Since entering the 1990s, the development of steel structure buildings in China has been very rapid, especially the completion of some iconic high-rise buildings representing the city, which has opened a new page for the development of steel structures in China. For example, the 88-story, 420m-high Jin Mao Tower in Shanghai, which is the third tallest in the world, has been completed and is now in operation. Shanghai Pudong World Financial Center, which is said to be the world ’s highest height, has a 95-story 460m high and a building area of 310,000 m2. Construction is now being stepped up. The Dalian Presidential Building, which is invested by foreign investors, is being stepped up and is under construction, with a total of 95 floors. After its completion, it is said that its height will be among the best in the world.
The development of light steel structure is in full swing, especially in the construction of industrial plants. It can be seen 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 dozens of originals to more than 400 at present, and Baosteel in Shanghai alone has Nearly one hundred steel structure manufacturing plants. Under the great situation, how to make good use of the situation and grasp the quality of design and construction is a very urgent issue at present. This article intends to give a brief introduction to the advantages of light steel structure, material selection and design points, plastic design and the use of web buckling strength and skin effect, which will help readers have a more comprehensive understanding of light steel structure.
Light steel structure industry and its scope of application
The so-called light steel structure usually refers to the structure composed of the following steel materials:
① cold-formed thin-walled steel structure; ② hot-rolled light-weight steel structure; ③ welded or high-frequency welded light-weight steel structure; Structure composed of welded composite beams and welded composite columns.
1. Scope of application
According to the current situation in China, this structure has been widely used in single-storey industrial plants, multi-storey industrial plants, office buildings, and non-load-bearing components in high-rise buildings due to its wide utility and obvious advantages. For single-storey industrial plants, usually H-shaped steel, welded connection is used as beam and column, C-shaped or Z-shaped light steel plate is used for purlin, roofing system or floor system is made of pressed color steel plate, and the top can be poured Concrete and profiled steel plates can be used as rebar, and can also be equipped with rebar if necessary. The wall enclosure can also be made of single-layer or sandwich-laminated steel plates, which 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 spliced and installed on site by the factory. It takes only 45d to 2 months for a small industrial plant, and 8 to 12 if reinforced concrete is used. About months.
⑵Good comprehensive economic benefits: due to the short construction period, it can be put into use in advance to obtain investment benefits in advance; more due to the use of brightly colored color profiled steel plates, which are beautiful and gorgeous, improving the dynamic sense of the surrounding environment; because the building itself has light weight In general, there is no need to make pile foundation, which can save investment; due to the use of polystyrene foam sandwich board or veneer plus insulation cotton and other measures, the effect of heat preservation, heat insulation and chapter insulation is good. The color steel plate is made of galvanized as the substrate and silicone as the surface, processed by two-divided and two-baked, and the durability is also good. According to the current market price in China, the cost of the light steel structure is already 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 on which it relies on competition.
(3) Good seismic performance: Because the steel structure is a flexible structure and light weight, it can effectively reduce the earthquake response and disaster impact, which is very conducive to earthquake resistance. China is a country with many earthquake areas. Steel structures should be widely used in the construction of earthquake areas, which can greatly reduce earthquake disasters and casualties. The painful lessons of the Tangshan earthquake should be remembered. At present, Tianjin has officially launched light steel structure housing.
⑷ is suitable for demolition and relocation: once the owner is not satisfied with the factory site or unexpected changes in the external environment, the entire building can be demolished within a short period of time with minimal loss, and all of these 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 anticorrosion in recent years, the shortcomings of poor corrosion resistance of light steel structure have been better solved, making it obtained in industrial plants and civil facilities Widely used.
Steel structure is generally divided into ordinary steel structure and light steel structure, and there is no obvious boundary between them. The calculation rules are the same. The so-called light steel structure is generally a type of structure with a small structural load, a small structural rod and a thin wall thickness. The main load-bearing structure is generally a portal steel frame, roof frame and net frame. Because the load acting on the light steel structure is small, the internal force generated by the structural effect is generally small, which makes the strength of the structure often not a problem. However, due to the small cross-section of the member and the small cross-sectional inertia, the structural As the stiffness also decreases, the overall and local stability of the structure becomes the main problem that must be paid attention to in the design. This is the characteristic of light steel structure. Knowing this feature, we can take corresponding measures, such as adding support and bracing to meet the slenderness ratio requirements of the rod, and adding stiffeners to meet the local stability of the component.
