Eight basic knowledge of steel structure

First, the characteristics of steel structure

1. steel structure light weight

2. the reliability of steel structure work is high

3. steel vibration (shock) resistance, good impact resistance

4. Steel structure manufacturing has a high degree of industrialization

5. the steel structure can be assembled accurately and quickly

6. easy to make a sealed structure

7. the steel structure is easy to corrode

8. Poor fire resistance of steel structure

Second, the grade and performance of steel commonly used in steel structure

1. Carbon structural steel: Q195, Q215, Q235, Q255, Q275, etc

2. low alloy high strength structural steel

3. high-quality carbon structural steel and alloy structural steel

4. special purpose steel

Third, the material selection principle of steel structure

The material selection principle of steel structure is to ensure the bearing capacity of the load-bearing structure and prevent brittle failure under certain conditions, according to the importance of the structure, load characteristics, structural form, stress state, connection method, steel thickness and working environment and other factors.

The four types of steel models proposed in the "Steel Structure Design Code" GB50017-2003 are "appropriate" models to use, and are the first choice when conditions permit, and the use of other models is not prohibited, as long as the steel used meets the requirements of the code.

Four, the main steel structure technical content

(1) High-rise steel structure technology.

According to the building height and design requirements, frame, frame support, cylinder and giant frame structure are adopted respectively, and its components can be made of steel, reinforced concrete or concrete filled steel tubes. The steel component is light in weight and good in ductility, and can be used as welded or rolled steel, which is suitable for ultra-high building. The rigid reinforced concrete member has high rigidity and good fire resistance, which is suitable for middle and high-rise buildings or bottom structures. Concrete filled steel tube is simple in construction and is only used for column structures.

(2) Space steel structure technology.

The space steel structure has light weight, high stiffness, beautiful shape and fast construction speed. The spherical joint flat grid with steel pipe as the rod, multi-layer variable section grid and reticulated shell are the structure types with the largest amount of space steel structure in China. It has the advantages of large space stiffness, low steel use, in design, construction and inspection procedures, and can provide complete CAD. In addition to the grid structure, there are long span suspension cable structure, cable membrane structure and so on.

(3) Light steel structure technology.

The new structural form is accompanied by light colored steel plates made of walls and roof enclosures. By more than 5mm steel plate welded or rolled large section thin wall H-beam and roof purlin, round steel made of flexible support system and high-strength bolt connection composed of light steel structure system, column distance can be from 6m to 9m, span up to 30m or larger, height up to ten meters, and can be set up light lifting four. The steel quantity is 20 ~ 30kg/m2. Now there are standardized design procedures and specialized production enterprises, good product quality, fast installation, light weight, less investment, construction is not subject to seasonal restrictions, suitable for a variety of light industrial plants.

(4) Steel concrete composite structure technology.

The beam and column bearing structure composed of section steel or steel management and concrete members is a steel-concrete composite structure, which has been widely used in recent years. The composite structure has the advantages of both steel and concrete, the overall strength is high, the rigidity is good, the seismic performance is good, when the external concrete structure is used, it has good fire resistance and corrosion resistance. Composite structural members can generally reduce the amount of steel used by 15 to 20%. The composite floor and concrete-filled steel tube members also have the advantages of less or no mold support, convenient and rapid construction, and have great promotion potential. It is suitable for frame beams, columns and floors of multi-storey or high-rise buildings with large loads, industrial building columns and floors, etc.

(5) high-strength bolt connection and welding technology.

The high-strength bolt transmits the stress through the friction force and is composed of three parts: bolt, nut and washer. High-strength bolt connection has the advantages of simple construction, flexible removal, high bearing capacity, good fatigue resistance and self-locking, and high safety, and has replaced riveting and partial welding in the project, and has become the main connection means in the production and installation of steel structures. For steel members made in the workshop, automatic multi-wire arc welding should be used for thick plate, and melting nozzle electroslag welding should be used for box column partition. In the field installation and construction, semi-automatic welding technology and gas shielded flux-cored wire and self-shielded flux-cored wire technology should be adopted.

(6) Steel structure protection technology, steel structure protection including fire prevention, corrosion prevention, rust prevention, is generally used in the fireproof paint treatment without rust treatment, but in the corrosive gas in the building still need to be anti-corrosion treatment.

