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Construction of welded ball space frame in plateau environment with large temperature difference

space frame lecture hall

Welding is a high-efficiency, low -cost high -efficiency technology, and is widely used in large-scale engineering construction projects. In a large temperature difference environment, the role of welding ball space frame is to pass the large gravity load to a high altitude to achieve a higher stability and high efficiency; At the same time, the diameter of the steel pipe decreases because the high-pressure gas has a great influence on the welding wire. First of all, a comprehensive understanding and study of the welder is required; Secondly, according to the actual situation to design a reasonable and effective process flow and equipment layout; Finally, it is necessary to develop scientific, feasible, safe and reliable, economical and easy to operate management modes and methods, so as to ensure construction quality and reduce unnecessary losses.

1 Basic theory of welding ball space frame

1.1 The force mechanism of welding ball space frame

The stress feature of the welding ball space frame are:

 (1) During the welding process, the workpiece produces large deformation along the surface of the weld under the action of gravity, which gradually reduces the section size, so that its section size gradually decreases. The stress field formed in this case will change over time. Therefore, when the steel component is at a high altitude, there is a series of non -linear effects such as the maximum stretching strain and the yield limit, deflection, and plastic deformation. At the same time, these linear effects also affect the connection between the overall stress state and the stable performance of the structure.

(2) Due to the strong impact of the workpiece surface after welding. The process of welding is composed of horizontal, vertical direction and ground composed of a force system, mainly acting on the rotating platform, it will travel in a straight line. Due to limited working space and many unstable factors in the horizontal direction. Therefore, in order to ensure that the vertical installation of uniform distribution of solder joints alignment joint position requirements must reach a higher standard to meet; At the same time, it is also necessary to ensure that the welding parts can be accurately spread from the straight rear and have good and smooth contact with the ground without loosening so that the quality of the connection can be improved.

1.2 Stress and deformation of welding ball space frame

After the welding work is completed, the structural deformation analysis should be performed to determine whether the performance of the steel is consistent with the design requirements. The two methods of thermal return, rebound method, and theoretical calculation are used to detect the shape of welded seams, and the calculation of stress-strain for the high-temperature difference environment is used to use the limited element analysis software ANSYS Long welding parameters provide a theoretical basis for reducing the probability of crispy breaks and improving welding efficiency. Use the stress test method to measure fatigue intensity, to derive the stress-strain curve diagram and mechanical model parameter values; If there is a strain change may lead to deformation stress, resulting in crack growth and fracture phenomenon, fatigue damage, and brittle fracture, and may even become a common welding defect area at the solder joint.

1.3 Slipping construction technology

Sliding construction technology refers to the use of gravity to connect one end of the steel mesh to the pillar and the other end to the slider so that it moves along a certain track and angle. Flexible components such as steel wire rope and rubber tape use special processing equipment to fix them together to form an overall structure so that the sliding system has a certain stiffness and good elasticity. Set the bracket between the steel pipe and the support as the temporary support system to complete the mobile work of the entire project. At the same time, you can also combine the wire rope, roller or other objects as needed to achieve the overall synchronous movement and parallel upper and lower interception displacement. The construction process ensures that the steel space frame is not affected by horizontal, lateral deviation and longitudinal displacement and other factors to maintain a stable and reliable working state, also known as the lateral force balance or lateral deformation balance method.

For example, the overall idea of the main stage space frame of this project is to set up an assembly platform in the direction of the U-axis, to install a sliding track (50kg/m) on the roof ring beam of the 22-axis and 25 axes with the splintered sliding construction technology, and to set traction equipment outside the U-axis. The space frames assembled in the assembly area are slipped to the installation position by sliding in pieces (assembling and sliding in pieces), and then the mesh struts between pieces are reassembled.

