The characteristics of steel structure:
- steel structure light weight
- high reliability of steel structure work
- steel vibration (shock), impact resistance is good
- a high degree of industrialization of steel structure manufacturing
- steel structure can be accurately and quickly assembled
- easy to make a seal structure
- steel structure easy to corrosion
- steel structure fire resistance is poor
Grade and properties of steel for common steel structure:
- Carbon structural steel: Q195, Q215, Q235, Q255, Q275, etc.
- Low alloy high strength structural steel
- Quality carbon structural steel and alloy structural steel
- Special purpose steel
Steel structure material selection principles:
The material selection principle of steel structure is to ensure the bearing capacity of the load-bearing structure and to prevent brittle failure under certain conditions, According to the importance of the structure, load characteristics, structure form, stress state, connection method, steel thickness and working environment, etc.
The four types of steel proposed in Code for Design of Steel Structures GB50017-2003 are “appropriate” models, which are the first choice when conditions permit. The use of other types is not prohibited, as long as the steel used meets the requirements of the specification.
The main steel structure technical content:
- High-rise steel structure technology. According to the building height and design requirements, frame, frame support, tube and mega frame structures are adopted respectively, and the components can be steel, steel reinforced concrete or concrete filled steel tube. The steel components are light, light and good ductility. They can be welded or rolled steel, which is suitable for super-high buildings. The rigid reinforced concrete member has high rigidity and good fireproof performance, which is suitable for middle and high-rise buildings or bottom structures. Concrete-filled steel tube construction is simple, only used for column structure.
- Space steel structure technology. The space steel structure has light weight, large rigidity, beautiful appearance and fast construction speed. The spherical node plate space truss, multi-layer variable cross-section space truss and reticulated shell with steel pipe members are the most widely used space steel structures in China. The utility model has the advantages of large spatial stiffness and low steel consumption, and can provide complete CAD in design, construction and inspection procedures. In addition to the space structure, there are long-span suspension structure, cable-membrane structure and so on.
- Light steel structure technology. Accompanied by light-colored steel wall and roof envelope made of a new structural form. Large section thin-wall H-section wall beams and roof purlins welded or rolled by steel plates over 5mm, Made of round steel flexible support system and high-strength bolt connection composed of light steel structure system, column spacing from 6m to 9m, span up to 30m or more, up to ten meters high, and light crane can be installed. Steel weight 20 ~ 30kg / m2. Now there are standardized design procedures and specialized production enterprises, product quality is good, fast installation, light weight, less investment, construction is not subject to seasonal restrictions, suitable for various light industrial plant.
- Steel concrete composite structure technology. In recent years, the application of steel-concrete composite structure, which is composed of beam and column bearing structure of steel or steel management and concrete members, has been expanding. Combined structure has the advantages of both steel and concrete. It has high overall strength, good rigidity and good seismic performance. When encased concrete structure, it has better fire resistance and corrosion resistance. Generally, the steel consumption of composite structural members can be reduced by 15-20%. The composite floor and concrete filled steel tubular members also have the advantages of less or no formwork support, convenient and rapid construction, and have great potential for popularization. It is suitable for frame beams, columns and floors of multi-storey or high-rise buildings with large loads, industrial building columns and floor, etc.
- High strength bolt connection and welding technology. High strength bolt is to transfer stress through friction, which consists of bolt, nut and washer. The high-strength bolt connection has the advantages of simple construction, flexible dismantling, high bearing capacity, good fatigue resistance and self-locking, high safety, etc. In engineering, it has replaced riveting and partial welding and become the main connection means in the manufacture and installation of steel structure. Automatic multiwire submerged arc welding should be used for thick plates made in the workshop, and nozzle electroslag welding technology should be adopted for box-column separators. The semi-automatic welding technology, gas shielded flux cored wire and self shielded flux core wire technology should be adopted in field installation and construction.
- Steel structure protection technology. Steel structure protection includes fire prevention, corrosion, rust, generally used in the fire retardant coating treatment without rust treatment, but in the construction of corrosive gas still need corrosion treatment. There are many kinds of fire-retardant coatings in China, such as TN series, MC-10, etc. Among them, MG-10 fire-retardant coating has alkyd enamel paint, chlorinated rubber paint, fluorine rubber paint and chlorosulfonated paint. Suitable coating and coating thickness should be selected according to steel structure type, fire resistance grade and environmental requirements.
