Abstract This paper briefly introduces the general situation of the light steel structure that is booming today. It gives a brief introduction to its scope of application, main advantages, material selection and attention in design. For post-buckling strength utilization, skin effect, etc. Corresponding to the differences in general steel structure design. It has a relatively complete understanding of the characteristics of light steel structures.
Key words light steel structure, technical characteristics, skinning effect
I. Introduction
Since the beginning of the 1990s, the development of China's steel structure has been very rapid, especially the construction of some of the iconic high-rise buildings on behalf of the city, which has opened a new page for the development of steel structure in China. For example, the 88-storey Shanghai and the 420-meter-high Jinmao Tower, which is the third-highest in the world, has been completed and is now operational. Shanghai Pudong World Financial Center, which is said to be the world's highest height, has a 95-storey height of 460m and a building area of ​​310,000 m2. It is now under construction. The Dalian Presidential Building, which is invested by foreign investors, is under construction, with a total of 95 floors. It is said that its height will be among the best in the world after its completion.
The development of light steel structures is even more in full swing, especially in the construction of industrial plants. From the substantial increase in the number of steel structure manufacturing and construction companies, for example, Shanghai's steel structure manufacturing and construction units have grown from the original dozens of companies to more than 400 now, and there is a single Shanghai Baosteel area. Nearly one hundred steel structure manufacturers. Under the good situation, how to take advantage of the situation and grasp the quality of design and construction is a very urgent issue. This paper intends to give a general introduction to the advantages of light steel structure, material selection and design points, plastic design, the use of web post-buckling strength and skin effect, which will help readers have a more comprehensive understanding of light steel structure. .
Second, light steel knot and its scope of application
The so-called light steel structure usually refers to the structure consisting of the following steel: 1 cold-formed thin-walled steel structure; 2 hot-rolled lightweight steel structure; 3 welded or high-frequency welded light steel structure; 4 light-duty steel pipe structure; 5 thinner wall A structure formed by welding a composite beam and welding a combined column.
1 Scope of application
According to the current situation in China, this structure has been widely used in single-story industrial plants, multi-story industrial plants, office buildings and non-load-bearing components in high-rise buildings due to its wide use and obvious advantages. For single-storey industrial plants, H-beams are usually used as welded beams and columns, and C-shaped or Z-shaped light steel plates are used as the purlins. The roof system or floor system is made of pressed color steel plates as the surface layer. Concrete, profiled steel can be used as reinforcement, and if necessary, with reinforcement. Wall cladding can also be used with single-layer or laminated laminated steel panels, and the interior of the sandwich panels can be filled with various insulation layers.
2 main advantages
(1) Short construction period: The biggest advantage of light steel structure is that all components can be installed on site by factory. It takes only 45d to 2 months for the smaller industrial plants, and 8-12 for reinforced concrete buildings. About a month.
(2) Good comprehensive economic benefits: Due to the short construction period, it can be put into use in advance, and the investment benefits can be obtained in advance; moreover, the use of colorful color profiled steel plates is beautiful and gorgeous, improving the dynamics of the surrounding environment; Under normal circumstances, it is not necessary to do the pile foundation, which can save investment. Because of the measures such as polystyrene foam sandwich panel or single board plus insulation cotton, the effects of heat preservation, heat insulation and separate chapters are good. The color steel plate is made of galvanized as the substrate and silicone as the surface. It is processed by two-times and two-bake process, and the durability is also good. According to the current market price in China, the cost of the light steel structure is lower than that of the reinforced concrete structure. The greater the span of the plant, the more obvious its advantages, which is also a major advantage of its competition.
(3) Good seismic performance: Because the steel structure is flexible structure and light in weight, it can effectively reduce the seismic response and the degree of disaster impact, which is very beneficial to earthquake resistance. China is a country with many earthquake zones. It is necessary to promote the application of steel structures in earthquake-stricken buildings, which will greatly reduce earthquake disasters and casualties. The painful lessons of the Tangshan earthquake should be remembered. At present, Tianjin has officially launched a light steel structure residential building.
(4) It is advisable to dismantle and relocate: once the owner is dissatisfied with the site or unexpected changes in the external environment, the entire building can be demolished in a short period of time with minimal losses, all of which are not available in reinforced concrete buildings.
It is precisely because of the many advantages of light steel structure, and with the continuous emergence of new products of fire prevention and anti-corrosion in recent years, it has solved the shortcomings of poor corrosion resistance of light steel structure, which has obtained it in industrial plants and civil facilities. A wide range of applications.
Third, the material selection and design considerations
The light steel structure is a derivative structure of ordinary steel structure, and its basic calculation theory is basically the same as the latter. For details, please refer to Shanghai Standard DBJ08-68-97 "Light Steel Structure Design Regulations" and China Engineering Construction Standardization Association Standard CECS102:98 "Gate Steel Frame Light House Steel Structure Technical Regulations", only highlighting several points here:
(1) “H†shaped steel should be used preferentially on the materials, which is reasonable in force, easy to splicing and easy to process. For the load-bearing structure, 16Mn, 15MnV or 15MnV steel in Q235 steel and low-alloy steel should be used, but it should be noted that the carbon content of Q235-A steel is not a delivery condition, and the weldability is not guaranteed, so it is not suitable for welding structure. For beam flanges and columns with a thickness greater than 25 mm, Q235-BF should not be used for beam-column joints welded in the field. Q235-B or Q235-Bb should be used as much as possible. For particularly important structures, Q235-C or Q235-D should be used.
