1 construction method of long-span continuous beam bridge
Long-span continuous beam bridge is the most widely used bridge type in China at present, and there are three main construction methods for long-span continuous beam bridge:
1. 1 cantilever construction method
The basic principle of cantilever construction method is to gradually increase segments from the top of pier to both sides to form concrete beams. The weight of the next segment and construction tools shall be borne by the completed adjacent segment. When this section reaches the design strength, apply proper prestress to connect it with the previous section into a whole, and then continue the construction of the next section. Compared with previous construction methods, cantilever construction has the following advantages: it can save a lot of steel, supports and templates during construction, and can better ensure the quality of concrete. Secondly, cantilever construction does not need to use hanging baskets for concrete pouring and maintenance, and only needs to simply move the brackets. Segmental prefabrication can be carried out at the same time as the bridge substructure, which can not only greatly speed up the construction progress, but also appropriately reduce the negative impact caused by early creep of concrete and give full play to the performance of stressed members. In addition, the installation of segments can give full play to the advantages of mechanized equipment, and construction can be carried out when the traffic flow or traffic flow is small, so as to minimize the impact on traffic.
1.2 incremental launching method Construction
The construction of incremental launching method is to open a prefabricated site behind the abutment along the longitudinal axis of the bridge, prefabricate concrete beams in sections, connect them into a whole with prestressed tendons, and then prestress them with jacks. With the help of sliding devices of stainless steel plates and PTFE templates, the beams are lifted up section by section, and then the frames are lowered after each section is in place, and the formal supports are installed at the bottom of the bridge to complete the bridge construction. The construction in incremental launching method has the following characteristics: the cost of temporary equipment is low, the small formwork can be recycled, the labor intensity of the project is low, the traffic under the bridge is not affected, and the construction operation is safe. If the shed cover is directly installed on the bridge construction platform, it can be closed and not affected by the external environment, which is conducive to ensuring the quality of bridge construction. However, each section will be subjected to repeated bending moments in the process of jacking, so the corresponding reinforcement amount will increase. Pushing construction has been widely used in the construction of long-span prestressed concrete continuous beam bridge.
1.3 hole-by-hole construction method
Hole-by-hole construction method is a common construction method for medium-span prestressed concrete beam bridges, and sometimes it is also used for the construction of long-span continuous beam bridges. The basic principle is that the continuous beam is divided into several beam segments, and proper prestress is applied to the beam segments during prefabrication to make them bear their own gravity. Then, special equipment is used for hole-by-hole construction from one end of the bridge, mechanized support and formwork are supported on the load-bearing beam, and cast-in-place concrete construction is carried out in the bridge span. After the concrete reaches the strength, demoulding is carried out, and the whole hole formwork is moved to the next pouring bridge hole along the guide beam, and then it is pushed hole by hole until the whole bridge construction is completed. The main feature of hole-by-hole construction is that it can be operated continuously during bridge construction. The longer the bridge body is, the more times the construction equipment is turned over, and the higher the economic benefit of the hole-by-hole construction method is. For multi-span bridges with spans below 75m, hole-by-hole construction is the most economical construction method. The hole-by-hole construction method does not need to set up ground support, and does not affect the navigation and traffic of the river under the bridge. The construction is safe and reliable, and the construction environment is good, which can ensure the construction quality. The degree of mechanization and automation is high, and the upper and lower structures can work in parallel, which can reduce the labor intensity and shorten the construction period.
2. Key construction technologies of long-span continuous beam bridge
2. 1 cantilever construction
When constructing a long-span continuous beam bridge by cantilever method, precise elevation control network and plane control network should be established by using linear control software technology before prefabrication. When prefabricating segments by butt joint method, the deviation of the previous segment in production should be corrected when prefabricating the next segment. It should be noted that it is not advisable to rectify the deviation too much at one time. Pay attention not only to the centerline and elevation of the segment, but also to the inclination of the segment, so as to avoid repeated rectification, make the laying work go smoothly, and ensure the accuracy of the centerline and elevation when closing.
