The following are three construction methods for steel structure grids brought to you by Zhongda Consulting for your reference.
High-altitude assembly method
The steel grid is installed using the high-altitude assembly method. The assembly bracket is first set up at the designed position, and then a crane is used to divide the grid components into parts (or blocks). ) is hoisted to the designed position in the air and assembled on the bracket. This method sometimes does not require large lifting equipment, but requires a large amount of assembly brackets and requires a lot of high-altitude operations. Therefore, it is more suitable for steel grids connected by high-strength bolts and made of section steel or steel pipe grids with bolted ball joints. There are still some steel grids constructed using this method.
2 Overall installation method
The overall installation method is to first assemble the grid as a whole on the ground, and then use lifting equipment to lift the whole to the designed position and fix it. This construction method does not require tall assembly brackets, requires less high-altitude operations, and can easily ensure welding quality. However, it requires heavy lifting equipment and is technically complex. Therefore, this method is more suitable for steel grids with ball joints (especially grids with many rods such as three-way grids). According to the different equipment used, the overall installation method is divided into multi-machine lifting method, pull rod lifting method, jack lifting method and jack lifting method.
1. Multi-machine lifting method
This method is used for medium and small grid structures that are not large in height and weight. Before installation, perform staggered assembly operations on the grid on the ground (that is, the assembly position is staggered by a certain distance from the installation axis to avoid the position of the pillars). Then use multiple cranes (mostly crawler cranes or truck-type cranes) to lift the assembled grid as a whole above the top of the column, shift it in the air and then drop it down to fix it.
(1) Grid assembly
In order to prevent the grid from colliding with the columns when the grid is lifted as a whole, the staggered distance depends on the grid and the columns or column corbels during the grid lifting process. The clear distance between them should generally not be less than 10 ~ l125px. At the same time, the convenience of grid assembly and the convenience of the crane during aerial shifting should be considered. If necessary, you can negotiate with the design unit to leave some edge rods of the grid for welding after the grid is lifted, or to change some column corbels that affect the lifting of the grid.
After the steel grid is processed in the metal structure factory, the single pieces are assembled into small unit plane trusses or three-dimensional trusses and transported to the construction site. The construction site assembly means that the small unit trusses are assembled into the entire grid at the assembly position. The key to grid assembly is to control the size of the grid frame axis support (the amount of welding shrinkage must be pre-calculated) and the arching requirements.
Grid welding is mainly the welding of spheres and steel pipes. Generally, equal strength butt welding is used. For safety reasons, a 6-8mm fillet weld is added to the butt weld. For weldments with pipe wall thickness greater than 4 mm, the interface should be grooved. In order to make the butt weld uniform and the length of the steel pipe slightly adjustable, a casing can be added. When assembling, install the upper and lower chords first, and then install the oblique web rods. After all the steel pipes between the two trusses are placed and corrected, the steel pipes can be welded one by one.
2) Grid hoisting
This kind of medium and small grids are usually lifted by four crawler cranes (or truck-type or tire-type cranes), and two are also used. Hoisted by a crawler crane or a lever.
If the weight of the grid is small, or the lifting capacity of the four cranes meets the requirements, it is advisable to arrange the four cranes on both sides of the grid, so that as long as the four cranes rotate at the same time, the aerial lift of the grid is completed. In order to meet the shifting requirements, the key to multi-machine lifting is that the lifting speed of each crane is consistent. Otherwise, some cranes will be overloaded, the grid will be twisted, and the welds will crack. For this reason, it is necessary to measure the lifting speed of each crane before lifting so that it can be mastered during lifting.
When the grid is lifted to about 750px higher than the height of the column top, perform an air shift to move the grid above the column top. When the grid is in place, in order to make the center line of the grid accurately match the center line of the column top, tie a steel wire rope to each of the four corners of the grid in advance, and use the inverted chain to align the lines.
2. Pull-rod lifting method
For the installation of large-scale steel pipe grids with ball joints, the pull-rod lifting method is currently used in my country. When constructing with this method, the grid is first assembled in misplaced positions on the ground, and then multiple one-legged poles are used to lift the grid as a whole above the top of the column, shift it in the air, and install it in place.
(1) Principle of aerial shift
Aerial shift is the key to this method. Aerial displacement uses the unequal horizontal forces in the lifting pulleys on both sides of each pulling rod to move the grid horizontally.
When the grid is shifted in the air, it is required to have at least two pulling rods to hang the grid, and the lifting pulley group on the same side does not move. Therefore, when the grid is shifted in the air, only Pan without tilting. Since the pulley block on the same side does not move, in addition to translation, the grid also produces a controlled circular motion, causing the grid to drop slightly. The direction of the aerial displacement of the grid is related to the arrangement of the pull-out rods.
(2) Selection and arrangement of lifting equipment
The selection and arrangement of lifting equipment is an important issue in the construction of grid lever lifting. The content includes: selection of pull-out rods and arrangement of lifting points, arrangement of cable wind ropes and ground anchors, threading method of lifting pulleys and lifting point rigging, winch arrangement, etc.
