1 Introduction
At present, there are many construction methods to choose from during the construction of cable-stayed bridges, and the hanging basket construction technology does not need large-scale hoisting equipment, and is more suitable for situations with large contract traffic volume and complex surrounding environment. Hanging basket can make better use of the cable force of stay cables to reduce the load it bears. The design of hanging basket is directly related to the construction progress and safety of the whole bridge.
The main girder of the superstructure of the main bridge of Guangzhou Bridge adopts a single box, four rooms and three-way prestressed concrete box girder structure with a total length of 245m, and the design requires the cantilever pouring technology of cable-stayed hanging basket. The main girder * * * is divided into 20 hanging basket cantilever casting sections, the construction length of the standard section is 6m, and the beam weight of the standard section is about 364t. According to the overall construction technology arrangement, the main girder is constructed with cable-stayed hanging baskets, and 1 pair (2 sets) of hanging baskets are put into two working faces of the whole bridge.
2 hanging basket design
2. 1 Classification and characteristics of hanging baskets
At present, there are many kinds of hanging baskets, and there are many kinds according to different classification methods. Among them, there are only four kinds of load-bearing structures: truss type, cable-stayed type, steel plate beam type and cable-stayed type. The hanging basket used in the construction of Guangzhou Bridge is cable-stayed. Using stay cables as temporary front fulcrums in the hanging basket makes the cantilever beam in the ordinary hanging basket become a simply supported beam, which reduces the peak value of temporary internal force of the main girder during construction, greatly improves the hanging span and bearing capacity of the hanging basket and speeds up the construction progress. The disadvantage is that Zhang Lala cable should be installed in stages during the construction process, and the construction process is complicated and technically difficult.
2.2 Design of hanging basket
1. Design parameters: (1) The concrete weight of box girder segment is 364t；; (2) The crowd and construction load are1.5kn/m2; (3) Working wind pressure of the wind load hanging basket, wind speed: 13.6m/s non-working wind pressure of the hanging basket, wind speed: 28.6m/s; (4) According to the preliminary calculation of the monitoring unit, the maximum cable force of a single hanging basket is 275t.
2. Design features of hanging basket
This hanging basket is 6m long, 33.5m wide and weighs 364t. Its structural design has the following characteristics: (1) The system is a cable-stayed front fulcrum hanging basket, and the lower bearing platform, upper walking main truss and accessories form two stress systems of hanging basket pouring and walking, with clear division of labor, clear stress and simple structure, which well realizes the concept that bridge design needs front fulcrum construction; (2) The overall rigidity of the hanging basket is good, meeting the requirements of anti-tilting and wind stability. The bearing platform, the main component of the hanging basket, is a plane rigid frame structure with simple structure and clear force. In order to reduce the structural size, the bearing platform will adopt carbon structural steel, and the external dimensions of the components are small. On the premise of ensuring the rigidity and strength of the platform, reduce the quality of the platform as much as possible. The hanging basket structure and components are assembled and can be reused; (3) The formwork system is mainly composed of bottom formwork, side formwork and formwork support frame. All adopt large formwork structure to improve the appearance quality of concrete, effectively control the bridge type and shorten the construction period.
2.3 hanging basket structure
The bridge is designed as a cable-stayed bridge with a central cable plane. The main girder is a single-box and four-room prestressed concrete box girder structure. The standard beam section is 3.60m high, the beam top surface is 33.50m wide and the pouring length is 6.00m. The hanging basket is mainly composed of bearing platform, cable system, walking system, positioning system, anchoring system, formwork system, propulsion system, operating platform and embedded parts system. The self-weight of the front fulcrum hanging basket shall be controlled below 190t (including formwork and hanging basket safety protection structure, etc.). ), and the carrying capacity of the hanging basket is not less than 484t.
3 Spatial structure analysis of hanging basket
3. 1 stress of hanging basket
In the process of hanging basket construction, the most unfavorable working conditions can be selected, and the overall structure and local position of hanging basket can be analyzed by finite element software. There are two main situations: one is the no-load cantilever state, which only considers the internal force of the hanging basket under its own load and checks its stress state; Second, in the working process of hanging basket, the internal force of the whole structure of hanging basket is always influenced by the weight of hanging basket, construction load and concrete. The most ideal situation is to analyze the bean sprouts in each pouring stage, because the anchorage position and direction of the front fulcrum of the hanging basket will change every time, so the stress of the hanging basket will also change.
