Precautions for safe construction of the hottest l

2022-08-06
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Precautions for safe construction of long-span continuous beam

according to the construction characteristics of the bridge continuous beam in this bid section, combined with the actual situation, construction environment, construction season and other characteristics of the project, from the comprehensive analysis of human, machine, material, method, environment and other factors, the following hazard sources that may cause personal injury and property loss are identified and confirmed:

1. The design, manufacture, acceptance and other requirements of large equipment do not meet; 2. The strength, stiffness stability and foundation bearing capacity of the support are insufficient; 3. The support is not preloaded or the preloading load cannot meet the design requirements; 4. The assembly and removal of the hanging basket do not meet the design requirements; 5. The hanging basket is not firmly anchored, and is shifted to an infinite position for installation; 6. Mobile cantilever crane has no braking device; 7. No lightning and wind protection measures

8. The segmental beam of cantilever crane is not firmly anchored after assembly; 9. There are no protective and life-saving measures for water construction; 10. No protective measures are taken during highway crossing construction; 11. Anti overturning measures for hanging basket construction; 12. The support is not erected according to the design specification

(I) safety technical measures for continuous beam construction

1. Support construction safety:

(1) the support structure shall have sufficient strength, stiffness and stability, the member stress safety factor shall be greater than 1.3, and the stability shall be

(2) the support foundation must have sufficient bearing capacity, no uneven settlement shall occur, and the ground longitudinal and transverse drainage treatment shall be done well. Foundation reinforcement measures shall be taken for soft soil subgrade

(3) at the bottom of the vertical pole of the support, a pedestal or a base plate shall be set, and vertical and horizontal sweeping poles must be set. The vertical sweeping pole shall be fixed on the vertical pole not more than 200mm away from the upper surface of the pedestal with right angle fasteners, and the horizontal sweeping pole shall also be fixed on the vertical pole close to the vertical sweeping pole with right angle fasteners

(4) except that the top step of the top layer can be overlapped at the pole extension, the joints of other layers must be connected with butt fasteners. The butt fasteners on the force bar shall be staggered, the lap length shall not be less than 1 m, not less than 2 rotating fasteners shall be fixed, and the distance from the edge of the end fastener cover plate to the rod end shall not be less than 100 mm

(5) the longitudinal horizontal bar should be set at the inner side of the vertical bar, the length should not be less than 3 spans, and the spacing should not be greater than 400 ㎜; The longitudinal horizontal bar should be connected by butt fasteners or lap joints, and the butt fasteners should be staggered; The lap length shall not be less than 1m, and three rotating fasteners shall be set at equal intervals for fixation

(6) a horizontal bar must be set at the main node, which shall be fastened with right angle fasteners and shall not be removed; Horizontal bars at non main nodes shall be set at equal intervals, and the maximum spacing shall not be greater than 1/2 of the longitudinal spacing

(7) the extension length of the end of the horizontal horizontal bar against the wall shall not be greater than 0.4 horizontal spacing, and shall not be greater than 500 ㎜; The extended end shall be provided with isolation measures every three steps

(8) diagonal bracing shall be set outside the support. The width of the diagonal bracing shall not be less than 4 spans and shall not be greater than 6m. The inclination angle between the inclined rod and the ground should be 45 ~ 60, and the diagonal bracing must be set continuously; The extension of diagonal bridging rods should be overlapped. The overlapping length should not be less than 1m, and not less than 2 rotating fasteners should be fixed. The distance from the edge of the end fastener cover plate to the rod end should not be less than 100 mm