There are many kinds of domestic fire retardant coatings, such as TN series, MC-10, etc., among which MC-10 fire retardant coatings include alkyd enamel, chlorinated rubber paint, fluoro rubber paint and chlorosulfonated paint. In the construction, the appropriate coating and coating thickness should be selected according to the steel structure type, fire resistance grade requirements and environmental requirements.

Fifth, steel structure objectives and measures

Steel structure engineering involves a wide range of technical difficulties, in the promotion and application must follow the national and industry standards. Local construction administrative departments should pay attention to the construction of the specialized stage of steel structure engineering, organize the training of quality inspection teams, and summarize the work practice and new technology application in a timely manner. Colleges and universities, design departments and construction enterprises should accelerate the training of steel structure engineering and technical personnel, and promote the mature steel structure CAD. Mass academic groups should cooperate with the development of steel structure technology, extensively carry out academic exchanges and training activities at home and abroad, and actively improve the overall level of steel structure design, production, construction and installation technology in the near future.

Six, the connection method of steel structure

The connection method of steel structure has three kinds: weld connection, bolt connection and rivet connection.

(1) Weld connection

The welding joint is the partial melting of the welding rod and the weldment through the heat generated by the arc, and the welding seam is condensed by cooling, so that the weldment is connected as a whole.

Advantages:

It does not weaken the section of the component, save steel, simple structure, convenient manufacturing, large connection stiffness, good sealing performance, easy to use automatic operation under certain conditions, high production efficiency.

Disadvantages: The heat affected zone of the steel near the weld due to high welding temperature may be brittle in some parts; During the welding process, the steel is subjected to unevenly distributed high temperature and cooling, resulting in residual welding stress and residual deformation of the structure, which has a certain influence on the bearing capacity, stiffness and service performance of the structure. Due to the high stiffness of welded structure, once the local crack occurs, it is easy to expand to the whole, especially at low temperature. The plasticity and toughness of the weld joint are poor, which may produce defects during welding and reduce the fatigue strength.

(2) Bolt connection

The bolted connection is to connect the connector into one body through bolts such fasteners. Bolt connection can be divided into ordinary bolt connection and high strength bolt connection.

Advantages: simple construction process, easy installation, especially suitable for site installation connection, but also easy to disassemble, suitable for the need to install and disassemble structures and temporary connections.

Disadvantages: the need to open holes in the plate and assemble holes, increase the manufacturing workload, and the manufacturing accuracy requirements are high; The bolt hole also weakens the section of the member, and the connected parts often need to be overlapped with each other or an auxiliary connecting plate (or Angle steel), so the structure is more complicated and costs more steel.

(3) Riveted connection

Rivet connection is the rivet with a semi-circular prefabricated nail head at one end, the nail rod is burned red and quickly inserted into the nail hole of the connector, and then the other end is also rivet into the nail head with the rivet gun to make the connection tight.

Advantages: reliable riveting force transmission, plasticity, toughness are good, quality is easy to check and guarantee, can be used for heavy and directly bear the dynamic load structure.

Disadvantages: The riveting process is complex, the manufacturing cost of labor and materials, and the labor intensity is high, so it has been basically replaced by welding and high-strength bolt connection.

Seven, welding connection

(1) Welding method

The common welding method of steel structure is arc welding, including manual arc welding, automatic or semi-automatic arc welding and gas shielded welding.

Manual arc welding is the most common welding method in steel structure, its equipment is simple, flexible and convenient operation. However, the working conditions are poor, the production efficiency is lower than that of automatic or semi-automatic welding, and the variability of the weld quality is large, which depends on the technical level of the welder to a certain extent.

The weld quality of automatic welding is stable, the internal defects of the weld are less, the plasticity is good, the impact toughness is good, and it is suitable for welding longer direct welds. Semi-automatic welding due to manual operation, suitable for welding curves or arbitrary shape of the weld. Automatic and semi-automatic welding should use the welding wire and flux suitable for the main metal, the welding wire should meet the provisions of the national standard, and the flux should be determined according to the welding process requirements.

Gas shielded welding uses inert gas (or CO2) gas as the protective medium of the arc to isolate the molten metal from the air to maintain the stability of the welding process. Gas shielded welding arc heating concentration, welding speed, deep penetration, so the weld strength is higher than manual welding. And good plasticity and corrosion resistance, suitable for thick steel plate welding.

