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2023.01.13

Quality Control Technology of Large Steel Structure Truss Fabrication

steel pipe truss structural roof

1. Overview Project

The main structure of the all-weather dock warehouse is a steel structure truss, 108 meters long, 47 meters wide and 36 meters high. The main structure adopts a pipe truss structure, which is different from the traditional open-section (H-beam and I-beam) steel truss. In comparison, the section of the tube truss structure is more evenly distributed around the neutral axis, and the section has good compressive and torsional bearing capacity and greater stiffness at the same time. The structure is simple, the appearance is beautiful, and the economic effect is good, but the various rods in the pipe truss intersect in the form of intersecting lines, and the shape of the ends of the rods is complicated, so the manufacturing difficulty is greatly increased.

2. the production control points

(1) Preprocessing

The pretreatment process refers to the process of carrying out surface shot blasting or sandblasting to remove rust and applying a layer of protective primer before steel processing. The purpose is to improve the corrosion resistance of steel structural parts, improve the fatigue resistance of steel plates, and prolong their service life. In addition to controlling the size and type of sand grains in pretreatment, the focus of control should be on environmental conditions, because two specific environmental conditions need to be met in pretreatment: (1) the relative humidity of the environment should not be greater than 85%; (2) the temperature of the steel plate It should be more than 3°C above the dew point temperature. Therefore, the focus of pretreatment is to monitor the environmental conditions. Only when these two conditions are met simultaneously, can the quality of steel rust removal be ensured. In the absence of a constant temperature and humidity painting room, the winter construction can only be done between ten o’clock in the morning and three o’clock in the afternoon when the weather is fine. During this time, the humidity is low and the temperature is relatively high, which is also the best for steel pretreatment construction. opportunity. After the rust removal reaches the smoothness and roughness required by the drawing, it is necessary to apply a sealing primer within 4 hours to prevent the steel from rusting back. Due to the limited time period that can be applied every day, the one-time rust removal rate is also significant.

(2) Cutting

The most important thing before cutting is to further deepen the design drawing. The pipe trusses are intersected by intersecting lines, the shape and size of the end faces are complex and changeable, and the length of each part is different, so the steel structure should be strengthened before cutting. 3D modeling of grid drawings, and accurate calculation of the specific size of each part. In actual cutting and blanking, mechanical numerical control cutting is used instead of manual cutting, which can greatly improve the productivity and the precision of cutting and blanking. Check the condition of the equipment before cutting and unloading. When cutting, the parts to be cut should be sampled. At least 10% of the cutting amount should be sampled every day. Once the size deviation of the parts to be cut is found to be too large, the cutting should be stopped immediately and the problem should be found to eliminate the problem. Only then can you continue to cut to ensure the accuracy of the part size.

(3) Assembly of components

Assembly is a process in which components are assembled from parts into components, and the technical requirements are high. It is not simply to assemble the parts into a whole according to the drawings but to comprehensively consider the size of the parts and the overall size of the components. The parts are assembled into a whole. Below I will describe the main points that need to be controlled in the assembly.

 

1)Tire frame control

Steel structure trusses are all spatial three-dimensional structures. The precise positioning of the spatial structure assembly requires a tire frame as support. The tire frame must not only have sufficient rigidity and strength but also have accurate dimensions. In the production of the tire frame, use the heavy vertical line to determine the vertical position, and the elastic line to locate the horizontal position size. After drawing the money, repeatedly measure the size of the tire frame with a spirit level and a total station, and make sure that the tire frame is accurate.

2)Control of assembly clearance

No matter how precise the cutting size of the instrument is, it is impossible to be 100% accurate. During assembly, there will be more or less mismatches. If the size of the part is long, it can be cut off according to the line and pipeline arc, but in the steel structure space frame, The assembly problem encountered in the production of the frame is often the case that the assembly gap is too large. There are two ways to control the excessive gap: one is to “borrow” the gap on both sides, that is, move the rod a little to the side where the gap is small, so that the two sides are “borrowed”. The size of the gap is similar so that the gap is reduced by half; another method is to “borrow” a little gap through the parts that need to be assembled later, that is, to “borrow” the front part by controlling the gap between the parts that need to be assembled later and the parts that have been transferred before. Clearance.

3)There is a margin for the overall size of the component

One of the most important dimensional controls of steel structures is welding deformation, and the shrinkage after welding will affect the overall size of the components, and the shrinkage of rods with different lengths and sizes will be different after welding. For each additional intersection of the chord rods, an extra 1mm of allowance should be reserved. After welding, the 1mm allowance will shrink to obtain the desired length. Moreover, the margin should be more than less. The excess margin can be cut off after welding. If the size is short, the gap can only be “neutralized” by subsequent parts, so that the weld will become larger and affect the welding. time and increased costs.