2 Construction scheme of high space welding ball space frame under large temperature difference environment

2.1 Project Overview

This project is a roof space frame steel structure project in the Tibet Cultural Radio, Television Art Center project. The main functions of the project include radio and television and theater. Among them, the radio and television studio, opera house main stage, and audience hall above the welding ball are placed quadrangular steel space frame structure, the top string of the space frame is connected with the rectangular tube and cast-in-place reinforced concrete roof plate with bolts. The steel space frame over the big studio is rectangular, with a plane size of 54.0mX36.0m. The roof adopts the form of welded ball nodes, and the upper chord node of the outer ring of the space frame shell is supported on the roof frame. The steel space frame over the main stage of the opera house has a rectangular plane with a plane size of 24.5mX34.9m. The roof adopts the joint form of welded balls, and the upper chord node of the outer ring of the mesh shell is supported on the shear wall. The plane of the steel space frame over the Opera hall is polygon (nearly circular), the plane size is 32.7mX37.1m, the roof adopts the form of welded ball nodes, and the upper chord node of the outer ring of the space frame shell is supported on the frame column leg. In addition to bearing the weight of the roof and itself, the steel structure roof also serves as the load-bearing structure of the mechanical hanging system of the lower ceiling, light bridge bridleway and stage space frame layer. The space frame node is: the ball space frame structure is classified by the type of the rod, including the welding ball node (including the support node) and the space frame rod; Support joints include: welding ball, support rib, support bottom plate and anchor bar, etc.

hall construction

2.2 Analysis and Solution of construction difficulties

(1) This is a square quadrilateral pill welding ball single-layer graphic space frame structure and the boundary condition of the space frame is on the upper chord support on both sides of the support. The roof welding ball space frame should be constructed after the completion of the construction of the roof steel structure, and the space frame weight is up to 200kg/ ㎡ per square meter, which is too large to be lifted away from the distance, and the crane cannot be lifted in the field. Therefore, the construction of the space frame adopts cumulative slip. The construction of cumulative slip of space frame is the key point of this project. According to the form of the space frame structure and site construction conditions, the project studio and the main stage of the space frame adopt the way of sliding construction: (1) assembly platform setting: at the end of the space frame scaffolding as a sliding unit of the assembly area, used to assemble the space frame. (2) Sliding track setting: lay 2 sliding tracks (50kg/m) on the concrete column leg of the axis on both sides of the space frame frame, and set traction equipment (2 10t electric hoist) on the corresponding track position at the other end of the space frame by sliding the assembled space frame in the assembly area to the installation position.

(2) The roof space frame of the auditorium is limited by the construction conditions at the scene. The area of the area is constructed by setting up a scaffolding of a space frame to set up a space frame assembly platform. The height of the full -scale scaffolding reaches 24m. How to set up a full frame to meet the requirements of the space frame load. Its setting method and construction safety are the focus of this project. Solution: The design of the poles of the scaffolding, load calculation, and on -site construction safety control requirements to ensure the construction meetings of the scaffolding.

(3) The span of the space frame is large, the precision of the installation of the support directly affects the quality of the installation of the steel structure, and the precision of the installation is very high. And the embedded parts connected with the support are arranged on the top of the concrete column or the concrete leg. If the relationship between the support and the embedded parts and the diameter of the whole concrete supporting column is not handled well, the quality of the whole project will be greatly affected. Solution: ①According to the axis and elevation control point, the cross line of the support is drawn on the buried part. When installing, the center line of the support is aligned with the cross line, the coordinates of the center point of the upper surface of the support are checked, and the support is spot welded on the buried part. ② The rubber bearing and the embedded parts are connected by the method of peripheral welding. In order to control the welding deformation, the welding points are arranged symmetrically in a reasonable welding sequence and welded in sections. ③ The upper string ball bearing node is welded into a hole in the factory before installation. When placed, align the center line, check the coordinates of the center of the upper string ball, adjust and correct the upper string ball support, and spot weld the upper string ball support on the rubber support after the measurement is qualified.

(4) This project is a spatial space frame structure, and each node has different spatial coordinates, and there are many control points. And the construction area is large, and the internal environment of the venue is complicated.

① Because the construction accuracy of each part of the structural system of the project directly affects the installation error accuracy of the next process, it is necessary to formulate a strict measurement scheme from the factory manufacturing to the site assembly and installation, the use of scientific measuring instruments and measuring means to control the construction accuracy of each way.