(2) Form of weld

The weld joint form can be divided into four forms according to the mutual position of the connected members, such as butt joint, lap joint, T-joint and Angle joint. The welds used for these connections have two basic forms: butt welds and fillet welds. In the specific application, it should be selected according to the force situation of the connection, combined with the manufacturing, installation and welding conditions.

(3) Weld structure

1. Butt weld

The butt weld transmits force directly, smoothly, and has no significant stress concentration phenomenon, so the force performance is good, and it is suitable for the connection of components under static and dynamic loads. However, due to the high quality requirements of the butt weld, the welding gap between the welds is more strict, and it is generally used in factory manufacturing connections.

2, fillet weld

The form of fillet welds: according to the length direction and the direction of external force, fillet welds can be divided into side fillet welds parallel to the direction of force action, positive fillet welds perpendicular to the direction of force action and oblique fillet welds oblique to the direction of force action and circumferential welds.

The section form of fillet weld is divided into ordinary type, flat slope type and deep penetration type. hf in the figure is referred to as the weld foot size of fillet weld.

The common section welded foot side ratio is 1:1, which is similar to the isosceles right triangle, and the bending of the force transmission line is severe, so the stress concentration is serious. For the structure directly subjected to dynamic load, in order to make the force transfer smooth, the front fillet weld should adopt the flat slope type with the size ratio of the two welding angles 1:1.5 (the long side is in the direction of the internal force), and the side fillet weld should adopt the deep penetration type with the ratio of 1:1.

Eight, bolt connection

(1) The structure of ordinary bolt connection

1, the form and specifications of ordinary bolts

The common form of the steel structure is a large hexagonal head type, and its code is indicated by the letter M with the nominal and diameter (mm). M18, M20, M22 and M24 are commonly used in engineering. According to international standards, bolts are unified with bolt performance grades, such as "4.6", "8.8" and so on. The number before the decimal point indicates the lowest tensile strength of the bolt material, such as "4" for 400N/mm2 and "8" for 800N/mm2. The number after the decimal point (0.6, 0.8) indicates the yield ratio of the bolt material, that is, the ratio of the yield point to the lowest tensile strength.

According to the processing accuracy of bolts, ordinary bolts are divided into A, B, C three levels.

Class A and B bolts (refined bolts) are made of 8.8 grade steel and are turned by machine tools. The surface is smooth, the size is accurate, and Class I holes are used (that is, the bolt holes are drilled or redrilled on the assembled members, the hole walls are smooth and the holes are accurate). Because of its high processing precision, close contact with the hole wall, its connection deformation is small, good mechanical performance, can be used to withstand large shear and tension connection. However, manufacturing and installation are more laborious and costly, so they are less used in steel structures.

Class C bolts (rough bolts) are made of 4.6 or 4.8 grade steel, rough processing, the size is not accurate enough, and only Class II holes are required (that is, bolt holes are punched in a single part at a time or drilled without a drill die. The general aperture is 1~2mm larger than the bolt rod diameter). In the transfer shear force, the connection deformation is large, but the performance of the transfer tension is good, the operation does not need special equipment, and the cost is low. It is commonly used in bolted connections under tensile forces and in secondary shear connections in structures under static loads or indirect dynamic loads.

2, ordinary bolt connection arrangement

The arrangement of bolts should be simple, unified and compact, meet the force requirements, reasonable structure and easy to install. There are two kinds of arrangement (as shown in the figure) : parallel and staggered. Parallel is simpler, staggered is more compact.

(2) Stress characteristics of ordinary bolt connection

1. Shear bolt connection

2, tension bolt connection

3, tensile shear bolt connection

(3) The stress characteristics of high-strength bolts

High strength bolt connection can be divided into friction type and pressure type according to the design and force requirements. When the friction type connection is subjected to shear, the external shear force reaches the maximum possible friction resistance between the plates. When the relative slip occurs between the plates, it is considered that the connection has failed and is damaged. When the pressure type connection is shear, the friction is allowed to be overcome and the relative slip between the plates occurs, and then the external force can continue to increase, and the final failure of the screw shear or the hole wall pressure that occurs thereafter is the limit state.