Steel structure goals and measures:
Steel structure engineering involves a wide range of technical difficulties, in the promotion and application must follow the national and industry standards. The administrative departments in charge of construction should pay attention to the construction of steel structure engineering specialization stage, organize the training of quality inspection team, and summarize the work practice and the application of new technology in time. Colleges and universities, design departments and construction enterprises should speed up the steel structure engineering and technical personnel training, popularize mature steel structure CAD. Mass academic groups should cooperate with the development of steel structure technology, widely carry out academic exchanges and training activities at home and abroad, and actively put the overall level of the steel structure design, production and construction installation technology, in the near future can be rewarded.
Connection method of steel structure:
The connection method of steel structure has three kinds of welding seam connection, bolt connection and rivet connection.
Welding seam connection
The welding joint is that the heat generated by the arc makes the welding rod and the welding parts melt locally, then the cooling condenses into the welding seam, thus the welding components become a whole.
The utility model has the advantages that the section of the member is not weakened, the steel is saved, the structure is simple, the manufacture is convenient, the connection rigidity is big, the sealing performance is good, the automatic operation is easy to be adopted under certain conditions, and the production efficiency is high.
Disadvantages: The heat affected zone of steel near the weld may be brittle in some parts of the steel due to high welding temperature; The uneven distribution of high temperature and cooling of steel in the welding process results in welding residual stress and residual deformation of the structure, which affects the bearing capacity, stiffness and service performance of structure. Because of the large stiffness of welded structure, the local cracks are easy to spread to the whole structure once they occur, especially brittle fracture at low temperature. Because of the poor ductility and toughness of the welded joint, defects may occur during welding and the fatigue strength decreases.
The bolt connection is to connect the connector into a whole through the bolt fastener. Bolt connection points ordinary bolt connection and high strength bolt connection two kinds.
The utility model has the advantages of simple construction technology, convenient installation, especially suitable for installation and connection on site, convenient disassembly, and suitable for assembling and disassembling structures and temporary connection.
Disadvantages: need to open holes in the plate and assembly hole, increase the manufacturing workload, and the manufacturing precision requirements; The bolt hole also weakens the section of the member, and the joints often need to lap each other or add auxiliary connecting plate (or angle steel), so the structure is more complicated and more expensive steel.
The rivet connection is to make a semi-circular precast nail head rivet on one end, and insert the nail rod into the nail hole of the connector after burning red, and then use the rive gun to make the other end rivet into a nail head to make connection tight.
Advantages: riveting force transmission is reliable, plastic, toughness are good, the quality is easy to check and guarantee, can be used for heavy and direct bearing dynamic load structure.
Disadvantages: riveting process complex, manufacturing labor and materials, and high labor intensity, it has been basically replaced by welding and high-strength bolt connection.
(I) Welding methods
The common welding method of steel structure is arc welding, including manual arc welding and automatic or semi-automatic arc welding as well as gas shielded welding.
Manual arc welding is the most commonly used welding method in steel structure, its equipment is simple, flexible and convenient operation. But the working condition is poor, the production efficiency is lower than automatic or semi-automatic welding, the variability of the weld quality is large, to some extent, depends on the level of the welder.
Automatic welding has stable weld quality, less internal defects, good plasticity, good impact toughness, suitable for welding long direct weld. Semi-automatic welding is suitable for welding curve or arbitrary shape weld because of manual operation. For automatic and semi-automatic welding, the welding wire and flux adapted to the main metal shall be adopted. The wire shall conform to the provisions of the national standard, and the flux shall be determined according to the welding process requirements.
Gas shielded welding is to use inert gas (or CO2) gas as the arc protection medium, so that molten metal and air isolation, in order to maintain the stability of the welding process. GMAW has the advantages of concentrated arc heating, fast welding speed and large penetration, so the weld strength is higher than that of manual welding. And good plasticity and corrosion resistance, suitable for thick steel plate welding.