 ② For the treatment of cumulative errors, the method of setting compensation amount in each space frame to adjust, eliminate section by section, to prevent the excessive cumulative amount of one-time elimination and impact on the structure. The measurement data should be used after adding the pre-deformation value on the basis of the design value, and adjust the pre-deformation value in time according to the construction synchronous monitoring data.

2.3 Welding ball space frame construction process

Construction technology of welding ball space frame: Before welding, we must make full preparations for welders, and reasonably formulate a complete and effective technical organization plan according to the quality and use requirements. At the same time, we must also pay attention to strict control in terms of ensuring safety, construction, and installation.

(1)After the completion of one space frame, unified welding is carried out, and the next space frame is assembled at the same time. The welding sequence is expanded from the middle to both sides, which is conducive to the release of internal and external stress in the structure. During welding, first weld the lower string ball, then weld the upper string ball. Weld the lower chord ball, lower chord bar, and lower end of the belly bar on the assembly platform, and weld the upper chord ball, upper chord bar and upper end of the belly bar on the movable welding platform. The center of the upper string ball support is aligned with the center line of the track, and the lower plane of the support is slightly lower than the upper plane of the lead rubber support (5-10mm). After all, components are welded and formed, they shall slip after being checked and qualified.

(2) Lap, fixing, and repair welding procedures. In the butt process of steel pipe and ball frame, due to the influence of various factors, two steel plates need to be stretched before welding. In order to ensure the connection quality to meet the requirements and construction technology level. At the same time, attention should be paid to maintaining balance, continuity and uniformity. In order to ensure that the corrugated lines in the steel pipe can pass smoothly, and make it smooth, beautiful, and correct position, it is necessary to prepare for the lap work: first, after wearing the guard pipe, and leading it to the lowest place with the steel pipe on the large diameter cylinder. At the same time, the uniform lifting method is adopted to reduce the rapid adjustment of the welding gun and joint parts during the welding process. Finally, a hole punch is used to ensure the welding quality.

2.4 Construction influencing factor of welding ball space frame

(1) The size parameters of steel structure have a great influence on the length and diameter of the weld. Because of the embedded parts before welding, the diameter and quality of the welded steel should be selected. At the same time, the surface finish, flatness, and corrosion resistance of the weld should be considered. In the actual construction process, due to the influence of various factors, the space frame structure is damaged to different degrees. Therefore, in order to avoid the adverse effects of the above problems on the project, we must strictly control the large temperature difference environment and high space high connection technology.

(2) Due to the large temperature difference, the welding ball space frame is a special material. Its own gravity will change greatly under different temperatures and certain pressure. When the external atmospheric pressure is lower than the critical value, the phenomenon of stress concentration is easy to occur. When the external pressure is higher than the critical value, it is easy to cause the thermal bridge, airbag, and other components deformation and damage. Therefore, in the actual project, the influence of these factors should be fully taken into account and effective measures should be taken to ensure the construction quality and safety of production, so that the welding space frame can achieve the expected goal, in the welding, the length of the arc, distance and other factors will affect the weld quality.

 (3) Materials, machinery and equipment, and technological conditions. Different types of ball materials have different characteristics and specifications. An electric arc is used in welding, so the weld must meet the requirements of high strength, strong wear resistance, and high hardness. In an environment of large temperature differences, welding quality of the weld is one of the most important factors. Due to special reasons, some welding heads with high strength, and strong wear resistance but low hardness are easy to crack.

Welded ball space frame support node

Welded ball space frame support node

2.5 Welding ball space frame construction process control

(1) The positioning of the welding ball space frame according to the actual situation of the construction drawings and the actual situation of the project, control the axis of the center frame before welding, accurately adjust the horizontal error value with the latitude and weft meter, and the technical disclosure of the welder is done on the site.

(2) After installing the steel pipe, fix the central axis and location of the hole with a wooden hammer to keep it in a horizontal state; after the center line of the hole is stable, use the electric iron nail to close the upper end of the heat conductive plate. When the spiral rod rotates a certain angle, move the axial direction to the specified direction to ensure that the catheter is parallel to the weld in the vertical direction.

