Xirong - take you to understand the steel structure

With the continuous development of the domestic construction market, the proportion of steel structure building market is increasing year by year, what are the advantages of steel structure building? Xirong steel structure takes you into the steel structure industry, let's feel the unique charm of steel structure!

First, the characteristics of steel structure

1. The self-weight of the steel structure is light

2. The reliability of steel structure work is high

3. Steel has good vibration (seismo) resistance and impact resistance

4. The

degree of industrialization of steel structure manufacturing is relatively high

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 commonly used steel structure steel

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

2. Low alloy high-strength structural steel

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

4. Special purpose steel

Third, the principle of material selection of steel structure

The

principle of material selection 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 are comprehensively considered.

The four steel models proposed in the "Steel Structure Design Code" GB50017-2003 are "appropriate" models, which are selected when conditions permit, and do not prohibit the use of other models, as long as the steel used meets the requirements of the specification.

Fourth, the main steel structure technical content

High-rise steel structure technology

According to the height of the building and the design requirements, frame, frame support, cylinder and giant frame structure are used, and its components can be steel, reinforced concrete or steel tube concrete. The steel components have good light weight and ductility, and can use welded section steel or rolled section steel, which is suitable for ultra-high building buildings; Rigid reinforced concrete components have large rigidity and good fire performance, which are suitable for medium and high-rise buildings or bottom structures; Steel tube concrete is easy to apply and is only used for column structures.

Space steel structure technology

The space steel structure has light weight, large rigidity, beautiful shape and fast construction speed. Ball node plate grid, multi-layer variable section grid frame and grid shell with steel pipe as member are structural types with large amounts of space steel structure in China. It has the advantages of large space stiffness and low amount of steel, and can provide complete CAD in the design, construction and inspection procedures. In addition to the grid structure, the space structure also has a long-span suspension cable structure, cable membrane structure, etc.

Steel-concrete composite structure technology

The load-bearing structure of beams and columns composed of section steel and concrete components is a steel-concrete composite structure, and the application range has been expanding in recent years. The combined structure has the advantages of both steel and concrete, with high overall strength, good rigidity, good seismic performance, and good fire resistance and corrosion resistance when the outsourced concrete structure is adopted. Composite structural components can generally reduce the amount of steel used by 15~20%. Composite building covers and steel pipe concrete components also have the advantages of less mold support or no mold support, convenient and fast construction, and have great promotion potential. It is suitable for frame beams, columns and building covers of multi-storey or high-rise buildings with large loads, columns and building covers of industrial buildings, etc.

High-strength bolting and welding technology

High-strength bolts are used to transmit stress through friction force and are composed of three parts: bolt, nut and washer. High-strength bolt connection has the advantages of simple construction, flexible dismantling, high bearing capacity, fatigue resistance and good self-locking, high safety, etc., and has replaced riveting and partial welding in the project, and has become the main connection means in the production and installation of steel structure. For steel components made in the workshop, the thick plate should adopt automatic multi-wire arc buried welding, and the box-shaped column separator should adopt molten nozzle electroslag welding and other technologies. In the on-site installation and construction, semi-automatic welding technology and gas shielded flux cored wire and self-protecting flux cored wire technology should be adopted.

Steel structure protection technology

Steel structure protection includes fire prevention, anti-corrosion, anti-rust, generally used in the fireproof coating treatment does not need to be anti-rust treatment, but in the building with corrosive gas still need anti-corrosion treatment. There are many types of domestic fireproof coatings, such as TN series, MC-10, etc., among which MC-10 fireproof coatings include alkyd enamel, chlorinated rubber paint, fluoroelastomer coating and chlorosulfonated coating. In the construction, appropriate coatings and coating thicknesses should be selected according to the steel structure type, fire resistance grade requirements and environmental requirements.

Fifth, the connection method of steel structure

There are three connection methods of steel structure: weld connection, bolt connection and rivet connection.

Weld seam connection

The weld connection is the local

melting of the electrode and the weldment through the heat generated by the arc, and the weldment is condensed into a weld by cooling, so as to connect the weldment into a whole.

Excellent: does not weaken the cross-section of the component, saves steel, simple structure, convenient manufacturing, large connection rigidity, good sealing performance, easy to adopt automatic operation under certain conditions, high production efficiency.

Deficiency: The heat-affected zone formed by the high temperature of the steel near the weld may be brittle in some parts; During the welding process, the steel is subjected to uneven distribution of high temperature and cooling, which causes welding residual stress and residual deformation of the structure, which has a certain impact on the bearing capacity, stiffness and performance of the structure; Due to the large rigidity of the welded structure, the local crack is easy to extend to the whole once it occurs, especially at low temperature, it is easy to cause brittle fracture; The plasticity and toughness of the weld connection are poor, and defects may occur during welding, which reduces the fatigue strength.

Bolted connection

Bolted connection is a fastener that connects the connecting parts into a single piece. Bolted connections are divided into two types: ordinary bolted connections and high-strength bolted connections. Excellent: simple construction process, easy installation, especially suitable for site installation and connection, but also easy to disassemble, suitable for the need to assemble and disassemble the structure and temporary connection.