(2) The design index of Q235 steel with thickness of 17~40mm is 5Mpa (Shanghai standard) higher than the current value specified in GBJ17-88 "Steel Structure Design Specification", and the weld strength is also adjusted accordingly.
(3) Considering the factors of technological advancement, the wall thickness of the main stressed members is reduced, that is, the wall thickness of the main stressed members in the current GBJ18-97 "Technical Specifications for Cold-formed Thin-walled Steel Structures" is not less than 2 mm. Small 1.5mm, frame beam member is not less than 3mm.
(4) Under the action of wind load, the lateral displacement of the portal frame is 1/150 of the height of the column top according to GBJ18-87 Technical Specification for Cold-Formed Thin-Wall Steel Structure. However, in these two procedures, the refinement rules have been made and adjusted accordingly, but the specific values ​​are not the same. Designers should pay attention when using it.
(5) When designing rigid frames, trusses and purlins, the adverse effects of changes in internal forces caused by wind suction should be considered. At this time, the permanent load partial coefficient is taken as 1.0. This regulation is mainly considered when the rigid frame, roof truss and purlins are designed to be lighter in the roofing material; if the wind suction is applied, the internal force of the component will change, and the lever will become the pressure bar. When the internal force changes, the permanent load acts as a load shedding. The Hefei City will take the permanent load partial factor to 1.2, which will result in a decrease in structural reliability and lead to unsafe factors.
Fourth, the plastic design of the structure or component and the strength of the web after buckling
The lightweight steel structure allows for a plastic design, but only for solid end beams, continuous beams, and single and 2-layer frame structures that are not directly subjected to dynamic loads. When a plastic-designed structure or member is designed according to the limit state of the bearing capacity, the design value of the load should be used, considering the development of the plasticity in the section of the component and the resulting internal force redistribution, using the plastic hinge theory considering the P-△ effect or Simple plastic hinge theory for internal force analysis. In plastic design, the steel and joint strength design values ​​shall be multiplied by a reduction factor of 0.9 in accordance with the values ​​specified in the relevant provisions of the Code. The aspect ratio of the plastic design section plate should also comply with the relevant regulations.
The web of the "work" section on the portal frame allows for consideration of the degree of buckling. Below we try to explain the distribution of the mid-surface stress after the uniaxial compression of the four sides, to explain why the plate can continue to bear after buckling. And the bearing capacity can be significantly improved. As shown in Figure 1 after the plate is bent, as long as the four sides of the plate remain straight, the compressive stress distribution on the load side is non-uniformly distributed, the stress on both sides is higher, and the intermediate stress is lower; the non-load side produces stress perpendicular to the edge line, and the middle part is pulled. stress. Due to the tensile stress, the development of the buckling deformation of the plate is limited, and the rigidity of the plate is improved, so that the plate can continue to bear after buckling. Carmen first proposed the concept of effective width, that is, the saddle-shaped stress distribution diagram of the plate under the pressure is replaced by two equivalent stress patterns instead of L (Fig. 2). The stress level of the equivalent stress pattern is fyc.
Fig.14 Distribution of the mid-surface stress after buckling of the simply supported plateFig. 2
Pu=fybet
Pu=∫σ(x)dx
Where: Pu is the ultimate bearing capacity of the plate; fy is the yield strength of the steel; be is the effective section width; t is the thickness of the plate.
When Pu is determined by theoretical analysis and experiment, the effective width can be used to express the ultimate strength after buckling of the compression plate, and then the effective section is used to consider the influence of local buckling of the plate. The structural design using the strength of the plate surface after buckling was first seen in the aircraft structure, because the aircraft structure design aims to reduce the structural weight under the premise of ensuring a certain degree of reliability. In the steel structure design of ordinary house construction, the plate is generally not allowed to be partially unstable. One of the characteristics of cold-formed thin-walled steel is that the wall is thin and the wall thickness is not more than 6mm. It is processed into a wide section with very little material to improve the section stiffness and overall stability of the member. Breaking through the steel structure design specification (GBJ17-88), the specification of the width-to-thickness ratio of the plate allows the plate to be partially unstable, and then the strength after buckling is improved.
The light steel structure portal frame is the main load-bearing structure, generally consisting of columns and beams with real-shaped variable cross-section. The portal frame can be formed on a single slope, a double slope and a multi-slope, and the intermediate column of the multi-span building is often connected with a swinging column. The webs of the steel frame members of the portal frame are mainly sheared, and the bending resistance is not as effective as the flange. The height of the web is increased to make the flange.
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