(9) bagged soil is used for preloading of the support, and the loading sequence is from the support to the middle of the span. The load holding time after full load shall not be less than 24h, and the preloading weight shall be 110% of the design load. During loading, the load shall be divided into three levels according to 50%, 100% and 110% of the design load. During loading, pay attention to the size of the loading weight and the loading rate to make it adapt to the strength growth of the foundation. After the foundation is under the action of the previous level of load and the foundation settlement speed has been observed to be stable, the next level of load can be applied. Especially in the later stage of loading, the loading rate should be strictly controlled to prevent the shear failure of the foundation due to excessive and rapid overall or local loading. The maximum settlement of the foundation shall not exceed 10 mm/d; The horizontal displacement shall not be greater than 4mm/d. The elevation of the bottom formwork shall be observed once before preloading, and it shall be observed once every 24 hours in the process of preloading after each loading level, until the settlement speed has dropped to 0.5 ~ 1.0mm/d. The elevation shall be observed again after unloading the preloading load according to the loading level. During the preloading process, accurate measurement shall be carried out to measure the elastic deformation value of the support and the foundation settlement value under the load of the beam section The foundation settlement value is superimposed with the pre camber required due to other factors proposed in the construction control to calculate the pre camber that should be used during construction, and the bottom formwork elevation is adjusted according to the calculated pre camber

2. Technical safety of bracket construction

(1) ensure that the verticality of the column is not greater than 1/1000

(2) ensure the weld quality, adopt reasonable welding sequence, ensure that the welding deformation is not too large, the weld size should be full, and avoid false welding

(3) strengthen the welding of weak links such as the intersection of column and I-steel. Before the preloading of bracket, carry out visual inspection on all welded parts according to the bracket design drawings and requirements. Inspection requirements: the weld shall not have defects such as incomplete welding, root shrinkage, undercut and poor joint; The weld shall be free of surface porosity, slag inclusion, arc scratch and other defects; The first transmission mode at the weld joint shall be stable and free of incomplete fusion, cracks and other defects. Unqualified welds in visual inspection shall be treated in time to ensure construction safety

(4) the gusset plate at the place where the welding deformation is too large shall be corrected, and the close fitting of the anchor plate connection must be ensured to ensure the quality of the anchor plate weld connection, so as to avoid the weld crack caused by the excessive welding gap at the anchor plate connection and safety accidents

(5) during the installation of double I-steel of longitudinal and transverse beams, the position shall be accurate to prevent cracks, and the upper and lower members shall be in the same plane to ensure balanced stress. Structural members and welds shall be free of cracks, and bracket structural members shall be free of bending deformation

(7) the preloading loading sequence of the bracket is from the middle to both sides. The load holding time after full load shall not be less than 24h, and the preloading weight shall be 120% of the design load. When loading, the load shall be divided into three levels according to 50%, 100% and 120% of the design load. The unloading shall be 120%, 10% without protection, 0% and 50%, and the unloading sequence shall be from both sides to the middle. When the bracket is preloaded, the settlement observation points shall be buried and observed with a level gauge. Observation frequency: before loading, 100% loading, 120% loading and after unloading

3. Technical safety of cradle construction

(1) the change of the total weight of the cradle shall not exceed 10% of the total design weight

(2) the safety factor against overturning and the safety factor of self anchoring system during concrete pouring and walking of beam section shall not be less than 2.0

(3) it is strictly forbidden to cut and enlarge the bolt hole at will during the assembly of the hanging basket. If it is clear that the hole needs to be enlarged, it must be approved by the hanging basket design unit. It is strictly prohibited to conduct electric welding and ignition on the threaded steel bar suspender. All finished rolled threaded steel bar suspenders must be locked with double nuts

(4) the cradles on both sides of the pier shall be moved symmetrically, and a brake device shall be set at the tail. The moving speed shall be controlled within 0.1m/min, and the cradles shall be anchored in time after moving in place

(5) when lifting the hanging basket components and other heavy objects, CM shall be lifted first, and the lifting can be continued only after it is checked and confirmed to be good. The lifting rod must be commanded by a fixed signal, with accurate flags and rapid rumors. No one is allowed to stand under the suspended parts

(6) the hanging basket can be installed on the bridge deck only after 0ne tensioning and grouting and tensioning of the vertical anti overturning reinforcement. During concrete pouring, the hanging baskets at both ends must be in a symmetrical position, but at this time, the difference between the two hanging baskets is allowed to be only 1/3 of the concrete weight of the section, and the pouring should be carried out from front to back