3.2 process flow
According to the calculation model, the hanging basket is divided into two units, namely the space truss bearing unit and the hook walking steel box unit. According to the design requirements of cable-stayed bridge, the stay cables can not be pretensioned at one time during construction, but the cable force is constantly adjusted during construction to control construction. Move the hanging basket forward to the design position of this section → survey and lofting, locate the hanging basket → hoist the hanging basket, adjust the template elevation, anchor the hanging basket → install stay cables, and perform the first cable tension according to the monitoring instructions. And recheck the template elevation → bind the bottom plate steel bar → bind the web plate steel bar → install the internal model → bind the top plate steel bar → pour 50% concrete → tension the stay cables for the second time according to the monitoring instruction → pour the remaining 50% concrete → cure the concrete → the concrete age is more than 7 days, and the strength is greater than 90% of the design strength → prestress and grout the main beam → tension the stay cables for the third time according to the monitoring instruction, and then carry out system transformation → unload the hanging basket anchor group.
3.3 Analysis of calculation results
1) Maximum internal force of longitudinal beam member
See table 1 for the maximum internal force of longitudinal beam members.
Table 1 Maximum internal force of longitudinal beam
2) Maximum internal force of front middle cross member
See Table 2 for the maximum internal force of the front center cross member.
Table 2 Maximum internal force of member in front middle cross member
In the above several checking computations, members should not only meet the requirements of strength, but also meet the requirements of stiffness, so as to ensure the stress stability of members. After calculation, all requirements are met before delivery for construction.
4 hanging basket construction technology research
4. 1 Manufacture, installation and preloading of hanging basket
This project is designed as cantilever pouring technology for the front fulcrum of hanging basket. The cantilever casting section is 6.00m long and 33.50m wide. Therefore, the design size of the hanging basket cap is 18.0 1m, and the length is 33.5m.
The preloading test of hanging basket is carried out after the hanging basket is installed, and after the stay cables are installed and tensioned according to the monitoring requirements, the actual suspension pouring construction process of the main girder standard section is simulated. The maximum load selected in the test is 1. 10 times of the load of the hanging basket pouring the heaviest section concrete, and its loading mode is realized by stacking sand bags. The elastic deformation and inelastic deformation of the hanging basket are obtained through the preloading test of the hanging basket which simulates the whole process of concrete pouring of the main girder. The inelastic deformation has been eliminated and the elastic deformation is adjusted by pre-lifting the elevation.
4.2 template engineering
The formwork system of beam hanging basket consists of bottom formwork, side formwork and internal formwork of main beam. The bottom formwork of main girder adopts steel formwork structure, which is welded into a whole with the bottom basket of hanging basket. The length of the bottom formwork along the bridge direction is 6.3m, with its front end supported on the front beam of the hanging basket and its rear end anchored on the poured beam section. The external mold adopts shaped steel formwork, and the internal mold adopts shaped steel formwork and wooden formwork, supported by steel pipes. After the construction of bottom plate, web plate, stiffening plate and cross beam reinforcement is completed, the pre-assembled internal formwork will be hoisted in place, and the side formwork of cross beam will be connected with the internal and external formwork of side box girder with opposite pull screws.
4.3 Construction of embedded parts
During suspension casting construction, attention should be paid to embedding embedded parts, including engineering embedded parts and construction embedded parts. Engineering embedded parts include traffic engineering embedded parts, landscape embedded parts, communication and power supply system embedded parts, drainage holes, ventilation holes, railings, etc. The embedded parts of construction measures mainly include embedded parts of hanging basket anchorage system, embedded parts of reserved holes of walking system and embedded parts of temporary anchorage system of tower beam.
4.4 concrete engineering
The design label of bottom web concrete in cantilever casting section of main girder is C50, and the panel is C50 steel drilled concrete with retarder, and the initial setting time is not less than 18 hours. The concrete quantity of a single standard section girder is 140m3. During the construction, the concrete of the main girder of the corresponding symmetrical section on the east and west sides of a single bridge tower is poured at one time, and the hanging baskets on the east and west sides of the bridge tower are hung symmetrically. Concrete production is provided by the mixing station, transported to the bridge deck by two symmetrical towed pumps, and delivered by pump pipes and hoses. When pouring concrete for a single hanging basket, the general principle is to proceed from the front end to the back end of the hanging basket, and follow the principles of symmetry, balance and synchronization to ensure the construction quality and safety. The allowable deviation of unbalanced weight on both sides of the hanging basket is less than 15t. In the process of concrete pouring, according to the monitoring instructions, when the concrete pouring is completed by 50%, the second cable tension is carried out, and the cable tension is mainly checked by controlling the elevation and cable force.
Through the analysis of box girder section, 50% of concrete pouring is the pouring of box girder bottom plate and web plate to the bottom of chamfer. During construction, the statistics of the mixing station shall prevail, and on-site review shall be conducted according to the pouring position. When the concrete pouring is 50% completed, the pouring construction will be stopped on site, and the stay cables will be tensioned for the second time. The second tensioning of stay cables is carried out in strict accordance with the monitoring instructions, with elevation control and cable force detection as the check.
After concrete pouring is completed, it shall be cured in time. Under normal circumstances, geotextile should be used to cover and water conservation to keep the concrete surface moist for more than 7 days.
The above are collected and sorted by Zhong Da Consulting Company.
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