, natural temperature change, humidity change and wind load; Pier displacement, foundation settlement, construction error, etc. Among these factors, there are many fuzzy and random factors, so it is difficult to monitor the deflection. In order to control the deflection and deformation of the beam more accurately, each piece should be observed in at least six stages, that is, before and after concrete pouring, before and after tensioning, and before and after hanging basket moving. Conditionally, all poured beam sections can be observed every day, and the deflection changes can be mastered in detail to provide a basis for the next piece of construction. 2. Plane position control
Before box girder pouring, a survey monitoring control network should be established, and the layout of the control network should follow the principle that deformation observation can reflect the actual deformation of the structure. We consider installing 1 working base points in the center of block 0 at the top of each pier. The working base point should form a control network with nearby traverse points, and should be reviewed regularly to ensure that the error of the working base point is within an acceptable range.
Through careful organization of construction, strict construction procedures and increased monitoring, the closure error of the left picture of Zhangpigou Bridge is controlled within an acceptable range.
Fourth, the advantages and disadvantages of cantilever construction
Cantilever construction method has been widely used only for more than 40 years since its invention, precisely because it has many unique advantages.
1. Cantilever construction is more suitable for long-span bridges, especially across rivers and valleys, and is less affected by topography.
2. Cantilever casting can reduce the lifting and other processes, and can be formed at one time, which simplifies the bridge construction procedure.
3. The cantilever casting has a high degree of mechanization, which reduces the labor input.
4. It can be applied to various bridge types, such as beam bridge, rigid frame bridge, arch bridge and cable-stayed bridge.
5. Due to mechanization and repeated operation, the process can be improved, the engineering quality can be improved, and the connection and mid-span closure can be easily realized.
6. The construction method of full-house support is cancelled, which provides spacious clearance under the bridge.
Although the cantilever construction method has many advantages, it also has some disadvantages: cantilever pouring can not be carried out until all piers are completed, and the next process can not be carried out until the concrete reaches the design strength, which will prolong the construction time; Construction monitoring is more complicated and construction errors are inevitable. There is no concrete leveling layer on the bridge deck, and the contact of blocks is prone to unevenness.
The successful closure of Zhangpigou Bridge is another example of long-span bridge construction. With the progress and perfection of cantilever construction technology, the improvement of construction mechanization, and the computer-aided internal force analysis and calculation of bridge structure and construction control, cantilever construction method has become the main construction method of modern long-span bridge construction, which also promotes the further development of bridges in the direction of high strength, light weight and long span.
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