4)Column base plate control

The lower column of the steel structure truss is composed of four seamless steel tubes with a diameter of 500mm. There is a bottom plate under each steel tube to connect the embedded bolts so that the column can be tightly anchored to the ground. Since the bolts are pre-buried in civil engineering, in order to avoid accumulated errors, the positions of the pre-buried bolts in civil engineering should be measured in advance and numbered and recorded. , and then keep the base plate number consistent with the corresponding embedded bolts, and also unify with the corresponding columns when the base plate is assembled, so as to reduce the accumulated error as much as possible. After the holes on the column base plate correspond to the embedded bolts, another thing that needs to be controlled is the flatness of the four base plates. This is like a chair. The chair will not tilt unless its four feet are on the same plane. In the same way, the four bottom plates of the column also need a plane to make them coplanar. In actual operation, first drill the four The bottom plate of the good hole is fixed on a flat steel plate according to the position and size of the column, and then the steel plate is lifted. The four bottom plates correspond to the steel pipes on the column, the center of the bottom plate and the center of the column coincide, and then let the four bottom plates They are all perpendicular to the corresponding pipes on the column. Finally, the stiffening plate is installed and fixed. After knocking off the flat steel plate, the assembly of the column bottom plate is completed.

5)Column Corbel Control

The corbel is a component located on the column to support the truss between the columns. On each column with a height of 30.5 meters, there are more than 20 corbels. Each inter-column truss needs 8 corbels to support. The spacing error needs to be controlled within 3mm after welding. The error of 3mm after welding means that the assembly error should be smaller. In order to assemble accurately, the assembly of the corbel should be completed after the welding of the column is completed, so as to prevent the corbel caused by the shrinkage of the column size. Location changes. Before assembling the corbel, it is necessary to check the size of the corbel, and it can continue to be used after the verification is passed. When assembling the corbel, first determine the center line of each column, and then measure the position of the corbel, and then measure and check it repeatedly. After confirmation, install the corbel and point it.

Steel structural truss

6)Truss end plate size control

The main control of the truss end is the flatness of the end plate and the length of the entire truss after the end plate is installed. This control method is similar to the flatness of the column backing plate. One is fixed and the other can be moved according to the length of the truss. Each shelf is composed of two small “columns”, and several corbels are welded on the “columns” according to the same size as the columns of the steel structure grid for pre-assembly. In this way, the length of the truss and the levelness of the end plate can be well controlled by simulating the method of pre-assembling and moving one end of the shelf.

7)Purlin pallet size control

The steel structure pipe truss roof and wall purlins and purlin brackets are connected by bolts with a diameter of 16mm, and the bolt holes are 17.5mm, so the left and right errors left to the purlin brackets are only 1.5mm. The diameters of the pipe trusses where different purlins are located are also different, which brings a lot of trouble to the assembly of the purlins. In order to control the position and size of the purlin support more accurately, the purlin support on the wall adopts different sizes to match the diameter of the pipe truss, so that the purlin support plate of the whole wall is kept on the same plane as accurately as possible; Set up the tire frame to control the camber of the roof truss, the camber of the roof truss is kept the same, and the purlin support above is naturally controlled in a plane; after controlling the flatness of the purlin support, the most important and most difficult thing is the purlin support. Spacing, among the 135 components made of steel structure grids, there are on average more than twenty purlin pallets per component. Among these more than 3,000 purlin pallets, the control of the purlin pallet spacing is only measured one by one. Checking can ensure the accuracy of the spacing. Although this work is tedious, as long as you are careful enough, you can still control it well.

8)Welding control

The most important thing to control in welding is to control welding deformation. Welding deformation is often unavoidable in the welding process, which will affect the accuracy of welding dimensions and the strength of welded joints. Once the control is not good, it will take a lot of manpower and material resources. Repair and calibration, in severe cases, the entire component will be scrapped. Selecting a reasonable smooth welding is a good measure to control the welding deformation, that is to arrange welding in different positions of the structure in advance, especially the staggered welding on both sides of the neutral axis, so as to reduce the deformation on both sides, such as the butt joint of the steel truss chord. , first select a welding point and weld it symmetrically from two sides to the other side. Another method for controlling deformation is the support method, that is, some auxiliary members are used to support the assembled components, and then they are removed after welding. For example, in the welding of trusses, the support method is the most effective way to control the change of the center distance of the chord. measure. The last method is the reverse deformation method, that is, the method of deliberately biasing the weldment before welding uses the shrinkage force to shrink the component to the required position after welding. This method is not easy to control, and is generally used in some structures with low precision requirements. .

Although the form of the steel structure truss is relatively complex, through reasonable measures and methods, the quality control can still be relatively high.