A. Features of portal frame structure
Let's talk about our common portal frame structure. The structure of the portal frame light-weight house is generally composed of the main frame and the supporting system. The main frame includes: 1) the main load-bearing member-the portal frame; 2) the wall frame; 3) the purlin, etc .; the supporting system includes: 1) Vertical support between rigid frame columns; 2) Horizontal support between rigid frame beams; 3) Rigid tie bars and braces; 4) Corner bracing, etc .;
As can be seen from the above structure, the support system occupies a large proportion, which is also the characteristic of light steel structure.
The load acting on the portal frame house is generally: vertical load (structural weight, snow load, ash load), horizontal load (wind load, crane braking force), and seismic load (horizontal and vertical). Due to the light weight of the light steel structure, the response to earthquakes is also relatively light, which is very beneficial for earthquake resistance.
B. Judge whether the structure is suitable for light steel structure
Whether is light steel can be determined according to the size of the structure under load and the type of structure. 1. Floor and roof structure system: if it is a reinforced concrete floor or roof slab, the light steel structure cannot be used when the load is large; 2. Crane load: if there is a large tonnage crane, it does not meet the requirements of light steel structure; 3. Structural system: If the building is multi-story, or a relatively high single-story structure, it should not be designed according to light steel.
C. "Domestic rigid frame light house steel structure technical regulations" CECS102: 2002 scope of application
It is suitable for single- or multi-span solid web portal frame, with light roof and light exterior wall, no bridge crane or only medium and light working bridge crane with lifting capacity not more than 20t or Steel structure of single storey house with 3t suspended crane.
D. The role of each member of portal frame
Portal type rigid frame, the various loads on the building are mainly borne by it and transferred to the foundation. The connection between the rigid frame and the foundation has two forms: rigid connection and hinged connection. Generally, hinged connection should be used. When the horizontal load is large, the house When the height is high or the rigidity is high, rigid connection can also be adopted. The rigid frame column and the inclined beam are rigidly connected. The characteristic of the rigid frame is that the rigidity in the plane is large and the rigidity out of the plane is very small, which determines that it can bear the load parallel to the plane of the rigid frame when the horizontal load is acting, but it can't help the load of the vertical rigid frame.
Wall frame is mainly responsible for the weight of the wall and the horizontal load (wind load) acting on the wall, and indirectly transmits it to the main structure.
Purlins, bear the roof load, and pass it to the frame. The purlins are also connected to each rigid frame by bolts, together with the wall frame beams and the rigid frame to form a spatial structure.
Because the purlin and the wall frame beam are connected by bolts, the connection point is similar to the hinge, and because the slenderness ratio of the purlin and the wall frame beam is larger, under the action of parallel to the longitudinal load of the house, its transmission stiffness Limited, so it is necessary to set a certain number of rigid tie bars between the rigid frames of the roof. We just said that the rigidity outside the plane of the rigid frame is very small, so we must set up the inter-column support between the rigid frame columns and the horizontal support between the rigid frame beams to form a structure with sufficient rigidity;
Because the out-of-plane rigidity of purlins and wall frames is small, it is necessary to set braces (increasing support) to reduce the slenderness ratio in this direction;
For rigid frame inclined beams, the upper flange is generally compressed, the lower flange is tensioned, and the upper chord is generally not unstable due to the purlin connection. However, when the roof wind load is applied, the lower flange of the inclined beam may change its sign. The pressure becomes unstable, so it is necessary to install the support.
In addition, the portal frame and the foundation are connected by anchor bolts. When the shear force formed by the horizontal load is large, the bolts must bear these shear forces. We generally do not want the bolts to bear this part of the shear force. It does not consider its role, but sets the shear key between the column foot of the rigid frame and the foundation to bear this part of the shear force.
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