During the suspension splicing stage, a leveling point shall be set at each fixed position at both ends of the bridge, and a leveling base point shall be set at the center line after each section is completed. Measuring point marks should be made on each beam and kept in the same line with the leveling points at both ends. Before construction, the reasonable curve of each beam segment should be calculated and drawn, and the deflection deviation value and deflection direction of each beam segment node should be dynamically monitored in real time. Once an abnormality is found, corresponding measures must be taken in time to ensure that the actual construction curve is as consistent as possible with the reasonable construction curve.
During the bridge closure stage, before locking the steel support in the closure section, the height and horizontal position of the two cantilever ends, the relative height difference of the two cantilever ends and the lateral deviation of the cantilever end beam axis should be measured, and compared with the theoretical calculation value and the specification requirements in time. If there is a big deviation, it should be adjusted to meet the requirements before locking, so as to ensure that the center line and elevation are the same as the design value when closing.
2.2 incremental launching method Construction
During jacking construction, the center line of the beam segment must be controlled within the allowable range, the deviation between the center line of the guide beam and the beam body should not be greater than 2mm, the deviation of the top height of the sliding device on the same side of two adjacent spans should not be greater than 65438±0mm, and the deviation of the top height of the sliding device at two points of the same pier should not be greater than 0.5mm In the process of propulsion, the beam body must move forward on the pad plug of the PTFE board, and both sides of the PTFE skateboard must be kept clean, and the white surface should be coated with lubricating silicone grease to reduce friction. During construction, the construction personnel should insert the gasket correctly, keeping the white side facing the stainless steel slide plate and the black side facing the box girder web. If the tetrafluoride plate fails to follow up in time during jacking, the jacking should be stopped immediately, and the box girder web should be jacked into the tetrafluoride plate before jacking. When advancing each section, advance 5cm first, then stop the oil return, then advance 5cm, then stop the oil return, repeat it for two or three times, and check whether the facilities in each part are qualified before the formal advancement. During jack-up, all jacks should keep in touch and apply force step by step. If the screw is loose, the guide beam bar is deformed, the concrete is cracked, and the joint between the guide beam and the box girder is deformed, the jacking should be stopped immediately.
In order to control the center line of the beam to meet the requirements of the code during jacking, a transverse guide device must be set. Especially when advancing on a circular curve, the lateral guide device is more important. A pair of deflectors are installed on both sides of the temporary pier in front of the precast pedestal to monitor the lateral position of the end of each beam and ensure the correct butt joint between the end of the beam and the precast formwork. In addition, two pairs of rectifying devices should be set in the advancing direction of the beam and advance alternately with the advancing direction of the beam segment. When pushing, we should also observe the lateral deviation, mainly observing the elastic lateral displacement of the main girder and the permanent pier.
2.3 hole-by-hole construction method
When temporary supports are used to assemble prefabricated segments hole by hole, if there is no large hoisting equipment, the beam can be divided into several segments in the prefabrication yard for segment production, and the self-weight generated by assembling segments during erection is temporarily borne by the supports, and the supporting beams are provided with tensioning prestressed tendons, and the installed beams are connected with the finished bridge structure according to the design requirements.
Bridge deck load is random, so construction management needs to be strengthened during construction. In addition to the necessary construction equipment, other equipment should be cleaned in time, and attention should be paid to maintaining the balance of bridge deck load. For the load that cannot be cleaned, the weight of the load and its action position should be carefully estimated, and it should be considered when calculating the deflection. Before construction, the construction load should be calculated and kept stable. Attention should be paid to collecting, sorting and summarizing relevant measured data and comparing them with calculated values. At the same time, we should pay close attention to the load changes during construction. The monitoring work should be based on the measured values of the elevation, weight, pretension, elastic modulus and concrete strength of the poured beam section, taking into account factors such as temperature influence, bearing deformation, pier settlement, etc., and using the analysis of the construction control program, the elevation value of the formwork for the next beam section should be put forward.
In the process of construction, it is necessary to improve the construction technology to reduce the error. Deflection control of bridge hole-by-hole construction is a cyclic process of construction → measurement → judgment → correction → construction. It is necessary to strengthen the monitoring of various construction conditions, keep abreast of the changes in bridge elevation, and adjust the construction in time when problems arise.
3 Conclusion
In recent years, due to the country's vigorous development of transportation infrastructure, bridge construction projects have developed rapidly, and the structural theory, stress analysis, design steps and construction technology of long-span continuous beam bridges have matured. The key to construction is to control the key technologies in construction.
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