(7) the hanging basket can only be moved after the new beam section is longitudinally tensioned and grouted. Before moving, check whether the removal and replacement parts are properly completed and whether temporary contact is added. When moving, it must be evenly balanced, left and right synchronized, and the direction is straight; The front end of the main beam shall be padded in time, and the rear end shall be set with anchors to prevent overturning. For the separation and adjustment of outer formwork and bottom formwork, chain fall should be used to hang the lower anchor beam on the outer beam. The top of the pull rod shall be jacked, and the safety pad shall be set, so that the stress of the two outside and two inside grade IV steel shall be similar

4. Technical safety of formwork construction

(1) when lifting the formwork, the vertical lifting shall not be less than two lifting points, and the horizontal lifting shall not be less than four lifting points

(2) the actual load of the installed formwork shall not exceed the design value. The supports and accessories that have been loaded shall not be removed or moved at will

(3) for the removal of the prestressed concrete structure or component formwork of the cast-in-place continuous box girder, the side formwork shall be removed before the prestressed tensioning, and the concrete strength can reach the removal conditions of the side formwork. The prestressed tensioning can be carried out only after the concrete strength reaches the design specified value, and the bottom formwork can be removed only after the prestressed tensioning is completed

(4) the demolition operation must be carried out step by step from top to bottom, and it is strictly prohibited to carry out both at the same time

5. Steel bar manufacturing and installation, electric welding technology safety

(1) operators must wear safety helmets that meet the requirements, use standard fire safety belts (safety belts meet the requirements of GB safety belts), with a length of no more than 2 meters, and wear protective rubber shoes. The hook of the safety rope on the safety belt shall be hung firmly

(2) it is forbidden to work at heights under climatic conditions such as strong wind, rain and fog above level 6

(3) before welding, check whether all tools, electric welding machines, power switches and lines are in good condition. The metal shell shall be safely and reliably grounded. The incoming and outgoing lines shall be provided with complete protective covers. The incoming and outgoing lines shall be firmly welded with copper joints

(4) the distance between the welding machine and the switch box shall not be greater than 3m, and the fusing current of the fuse shall be 1.5 times of the rated current of the machine. It is strictly prohibited to replace the fuse with metal wire. It is also equipped with a special electric shock protector

(5) the wires and ground wires shall not be in contact with steel wire rope and scaffold steel pipe, and it is not allowed to use steel wire rope or electromechanical equipment to replace the zero wire

(6) the transformer of the welding machine shall not be overloaded, and the temperature rise shall not exceed 60 ℃. Flammable materials shall not be covered on the casing

6. Safety of prestressed tensioning technology

(1) the groove of the fixture shall be 0.7 ~ 1mm larger than the diameter of the pulled reinforcement, and sufficient strength shall be ensured to prevent deformation of the anchorage

(2) before starting the high-pressure oil pump, loosen the regulating valves of each oil circuit, and then start the oil pump. After the no-load operation is normal and there is no

leakage in the oil circuit, the operation can be carried out after confirmation

the high-pressure oil pump shall not be overloaded. The safety valve shall be adjusted according to the rated oil pressure of the equipment. Arbitrary adjustment is strictly prohibited

(4) the prestressed reinforcement shall not exceed the specification of the tensioning mechanical equipment

(6) during the preloading of the bracket, the bracket members and welds shall be inspected when the preloading load reaches 100% and 120%. It is required that the bracket structural members and welds shall not have cracks, and the bracket structural members shall not have bending deformation

(7) the preloading loading sequence of the bracket is from the middle to both sides. The load holding time after full load shall not be less than 24h, and the preloading weight shall be 120% of the design load. When loading, the load shall be divided into three levels according to 50%, 100% and 120% of the design load. The unloading is 120%, 100% and 50%, which is the first time that new materials are regarded as an independent industry in China. The unloading sequence is from both sides to the middle. When the bracket is preloaded, the settlement observation points shall be buried and observed with a level gauge. Observation frequency: before loading, 100% loading, 120% loading and after unloading

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