(3) After completing the initial circle positioning, start the straight arc joint. First of all, the corresponding bending moment point is measured vertically along the center line of the steel pipe with a radius of about 5m and a distance of about 3cm from the base surface. Then place the axis of the horizontal section parallel to the vertical axis and adjust the Angle value as the included Angle between the longitudinal and transverse sections of the weld. When the included Angle between the center line of the weld and the substrate is adjusted to 1:40, the relative position of the corresponding bending moment point should be 3~5m. Then in the welding arc before the electric iron alignment, to ensure that the welder operation is correct, and do a good record.

3 Precautions for the construction of welding ball space frame under a large temperature difference environment

3.1 Selection of welding equipment

For large-scale complicated engineering projects, the welded tools that are appropriate and easy to assemble are needed to combine or linked; for small engineering projects, it is required to be welded fast and uniformly and uniformly, and with high efficiency.

① Arc welding. The double-headed spiral pond is used to replace the direction device. It is characterized by simple structure and good welding, but it affects the quality of construction due to poor structure and leakage and air entry. At the same time, it also exists on the surface of the steel surface. Large damage; problems such as disconnection accidents and impact wear at high temperature and high humidity environments.

② Drive device. When the common cyclone is used to pull the workpiece, it is necessary to apply a certain pressure on the feed rod. At the same time need to use the electric hoist through the coupling and the workbench fixed connection to control the speed of movement; During the driving process, due to the gravity effect, the welding quality will be affected by large shaking and impact torque.

3.2 Construction monitoring measurement measurement

There are two main aspects of construction monitoring: one is the monitoring of actual on-site, and the other is conducting overall, systematic, and comprehensive analysis of engineering projects. Because the actual measurement work is done under real points. Therefore, you should choose different specifications to record data according to the specific situation; at the same time, it is important to note that in order to ensure that the measurement results are accurate and reliable, there is no error. It is necessary to use a certain amount of upper -level value and the internal control network cloth setting report. Real-time monitoring; finally adjust the parameters in the environmental conditions of the real point to improve the construction accuracy and efficiency. The environmental conditions of the construction site mainly include atmospheric temperature and the level of the site. At the same time, it is necessary to consider the distance between the engineering structure and the surrounding buildings and the location of the entrance and exit of the building as well as the traffic road conditions and other factors. Finally, it is necessary to ensure the accuracy of the control point position by rationally arranging the measuring area. During the arrangement, the welding of adjacent components should be minimized in order to reduce the stress level at the weld and make it small deformation and high surface smoothness for the purpose.

3.3 Welding ball space frame construction security guarantee

During the construction of the welding ball space frame, it is necessary to strictly abide by the relevant state regulations, and carry out safety protection on the construction site to avoid high-altitude cranes and falling objects. Use an electric hammer or wire rope to fix it before welding, and check whether the connector is firm and reliable after welding.

①Strictly implement the operating rules and regulations, and it is strictly prohibited to work without a license. In the construction process, it is prohibited to throw and lift at high altitudes in violation of the safety technical specifications.

② Welders must wear gloves and helmet masks; before welding, they should be fully checked before they can be used inside; low-carbon steel should not be used instead of other materials (such as aluminum plates) as cushion work surfaces) and long rods that are connected to long rods. Connect the bolt and pull lines, and it is strictly forbidden to fix it directly with high-carbon steel plates.

③ In order to ensure the quality of the project, ensure safety, provide strict technical training for the construction personnel, and organize relevant skills examinations and assessments to improve the quality level of all personnel at the site.

This article analyzes the construction of welding ball space frame structure through the study of high-temperature difference environment engineering projects, and the construction process and solution, the construction process and reasonable organization of various types of work, and construction precautions.

The verticality and rotation accuracy of the welding working surface in the large temperature difference environment is high. During construction, adjustments should be made according to the actual situation. Because welders are all high-altitude operations and are mostly outdoor construction workers. Therefore, the welding structure design should be more reasonable, and should take into account the length, diameter and shape of the weld and other factors, the selection of materials and size matching degree also need to be further optimized, to do a good job of long arc frame and short arc groove connection work, so that it can meet the needs of the project.