Key points of long-span truss construction
1. Truss assembly In order to avoid error accumulation, the primary and secondary trusses are assembled as a whole, and the iron bench made of 16 channel steel is used as the assembly platform. In order to ensure the straightness accuracy of the truss, the chords should be leveled strictly with a level, and the outer ends of the upper and lower chords should be tightened with thin steel wires for straightening the chords. Measure the positioning sideline of the web member at the node position in the chord, and install the web member according to the sideline position. After the chord is adjusted, some web members are installed at the end, middle and joint positions immediately to fix the shape of the truss and avoid deformation when installing other web members.
2. Selection of assembling position and supporting position In order to improve the construction efficiency and avoid secondary transshipment and blocking the traveling route of the crane, the truss is assembled near the installation projection position, and the assembling platforms are arranged on both sides of the passage parallel to the direction of the passage. In addition, the frequency of crane displacement should be minimized during hoisting, so it is necessary to determine the position of crane bracket in advance. The principle is that the crane can lift two adjacent main trusses at the same position at the same time. When the truss is hoisted from the assembly position, the radius of gyration of the hook position is greater than that when the hook is in place, so that the actions of the crane in the hoisting process are hook lifting, arm turning and arm lifting. The radius of gyration is getting smaller and smaller, and the safety factor is getting larger and larger, which can ensure the safety of high-altitude hoisting to the greatest extent.
3. Construction sequence of main truss hoisting (1) Due to the limitation of site conditions, truss installation adopts the method of sequential construction from one side to the other. The construction sequence should meet the requirements of the construction organization design scheme and be managed in strict accordance with the construction disclosure. (2) Truss hoisting Before truss hoisting, the plane position and elevation of the bearing should be accurately adjusted, and after the adjustment, it should be welded firmly according to the requirements of drawings. Measure the positioning axis of the truss on the bearing surface. The whole truss is hoisted at two points. In order to avoid the lateral instability of a single truss, cable wind ropes are set on both sides of the truss at the distance from the end 1/3, and the truss is fixed with cable wind ropes after it is in place. When the truss is hoisted in two sections, it is also hoisted at two points. First, hoist the shorter segment, put the cantilever end on the top of the support frame and monitor and adjust the elevation with a level, then hoist the longer segment. Before the crane is unhooked, the upper and lower chords should be butt welded firmly, and then the web bar between butt joints should be welded. When lifting two machines, first lift the two sections at the end. The length of the middle part of the truss is greater than the clear distance between the concrete. In order to ensure that the truss will not interfere with the concrete structure during hoisting, the horizontal position of the truss should be inclined before the formal hoisting. In the hoisting project, if the actions of the two cranes are hoisting and swinging arms, the radius of gyration will be smaller and smaller, and the safety factor will be larger and larger. In addition, due to the different heights at both ends of the truss, the loads of the two cranes should be as consistent as possible. When lifting, lift it from the rear of the car, one point at a time. Immediately after positioning, weld the butt joints at both ends, and then weld the web between the butt joints.
4. Before hoisting the main truss of the secondary truss, measure the control sideline of the upper and lower chords of the secondary truss at the corresponding node position of the secondary truss, and hang the hanging basket to facilitate personnel operation. After two adjacent main trusses are hoisted, immediately hoist the secondary trusses between them, so that the primary and secondary trusses can form a stable whole and ensure the structural safety. After analysis, when hoisting the secondary truss, the crane boom can only be between the two main trusses, otherwise the boom will collide with the main truss due to the insufficient length of the boom. Through reasonable arrangement of component assembly site, optimization of truss segments, careful analysis of crane position, maximum performance of cranes, and reduction of crane number and crane displacement times, very good results have been achieved.
5. Preparation before welding of steel truss (1) Before welding, the interface area of 10 ~ 15 mm should be cleaned to remove corrosion and surface stains of steel. Before formal welding, the starting point and arc closing place of tack welding should be polished into a gentle slope shape, and no defects such as incomplete fusion and shrinkage cavity should be confirmed. Welding shrinkage should be reserved at both ends of the steel truss. Due to errors in processing and manufacturing, deformation may occur during transportation, which must be corrected before welding. (2) quality control 1) preheat the steel truss before welding to remove moisture and moisture; 2) Controlling the welding speed can appropriately increase the heat input; 3) control the fusion ratio to reduce the proportion of harmful substances in the base metal and molten metal in the weld metal; 4) The welding material of the root weld should try to choose materials with low sulfur, low phosphorus and high manganese content, so as to increase toughness and improve thermal cracking resistance. (3) Precautions for the first layer of welding Precautions Before welding, remove the raised part of the first layer and check whether there are unfused and sunken groove edges. If there is, it must be removed. When grinding solder joints and other parts, pay attention to avoid touching the edge of the groove. Large diameter covered electrode is selected for middle current vertical welding, and high current is selected for flat welding. Matters needing attention in surface welding When welding the surface layer, low current should be selected and the melting time should be extended at the edge of the groove. When replacing covered electrode, the time should be as short as possible to prevent welding interruption.