The selection of pull-out rods depends on the load they bear and the arrangement of hanging points. The calculated load when the grid is installed is:
Q=(K1Q1Q2Q3)·K(kN)
Where Q1——the self-weight of the grid (kN);
K1——Load factor 1.1 (if the weight of the grid is accurately calculated, it can be taken as 1.0);
Q2——The self-weight of additional equipment (including stringers, ventilation ducts, scaffolding, etc.) (kN);
Q3——The self-weight of the spreader (kN);
K——The uneven force coefficient caused by the difference in lifting. If the weight of the grid is basically uniform, the difference in lifting at each point is controlled. Below 250px, this coefficient takes the value 1.30.
The layout of the grid lifting points is not only related to the hoisting plan, but also related to the stress performance of the grid during lifting. During the lifting process of the grid, not only the internal force of some rods may exceed the calculated internal force during design, but also the sign of the internal force may change for some rods, causing the rods to become unstable. Therefore, the number and location of lifting points should be determined through grid lifting verification. However, on the premise that the lifting capacity, hoisting stress and grid stiffness are met, the number of pull-out rods and hanging points should be minimized.
The cable wind rope should be arranged so that multiple pulling rods are connected to each other as a whole to increase the overall stability. Each pull-out pole must have at least 6 cable wind ropes. The cable wind ropes must be calculated and selected based on the wind load, hoisting weight, pull-out pole deflection, cable wind rope initial stress and other loads, and the most unfavorable combination of conditions. Ground anchors also need to be calculated and determined.
The force calculation of the lifting pulley block can be carried out according to the actual force situation, and the specifications of the pulley can be selected based on the calculation results.
The specifications of the winch should be determined according to the internal force of the lifting wire rope. In order to reduce the difference in catching and lifting, it is best to use winches of the same specifications.
(3) Axis control
The position of the grid assembly pillars should be accurately measured according to the axis of the installed pillars to eliminate the accumulation of axis errors when the pillars are installed.
(4) Pull-out rod removal
After the grid is hoisted, the pull-out pole is enclosed in the grid and should be removed using the inversion method. This method is to hang two pairs of lifting pulleys at the upper chord nodes of the grid to hang the pull rods, and then remove the pull rods one by one starting from the bottom section.
3. Electric screw lifting method
The electric screw lifting method is similar to the lifting method. It is an electric screw hoist used in the construction of the lifting project, which will be assembled on the ground. The overall steel grid frame is raised to the design elevation. The advantage of this method is that it does not require large lifting equipment and is easy to construct. Use an electric screw hoist to lift the steel grid, which can only be lifted vertically and cannot be moved horizontally. For this reason, the design should consider setting up joists between the two columns, and the fulcrum of the grid falls on the joists.
Since the grid does not move horizontally when it is lifted, the grid assembly does not need to be dislocated and can be assembled in the original position.
3 High-altitude sliding method
In recent years, the use of high-altitude parallel sliding method for grid roof construction has gradually increased. It is especially suitable for theaters, auditoriums and other projects.
With this construction method, the grid is usually assembled with an assembly platform on the roof of the front hall of the building (an assembly platform can also be set up on the auditorium stand). After the first assembly unit (or first section) is assembled, That is to say, drop it onto the sliding track and use the traction equipment to slide forward a certain distance. Next, assemble the second unit (or second section) on the assembly platform. After it is assembled, it slides forward together with the first assembly unit (or first section). In this way, the assembly section by section continues to slide forward until The entire grid is assembled and slid into place.
To ensure the sliding movement of the grid after assembling the grid, rollers can be set under the grid support to allow the rollers to slide on the sliding track; a support bottom plate can also be set under the grid support so that the support bottom plate Slide along the embedded steel plate embedded in the reinforced concrete frame beam.
The grid sliding can be pulled by a winch or a lever hoist. Depending on the size of the traction force and the load-bearing capacity of the rods between the grid supports, one or more points of traction can be used. When the grid is sliding, the out-of-sync value at both ends should not be greater than 50mm.
When the sliding method is used to construct the grid, the following verification calculations should be made during the sliding and assembly process:
1) When there is no fulcrum in the middle of the span, the internal force of the rod and the mid-span deflection value;
2) When there is a support in the middle of the span, the internal force of the member, the fulcrum reaction force and the deflection value.
When the internal force of the rod changes due to the addition of intermediate slide rails in the grid sliding unit, temporary reinforcement measures should be taken to prevent instability.
The high-altitude sliding method is used to construct the grid structure. Since the grid assembly is carried out on the roof platform of the front hall, the danger of high-altitude operations is reduced; compared with the high-altitude assembly method, the assembly platform is small, which can save materials. , and can ensure the assembly quality of the grid; since the grid assembly uses sliding construction, it can flow parallel to the civil construction and cross at three levels, thus shortening the construction period of the entire project; the construction equipment of the high-altitude sliding method is simple and generally does not require large-scale cranes The equipment is reinstalled, so the construction cost can also be reduced.
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