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 of long-span continuous beam bridge:
1.1 Cantilever construction method
The basic principle of cantilever construction method is to gradually increase sections from the pier top section to both sides to form a concrete beam, and the weight of the next section and construction tools will be borne by the completed adjacent section. After this section reaches the design strength, Compared with the 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, the cantilever construction does not need to use hanging baskets for concrete pouring and maintenance, and only needs to simply move the brackets. The prefabrication of segments can be carried out at the same time as the substructure of the bridge, which can not only greatly speed up the construction progress, but also appropriately reduce the negative impact caused by the early creep of concrete and give full play to the performance of the stress bar. In addition, the installation of segments can give full play to the advantages of mechanized equipment, and construction can be carried out at a time when the traffic volume or traffic volume is small, so as to minimize the impact on traffic.
1.2 incremental launching method construction
incremental launching method construction is to open up a prefabrication site behind the abutment along the longitudinal axis of the bridge, prefabricate the concrete beam body in sections, connect it into a whole with prestressed tendons, then apply prestress with jacks, jack the beam in section by section with the help of the sliding device of stainless steel plate and PTFE die plate, and then drop the frame after each section is in place and install the formal support to 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 relatively low, and 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 of each section will be subjected to repeated bending moments during jacking, so the corresponding reinforcement amount will increase. Incremental launching method 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 to divide the continuous beam into several beam segments, apply a proper prestress to the beam segments during prefabrication to make them bear their own gravity, and then use special equipment to carry out hole-by-hole construction from one end of the bridge, support the mechanized support and formwork on the bearing beam, carry out cast-in-place concrete construction in the bridge span, demould after the concrete reaches the strength, and move the whole hole formwork along the guide beam to the next pouring bridge hole, and then push it 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, the more times the construction equipment turns, and the higher the economic benefit of hole-by-hole construction method. For multi-span bridges with spans below 75m, hole-by-hole construction is the most economical construction method. Hole-by-hole construction method does not need to set up ground supports, does not affect river navigation and traffic under the bridge, is safe and reliable in construction, has a good construction environment and 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 technologies of long-span continuous beam bridge construction
2.1 cantilever method construction
When using cantilever method to construct long-span continuous beam bridge, linear control software technology should be used to establish precise elevation control network and horizontal control network respectively before prefabrication. When prefabricating segments by butt joint method, the deviation produced by the previous segment in production should be corrected when prefabricating the next segment. It should be noted that it is not advisable to correct too much at a time when correcting the deviation. Attention should be paid 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 the bridge is closed.
a leveling point should be set at each fixed position at both ends of the bridge during the cantilever assembly stage, and a leveling base point should be set at the middle line of each section after the construction is completed, and the measuring points should be marked on the beams of each section, which should be kept in the same straight line with the leveling points at both ends. Before construction, the reasonable curve of each segment beam 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 any 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, the height and horizontal position of two cantilever ends, the relative height difference between the two cantilever ends and the lateral deviation of the beam axis at the cantilever ends should be measured before locking the steel support at the closure section, and compared with the theoretical calculation value and 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 centerline and elevation at closure are the same as the design value.
2.2 incremental launching method construction
During the jacking construction, the centerline of the beam segment must be controlled within the allowable range, the centerline deviation between the guide beam and the beam body should not be greater than 2mm, the top height deviation of the sliding device on the same side of two adjacent spans should not be greater than 1mm, and the top height deviation of the sliding device at two points of the same pier should not be greater than .5mm. In the process of pushing, the beam body must move forward on the pad plug of PTFE plate, and at the same time, both sides of PTFE slide plate must be kept clean, and the white surface should be coated with lubricating silicone grease to reduce friction. During the construction, the construction personnel should insert the gasket correctly, keep the white side facing the stainless steel slide plate, and the black side close to the box girder web. If the tetrafluoride plate fails to follow up in time in the process of jacking, the jacking should be stopped immediately, and the box girder web should be jacked up and put into the tetrafluoride plate before jacking. When each segment is pushed, it should advance 5cm first, then stop and return oil, then advance 5cm, then stop and return oil, and repeat it for two or three times, and check whether the movement of each part of the facilities is qualified, and then it can be officially pushed. During the jacking operation, each jack should keep in touch and apply force synchronously 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 centerline of the beam body to meet the requirements of the specification during the jacking process, a transverse guide device must be set. Especially when pushing 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 transverse position of the tail end of each beam and ensure the right joint between the tail end of the beam and the precast formwork. In addition, two pairs of rectification devices should be set in the forward direction of the beam and move forward alternately with the direction of beam segment pushing. 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 using temporary supports to assemble precast segments for hole-by-hole construction, if there is a lack of large-scale lifting equipment, the beam can be divided into several segments for sectional production in the prefabrication yard, and the self-weight generated by assembling segments during erection is temporarily borne by the support, and the supporting beam is equipped with tensioning prestressed tendons, and the installed beam is connected with the completed bridge structure according to the design requirements.
The load on the bridge deck is random, so the construction management should be strengthened during the construction. Except the necessary construction equipment, other equipment should be cleaned in time, and attention should be paid to maintaining the balance of the load on the bridge deck. For the load that can't be cleaned up, the weight of the load and its action position should be carefully estimated, which should be taken into account when calculating the deflection. Before construction, the construction load should be calculated and kept stable. Attention should be paid to the collection, collation and summary of relevant measured data, which should be compared with the calculated values, and the load changes during construction should be paid close attention to. The monitoring work should be based on the measured values of the elevation, weight, pre-tension, elastic modulus and concrete strength of the poured beam section, taking into account factors such as temperature influence, bearing deformation and pier settlement, and after analysis by using the construction control program, the vertical formwork elevation value of the next beam section should be put forward.
in the construction process, the error should be reduced by improving the construction technology. The deflection control of bridge construction by using the hole-by-hole construction method is a cyclic process of construction → measurement → judgment → correction → construction. We should strengthen the monitoring of various construction conditions, keep abreast of the changes of bridge elevation, and make timely and rapid adjustments to the construction when problems arise.
3 Conclusion
In recent years, due to the country's vigorous development of transportation infrastructure construction, 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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