Development and problems of space frame structure application in industrial workshops (Part 2)

2. Quality problems of high-strength bolts: After the installation of the bolted ball node space frame in a certain project in Beijing, it was found that some bolts were broken. After inspection, it was found that the bolts themselves were broken due to improper processing. After several production space frame the manufacturer checked the bolts in stock and found that a large number of high-strength bolts produced by a certain factory had microcracks. The reason may be that the raw materials did not meet the design requirements when they entered the factory, or it may be a problem caused by improper heat treatment. According to the manufacturer’s report, many high-strength bolts with large diameters are found to be impenetrable after heat treatment. Tens of tons of tension in the bolt ball joint space frame are transmitted by a single bolt, so it is the key point, and its quality should arouse our special attention.
3. Rod bending during installation: Recently, several projects have had rod bending during the installation of the space according to the steel structure construction acceptance specification, the initial bending of the rod is allowed to be <1/1000, and the bending is generally greater than this number when observed by the naked eye. There are many reasons for bending, including design reasons and construction reasons. The performance of the bolted ball joint space frame is more prominent. The value of the calculated length of the rod, the slenderness ratio of the rod, the length error of the rod, the bending of the rod during transportation, the angle deviation of the ball hole, the installation sequence and the installation error, etc. Important factors affecting the bending of the member. According to the appearance of bending of the actual engineering rods, it is not in the column space frame unit being installed, but when the second span or the third span is installed, the rods in the first span (or grid or cone) are bent Phenomenon. This shows that it is not only directly caused by the deviation of the length of the member, but also caused by the rotation of the node due to factors such as the length of the member, the deviation of the perpendicularity and the error of the angle, so the contradiction has not been exposed before the surrounding nodes are not tightened.After analysis, when the general node is rotated by 7°, the bending of the rod exceeds the allowable bending standard value of the rod. Of course, the angle deviation of the ball hole itself will also cause the rod to bend. This situation should be in the assembly of the space frame cone or the Occurs across time. Another characteristic of bar bending is that when the stiffness of the bar is very different at the junction of the nodes, the bar with a relatively small stiffness is prone to bending. The rods should also be coordinated with each other, and the slenderness ratio should not be too large. For the free side of some structural forms, the value of the calculated length of the member should be determined through specific analysis. In order to keep the joints from deflecting, the deviation of the length of the rod and the deviation of the ball hole should be strictly controlled so that the deviation of the ball hole cannot exceed the allowable range, and the installation sequence also has a certain influence. The best way to tighten a bolt is to position it first, and the bolts on the same node should be tightened gradually several times in turn, so as to adjust each other so that the error will not be concentrated on a certain rod.

After the construction of the space frame of a joint factory building was completed, it was found that there were five weld cracks in the short steel column on the upper part of the central column. According to the actual situation and analysis, the main reasons for weld cracking are:

1. The spaceframe is decomposed into individual cones for processing. The size of a single cone is 2~3mm smaller after processing. Although the thickness of the gusset plate is thickened by 2mm, the deviation problem cannot be solved. The entire space frame consists of more than 2,400 single cones. Its accumulated error is still a large negative deviation.
2. The short steel column on the top of the column was originally designed to be tightly welded to the top of the support plate. However, during processing, the short steel column did not touch the bottom plate and was not welded. It can be inserted with fingers, which changed the original design. power transmission path. At the same time, the groove on the short steel pipe is cut with oxygen, and the cut is very uneven, with different widths. The thickness of the weld seam connected to the cross plate is generally only about 5mm, which is about half of the thickness of the weld seam compared with the original design. From the weld seam at the crack Look, some are only 2~3mm thick. What’s more serious is that the weld is only welded on one side of the gusset plate, and the surface of the steel pipe is not welded at all.
3. During the construction, the four sides of the bottom plate of the support are welded around, so that there is no room for movement between the support and the top of the column, and arbitrarily changing the original design will cause hidden dangers.
4. The original design is to use the concrete column slip form to raise the space frame to the design level. After changing to the strip hoisting method, no effective measures have been formulated in accordance with the requirements of the “Space Frame Structure Design and Construction Regulations”. The spatial position of the space frame nodes does not even have an independent support point, so there is no guarantee for the accuracy of the spatial geometric position of the space frame nodes. At the same time, there are two angle steel splicing between the unit body and the unit body at high altitude without even a single installation bolt positioning. In this way, the hoisting and splicing must be welded there when it is installed. If it cannot be installed, it will be dragged. Blocks, the assembled spaceframe can be clearly observed by the naked eye that the upper and lower string planes of the space frame are high, low, crooked and crooked. According to the calculation, the maximum downward deflection of the space frame span is 2.36cm, but the actual measurement results show that some column space frame arch upwards as high as 10.2cm, and the deflection lines are very messy without any rules. The size error of the column network is also very large, and some even exceed the allowable error by 10 times in the “Space frame Design and Construction Regulations”. Later, the space frame of the newly built workshop was improved, and the construction quality was very good according to the design requirements. In short, these problems in the development are worthy of attention. As long as all links work together to ensure the quality, the application of the grid can be promoted smoothly.