(II) Form of weld
According to the position of the connected components, the welding joints can be divided into four types: docking, lap, T-shaped connection and corner connection. There are two basic types of welds for these joints: and fillet welds. In the specific application, the connection should be selected according to the stress situation, combined with manufacturing, installation and welding conditions.
(III) Structure of the weld
(1) Welding seam
The welding joint has the advantages of direct force transmission, smooth force transmission and no significant stress concentration, so it is suitable for the connection of components under static and dynamic loads. However, due to the higher quality requirements of the weld, welding gap between the welding parts is more stringent, generally used in factory manufacturing connection.
(2) Fillet weld
Fillet weld form: fillet welds according to its length direction and the direction of the external force is different, It can be divided into side fillet weld which is parallel to the force acting direction, oblique fillet welded which is perpendicular to the direction of force acting on the front side and the force working direction intersecting, and circumferential weld.
The fillet weld sections are divided into common type, flat slope type and deep penetration type. The HF in the figure is called the leg size of the fillet weld. The ratio of welding leg of ordinary section is 1: 1, which is similar to isosceles right triangle. The bending of the transmission line is severe, so the stress concentration is serious. For the structure that directly bears dynamic load, in order to make the force transmission smooth, the front fillet weld should adopt the flat slope type (the long side along the internal force direction) of the two fillet edge size ratio of 1: 1.5. The ratio of 1: 1 deep penetration should be used for side fillet weld.
(I) Structure of ordinary bolt connection
(1) the form and specification of ordinary bolts
The common form of the steel structure is the large hexagonal head type, its code name with the letter M and nominal diameter (mm). M18, M20, M22, M24 are commonly used in engineering. According to the international standard, the bolt unified with the bolt performance grade to express, such as “4.6,” “8.8” and so on. The number before the decimal point indicates the minimum tensile strength of the bolt material, such as “4” for 400N / mm2, “8” for 800N / mm 2. The numbers after the decimal point (0.6, 0.8) indicate the yield strength ratio of the bolt material, that is, the ratio between the yield point and the minimum tensile strength.
According to the processing precision of bolts, ordinary bolts are divided into three levels: A, B and C.
Grade A and B bolts (refined bolts) are made of grade 8.8 steel, turned by machine tools, The surface is smooth, the size is accurate, and the hole of class I (that is, the bolt hole is drilled on the assembled component or expanded, the hole wall is smooth and the holes are accurate). Because of its high processing precision, close contact with the hole wall, its connection deformation is small and its mechanical performance is good, and it can be used to bear large shear and tension connections. However, the manufacture and installation of labor, high cost, it is less used in the steel structure.
Grade C bolts (crude bolts) are made of grade 4.6 or 4.8 steel, rough machining, inaccurate dimensions, and only class II holes are required (that is, bolt holes in a single part or without die drilling. General hole diameter than bolt rod diameter 1 ~ 2mm). When transferring shear, the connection deformation is large, but the performance of transferring tension is good, the operation needs no special equipment and the cost is low. It is commonly used in bolted connections subjected to tensile forces and in secondary shear connections in structures subjected to static or indirect dynamic loads.
(2) the arrangement of ordinary bolt connection
The arrangement of bolts should be simple, uniform and compact, meet the force requirements, reasonable structure and easy to install. There are two kinds of arrangement: juxtaposition and staggered arrangement (as shown in the figure). The juxtaposition is simpler, the staggered column is more compact.
(II) the stress characteristics of ordinary bolt connections
(1) shear bolt connection
(2) tension bolt connection
(3) tension shear bolt connection
(III) Stress characteristics of high-strength bolts
High strength bolt connection can be divided into friction type and pressure type according to design and force requirements. When the friction connection is subjected to shear, the maximum frictional resistance between the plates is the limit state of the external shear force. When the relative slip occurs between the plates when exceeded, that is, the connection is considered to have failed and destroyed. In the case of shear, the friction is allowed to be overcome and the relative slip between plates occurs. Then the external force can continue to increase, and the ultimate failure of the screw shear or hole wall pressure is regarded as the limit state.