Missing: It is necessary to open holes and assemble holes on the plate, which increases the manufacturing workload and requires high manufacturing accuracy; The bolt hole also weakens the cross-section of the component, and the connected parts often need to be lapped with each other or add auxiliary connecting plates (or angle steel), so the structure is more complex and expensive steel.

Rivet connection

Rivet connection is a rivet with a semicircular 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 riveted into a nail head with a rivet gun to make the connection achieve fastening.

Excellent: riveting transmission is reliable, plasticity and toughness are good, the quality is easy to check and guarantee, and can be used for heavy and directly bearing dynamic load structures.

Missing: The riveting process is complex, the manufacturing is labor-intensive and material-intensive, and the labor intensity is high, so it has been basically replaced by welding and high-strength bolted connection.

6. Welding connection

Welding method

The commonly used welding methods for steel structures are arc welding, including manual arc welding, automatic or semi-automatic arc welding and gas shielded welding.

Manual arc welding is a commonly used welding method in steel structures, and its equipment is simple, flexible and convenient to operate. However, the working conditions are poor, the production efficiency is lower than automatic or semi-automatic welding, and the variability of weld quality depends to a certain extent on the technical level of the welder.

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

Gas shielded welding is to use inert gas (or CO2) gas as the protective medium of the arc to isolate the molten metal from the air to keep the welding process stable. Gas shielded welding arc heating is concentrated, welding speed is fast, penetration is large, so the weld strength is higher than manual welding. And good plasticity and corrosion resistance, suitable for the welding of thick steel plates.

Weld form

The

weld connection form can be divided into four forms according to the mutual position of the connected components: butt, lap, T-shaped connection and angle connection. The welds used for these connections are two basic forms: butt welds and fillet welds. In specific applications, it should be selected according to the force of the connection, combined with manufacturing, installation and welding conditions.

Weld construction

1. Butt weld

The butt weld has direct and smooth force transmission without significant stress concentration, so the force performance is good, and it is suitable for the connection of components bearing static and dynamic loads. However, due to the high quality requirements of butt welds, the welding gap between weldments is strict, and it is generally used in factory-made connections.

2. Fillet weld

The form of fillet weld: according to its length direction and the direction of external force, the fillet weld can be divided into a side fillet weld parallel to the direction of force action, a front fillet weld perpendicular to the direction of force and an

oblique fillet weld with an oblique intersection of the force direction and a perimeter weld.

The cross-sectional form of fillet weld is divided into ordinary type, flat slope type and deep penetration type. The ratio of ordinary cross-sectional welding foot edge is 1:1, which is similar to the isosceles right triangle, and its force transmission line is bent sharply, so the stress concentration is serious. For the structure directly bearing the dynamic load, in order to make the force transmission smooth, the front fillet weld should adopt a flat slope type with a ratio of 1:1.5 of the two welding corners (the long side follows the direction of internal force), and the side fillet weld should adopt a deep penetration type with a ratio of 1:1.

7. Bolted connection

Construction of ordinary bolted connections

1. The form and specification of ordinary bolts

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

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

A and B grade bolts (

refined bolts) are made of 8.8 grade steel, which is turned by machine tools, with smooth surface, accurate size, and equipped with Class I holes (that is, bolt holes are drilled or expanded on assembled components, the hole wall is smooth, and the holes are accurate). Due to its high processing accuracy, close contact with the hole wall, small connection deformation, good force performance, it can be used for the connection to withstand large shear and tensile forces. However, manufacturing and installation are labor-intensive and costly, so they are rarely used in steel structures.

Class C bolts (rough

bolts) are made of 4.6 or 4.8 grade steel, rough processing, inaccurate dimensions, and only require Class II holes (that is, bolt holes are punched once on a single part or drilled without drilling. Generally, the aperture diameter is 1~2mm larger than the bolt rod diameter). When transmitting shear, the connection deformation is large, but the performance of transmitting tensile force is good, and the operation does not require special equipment and the cost is low. It is often used for bolted joints subjected to tensile forces and secondary shear connections in structures subjected to static or indirect dynamic loads.

2. Arrangement of ordinary bolted connections

The arrangement of bolts should be simple, unified and compact, meet the force requirements, and the structure should be reasonable and easy to install. There are two types of arrangement: parallel and staggered. Parallel is simpler, staggered is more compact.

Force characteristics of ordinary bolted connections

1. Shear bolt connection

2. Tension bolt connection

3. Pull and shear bolt connection

Force characteristics of high-strength bolts

High-strength bolted connections can be divided into friction type and pressure-bearing type according to design and force requirements.

When the friction

type connection is subjected to shear, the external shear force reaches the friction resistance that may occur between the plates as the limit state; When the relative slippage occurs between the plates when it is exceeded, it is considered that the connection has failed and is broken.

When the pressure-bearing connection is sheared, the friction force is allowed to be overcome and the relative slip between the plates occurs, and then the external force can continue to increase, and the screw shear or the failure of the pressure on the hole wall that occurs afterwards is the limit state.