Discussion on the practice of residual stress in steel structure
The method of detection, the method of eliminating residual stress, and the matters that should be noted in planning and design. C 0 刖 目 General construction engineers are relatively new to the residual stress of steel structures. The main cause of residual stress is the cooling rate of steel. + The same. For example, in the rolling process of thick steel, the temperature of the steel bloom (SlaborBloom) after heating exceeds 1,200 * C, then it is rolled and cooled on one side, and gradually reduced to normal temperature 20 * C to 35 * C. The cooling rate is the fastest on the surface of the steel material, and the slowest inside. When the surface suddenly cools to normal temperature, the inside of the steel may still be hot and hot. Therefore, after the cooling is completed, there is residual pressure on the surface of the steel material, and there is residual tension inside the steel material. The stress distribution is like a contour map of topographic survey. As shown. (-> Residual pressure is warm. In addition, due to the easy accumulation of slag in the steel embryo, the center line cracks derived from the uncoordinated rolling of the rollers, and the concentration of impurities (such as phosphorus and sulfur) in the steel material to the high temperature area Segregation is very easy to contribute to the lamination of thick steel plates. This kind of interlayer phenomenon and the aforementioned internal residual tension make the central area of ​​the thick plate become a weak zone. As shown. Interlayer and segregation of a thick steel plate As far as hot-emulsion steel is concerned, relatively thick steel is prone to residual stress, especially at the junction of the wing plate and the web, its residual tension is the largest, commonly known as the "core" vulnerable area (Core ), As shown. according to. The residual stress of cross-section fracture leads to the damage of cross-section cracking. The residual stress is eliminated because the main reason for the residual stress is the different cooling rate (CoolingRate) after the steel material is heated. Therefore, if the cooling rate of the steel material can be made as consistent as possible during the construction process, the residual stress can be eliminated naturally. Therefore, the manufacturing and installation specifications of the steel structure clearly stipulate that the electric welding of thick steel materials must deal with preheating before welding and post-welding heat. The purpose of post-welding heat is to delay the cooling time of the steel surface to make the surface cooling rate as much as possible. Consistent with internal. The most commonly used post-heating method is to heat the gas fire row and gradually reduce its temperature. Only in this way can the residual stress inside the steel material be eliminated. ButtJoint welding of some thick steel materials is still used. The asbestos is coated with m asbestos to allow the steel material to cool down slowly after welding to eliminate the residual stress caused by high temperature welding. Especially in cold regions, because the temperature is too low, although the thickness of the steel material is not very thick, but the material of the steel material itself is very brittle at low temperature, if the father generates additional residual stress during welding, the phenomenon of cracking It will be more serious. 4 Matters needing attention in planning and design In view of the fact that thick steel materials have obvious residual stresses, some steel structure specifications also make the following provisions: The residual stress of steel structures is discussed in practice. * 1) Thick steel materials should not be designed as Tension members; 2) The welding of thick steel materials should not be welded by electric welding, and bolted joints should be used as much as possible. This article provides that it is more difficult for general business owners to accept, because when thick steel materials are bolted together, it will increase the weight of the steel plates and bolts for joining. Designers must persuade professional technical arguments. The increased weight varies depending on the design nature of the case, and is generally about 2% to 4% of the total weight of the steel structure; 3) If ultra-thick steel is used for electric welding, be sure to try to eliminate its residual stress. The method of elimination can be handled by the above or similar principles; 4) The so-called "ultra-thick steel", the thickness should be based on how thick, because the process method of each steel plant is involved, the residual stress value of each product may be endless the same. However, the "American Steel Construction Association" AISC has more clear regulations, that is, W14X233 to W14X730 steel and W12X210 to W12x336 (Group4 and Group5) are classified as ultra-thick steel. In other words, the thickness of the wing plate above 13/4 inch (82.55mm) is considered as ultra-thick section steel, and its design and construction must not be careless. 5Effect of electric welding on the length of the steel material After the electric welding is completed, due to the large amount of heat input during the electric welding process, there is residual stress in the steel material after cooling, which causes the length of the steel member to shrink. According to empirical statistics, after the continuous welding of the wing plate and the web is completed, the shortened length is proportional to the length of the original component, which is about. 3mmxL, as shown in (a). In the picture, L is the length of the section steel in meters. If welded stiffeners are added to the webs of combined H-beams or hot-emulsion H-beams, the greater the number of stiffeners, the shorter the length of the steel. Generally, when the stiffening plate is less than 3 pieces, the length of the member is not significantly shortened. When there are more than 3 stiffeners, the length of the component is shortened by about 0.3mm for each piece of stiffener welded, as shown in (b). The length reduction of the combined section steel is welded between the two steel plates by full penetration, and the length after joining will also be significantly shortened. The relevant relationship is shown below. The shortened length of the steel plate after butt welding is 6 'angle angle of the combined H-shaped steel. The combined H-shaped steel is still very common in Taiwan. After continuous welding of the wing plate and the web, in addition to the length of the aforementioned member will be shortened, due to the relationship of residual stress, the wing plate will also have a significant angular variation, and the thinner the wing thickness, the greater the angular variation, and the more welding The larger the multi-time angle variable, the size of the deformation angle is approximately as shown according to empirical statistics. 0 = deformation angle (radian) W = electrode weight per unit length of electric welding (g / cm) / = steel plate thickness (mm) combined H-shaped steel beam wing plate angle variable If the above angle variable exceeds the allowable tolerance, the combined H-shaped steel It must be reshaped before it can be used. The more advanced construction method adopts the pre-bent wing plate construction method. The wing plate is pre-compressed into reverse deformation according to the estimated angle variable. After continuous welding, the wing plate will be deformed due to the influence of residual stress, and the level will be automatically restored. status. The experience value of the reverse pre-bending is shown as 0. 7 The shrinkage deformation of the high-rise steel frame welding When the high-rise steel structure is installed, the site welding of the beam-column joints will shorten the length of the steel beam, and it will shrink up layer by layer, and it must be adjusted. Otherwise, except for the difficulty of installation, the structure will Presents the phenomenon of narrow up and wide down. The welding of the steel column will also shorten the height of the column, and the higher the accumulation, the more serious the shortening of the steel column, as shown in the figure. These phenomena are caused by residual stress after welding. Slotting angle 45 degrees 0 Wing plate pre-bending angle variable statistical chart Welding shrinkage of the vertical beam-column system 8 Conclusion Although the content described in this article is a natural phenomenon in metallurgy or materials science, many are empirical values, but the impact It is indeed the field of structure Ding 1 Cheng. "Jian r this" and the research and statistics of the structure of TiK is still very lacking, and relevant experts are still invited to discuss the correction. Uv Light Therapy Lamp,Red Light Therapy For Sad,Nature Bright Bright Light Therapy,Best Bright Light Therapy Lamp Shenzhen Bonliter Optoelectronic Co., Ltd. , https://www.beautyredlight.com