1. 1 construction site preparation
(1) scaffolding
Dismantling old cables, installing new cables and tensioning construction need to operate the platform. In order to facilitate personnel to go up and down and transport materials, steel pipe scaffolding is arranged around each tower column. Scaffolding materials are scaffolding steel pipes with a diameter of 48 ㎜× 3.5 ㎜, all of which are supported on the foundation of pile caps, and zigzag ladders are set in the scaffolding. In order to ensure the stability of steel pipe scaffold, a horizontal cross joint is set every 5 m to clamp the tower column.
(2) Cleaning work
This work is mainly aimed at cleaning the upper anchor cup, lower anchor box and upper and lower anchor pipes. First, open the original upper anchor cup steel sleeve to remove butter, old steel wire and rust. Then the outer sealing steel plate of the lower anchor box is removed by gas cutting, and the residual concrete in the lower anchor box is removed by air compressor and pneumatic pick. Finally, after cleaning the upper and lower anchor boxes, the upper and lower anchor pipes should be cleaned. First, the rubber shock absorber is removed, the concrete cable seat is chiseled off, the redundant anchor pipe is cut off, and the anchor pipe is cleaned by electric drill, pneumatic drill and steel drill, and then it is protected between the bottom anchor pipe and the cable pipe, and the joint is protected by injection molding to prevent rainwater from invading. Without cleaning, the anchorage on the cable can't be removed. Therefore, this work needs to be completed before replacing the cable.
(3) Check the cable parameters
After the tower beam is supported, open the cable anchor box, measure the exposed size of the cable anchor end, and calculate and check the cable length.
1.2 construction machinery preparation
The main construction machinery required for cable replacement includes cable unloading and cable hanging connector, tensioning screw lead-out, tensioning jack, oil pump, hoist, oil gauge, reaction frame, pad and so on. Monitoring equipment mainly includes cable tension meter and precision level.
1.3 preparation for construction monitoring
In the cable replacement project, it is necessary to carry out all-round monitoring, mainly aiming at the monitoring of cable force and elevation. In the process of cable unloading and new cable tensioning, cable tensiometer and precision level can be used to monitor the cable force of replaced cable and adjacent cable, the elevation displacement of bridge deck at the corresponding position, and the concrete stress at the bottom of beam or tower root.
2 Cable replacement construction technology
2. 1 Remove the old cable
2. 1. 1 process
Install the cable receiving (releasing) roller table and cable reel on the bridge deck, and remove the concrete in the steel casing of the anchor head on the beam. At the same time, a hoist is installed at the root of the cable tower, a crown block is installed at the top of the tower, and a cantilever hanging basket is installed at the cable changing position on the beam. Hoist the jack to the tower with a hoist, install tensioning equipment on the anchor head at the tensioning end of the tower, and loosen the cable. Looseness should be compared twice. When the readings of the two oil meters are not much different, the cable can be removed. Cable unloading should be carried out in stages. When there are 4-5 buckles left at the joint between the big nut and the anchor head, the tensioning equipment should be removed, the soft traction device should be installed, and the tension of the stay cable should be continuously relaxed until it is less than 5t. After the cable is loosened, use the winch to pull the sling of the pulley block, and slowly tighten it until the soft cable traction device is completely loosened, then remove the soft cable traction device, and slowly lower the cable with the winch and put it on the drum shaft. When the hanging basket enters the anchorage end of the beam, remove the nut at the fixed end and pull out the lower anchor head of the cable with a winch. Finally, fix the end on the empty tray and seal it, and transport it out of the field.
2. 1.2 Notes
(1) When unloading the cable, pay special attention to record the readings of the corresponding jack oil gauge when loosening the anchor nut, and compare them when loosening the cable twice. Only when there is little difference can the cable be unloaded;
(2) In the process of cable unloading, the elevation change of the top surface of the beam should be tracked and observed all the time, and compared with the theoretical calculation value. If there is any abnormality, the cable unloading work of stay cables should be stopped immediately, and it can be carried out only after the reason is found out;
(3) When the cable is unloaded, both sides of the tower body should be symmetrical, so as to ensure the stress balance on both sides of the main tower as much as possible, and there is no unbalanced load.
2.2 Installation of new cables
2.2. 1 process
Hoist the new cable to the vicinity of the anchorage section, with a length similar to that of the cable. Use the winch to unwind the cable and lay it flat on the drum. Fix the anchor head of the beam body, determine the installation fixture of the cable lifting point, put the lower anchor cup into the steel casing of the lower anchor head, and install the fixing nut of the lower anchor head to the specified position. Install a fixture at a distance of 1.5m from the cable end, connect the cable with the hoist, and slowly hoist the cable to the vicinity of the anchor head steel casing. Install the soft traction device until it can be connected with a new cable. Install the toothed plate and connector, adjust the jack and strut, and clamp the tool anchor. Align the conversion sleeve with the soft traction connector, and then screw the connector into the conversion sleeve to start soft traction tension. Slowly tighten the steel strand, and then remove the pulley block hook. After the jack moves 20cm, the locking tool anchors, the oil pump returns oil, and the tool moves the anchor back to its original position. Then repeat this process until the anchor head of the cable enters the steel sleeve and goes to the other end to expose three threads, and then tighten the fixing nut. Remove the soft traction device and install the tensioning device. Start tensioning after passing the inspection; The jacks on both sides are synchronously loaded step by step to the unloading tonnage of the old cable; Check the cable force and beam tower measurement by frequency measurement method, and adjust the cable force to meet the requirements; Screw on the cable nut.
matters need attention
(1) Before hanging the new cable, measure the bridge deck elevation with a precision level, and then hang the cable after measuring the bridge deck elevation.
(2) When the new cable is unfolded, the roller is used as bedding to protect the protective layer of the cable from being scratched or damaged.
(3) The tensioning of new cables should be carried out slowly and in stages; When the cable force reaches the design cable force, the load is 10min. At this time, it is necessary to check whether the cable force of the new cable force reaches the original cable force and whether the elevation of the bridge deck is restored to the elevation before cable replacement. When it is confirmed that the cable force and elevation are restored to the original cable force and elevation, the next cable can be replaced.
2.3 Cable replacement sequence
The cables are replaced one by one, using bidirectional symmetry, that is, one bundle is symmetrically replaced once on both sides of the tower column, and the other bundle is symmetrically replaced on both sides of the other tower column, alternately. Change the same cable number before changing the next cable number. When replacing stay cables, each bundle of stay cables shall be tensioned and anchored according to the cable force given in the design. After replacing all cable bundles with the same cable number, all cable bundles are tensioned at the same time to adjust the cable force, so as to achieve the given cable force.
3 cable force adjustment after cable replacement
The main monitoring indexes of cable replacement project are cable force and deck elevation. If the control of cable force and its distribution is deviated, the internal force distribution of the main girder of cable-stayed bridge will change, which may eventually make the structure deviate from the ideal state of the original design. Therefore, it is necessary to adjust the cable force after replacing the cable. The calculation methods of cable adjustment mainly include influence matrix method, approximation method and mathematical programming method, and the selection scheme should be adjusted repeatedly in combination with specific projects.
The principle of cable adjustment is to consider the changes of concrete stress, girder line shape and tower position. Specifically, the adjustment is required to alleviate the local deflection of the main girder and approach the bridge shape as close as possible; At the same time, the concrete stress at the upper and lower edges of the main beam should be moderately increased or decreased to avoid excessive tensile stress and ensure the safety of the main beam.