Truss demolition
1 scaffold removal (1) Before removal, the scaffold shall be thoroughly inspected and all unnecessary objects shall be removed. (2) The demolition sequence should be removed step by step from top to bottom, and the upper and lower floors should not be removed at the same time. (3) The dismantled steel pipes and fasteners shall be piled up in time for transportation and storage. (4) No loose fasteners shall be left at the end of each shift; Hidden dangers such as piling up steel pipes and fasteners on the frame. (5) When disassembling, it is forbidden to collide with the power cord near the scaffold to prevent electric shock accidents. (6) In the process of demolition, all loose bars and fittings shall be dismantled and transported away in time to avoid false support and false reliance on loose bars. (7) Before demolition, a temporary working platform shall be set up, and the platform shall be firm and reliable.
2 Truss Demolition (1) Due to the completion of box girder construction, the demolition work space becomes smaller. At the same time, it is necessary to ensure that the truss can not affect the normal use next time during the demolition process. Therefore, it is very difficult to dismantle the truss. (2) In order to facilitate the dismantling of the truss, holes with a diameter of 50mm are reserved at corresponding positions during box girder construction. And reinforce that steel bar at the reserved hole position. (3) When removing the 40a I-beam, weld the I-beam at the reserved hole position, and use chain block to lift the I-beam from the bracket from the bridge deck to make it close to the box girder bottom plate. (4) Fix and reinforce the I-beam as shown in the figure. (5) Install 12 sets 10t manual hoists at the opposite position of the truss. (6) Lift the truss with chain block and open the flange at the joint. After the flange is opened, one of the trusses will fall to the ground and be temporarily fixed with the pier. At the same time, lift another truss to the maximum lifting height. (7) Stagger the hoisted truss with another truss, then slowly pull the truss out of the pier with chain block, and fix one end on the pier at the opposite position. (8) Drop the truss on the I-beam laid on the ground in advance, and slowly lower the truss. After it is firmly put down, untie the connection with the handle crane. Drag the truss out of the box girder with a loader, and then load it and carry it away. And so on, from the outside to the inside one by one. Danger point analysis Because of the large installation size, small working space, many procedures and heavy weight of cast-in-place box girder, truss, as the supporting structure of cast-in-place box girder, is the foundation and key point of cast-in-place box girder construction. Truss installation mainly involves hoisting operation and aerial work, and there are many cross operations on the construction site, and the working conditions on the construction site are complicated.
3 Safety Assurance Measures (1) When hoisting trusses, the hoisting point, wire rope length, included angle and diameter shall be selected according to the center of gravity. Select the lifting point and calculate the rod force. (2) The parts should be horizontally and evenly lifted off the ground. The hook should be directly above the center of gravity, and the component will not swing back and forth or left and right after the hook is lifted. Check the stress of wire rope, and several wire ropes should be stressed evenly. Hook should be equipped with anti-skipping device, which will not interfere with rope arrangement. The lifting points of components shall be firm and free from sliding. Hooks are commanded by ground personnel, and operators only obey the command of full-time personnel. The lifting speed of the hook shall be slow, and the construction personnel shall not stand on the component. (3) The lifting height of the component should be within the reach of human hands, and it is not allowed to swing freely in the air. When hoisting the truss, two people are responsible for controlling the traction rope of the truss, and there shall be no obstacles in the hoisting channel of the components. Construction personnel should not stand in the direction of component movement. (4) Before dropping the hook, the position should be clear, and the components should be straightened to avoid being placed at will without purpose. Sleepers should be placed under members to facilitate the removal of wire ropes. Drop the hook slowly, and the wire rope can only approach the wire rope after the hook is fully stressed. Don't put your hand under the hook to take anything. When the wire rope exits, it is not allowed to pull directly with the hook, so as to avoid the wire rope popping up and hurting people. (5) After the truss is positioned, measures such as windproof shall be taken for the truss. The wind rope above and below the truss adopts steel wire rope, and the wind rope is fixed by ground anchor. (6) During the truss preloading construction, the construction personnel should fasten their seat belts. At the same time, the supervisor is arranged to monitor the stable state of the truss at any time and give an early warning in time.
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