Eight basic knowledge of steel structure-Part Two

Connection method of steel structure

There are three types of connection methods for steel structures: weld connection, bolt connection, and rivet connection.

Weld connection

The theory of welding seam connection is the partial melting of the welding rod and the part to be welded through the heat generated by the arc, and the welding is condensed into a weld by cooling, thereby connecting the two parts into a whole.

Advantages: The cross-section of the component won’t be weakened, steel could be saved. Simple structure, convenient manufacturing, large connection rigidity, good sealing performance, easy-to-use automated operations under certain conditions, and high production efficiency.

Disadvantages: The heat-affected zone formed by the welding high temperature of the steel near the weld may make some parts of the material brittle. The steel is subjected to unevenly distributed high temperature and cooling during the welding process, which causes the structure to produce welding residual stress and residual deformation, which is harmful to the structure. Due to the high rigidity of the welded structure, once local cracks occur, it is easy to expand to the whole, especially at low temperatures, which is prone to brittle fracture. The plasticity and toughness of the weld connection are poor, and when welding defects may occur, which will reduce fatigue strength.

Bolts Connection

Bolt connection is the connection of connecting pieces into a whole through bolts, which are fasteners. There are two types of bolt connections: ordinary bolt connections and high-strength bolt connections.

Advantages: The construction process is simple and the installation is convenient. It is especially suitable for site installation and connection, and it is also easy to disassemble. It is suitable for structures that require assembly and disassembly and temporary connections.

Disadvantages: It is necessary to make holes on the plate and align the holes when assembling, which increases the manufacturing workload and requires high manufacturing precision.

Bolt holes also weaken the section of the component, and the connected parts often need to be overlapped with each other, so the structure is more complicated.

Rivet connection

The 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 tight solid.

Advantages: The riveting force transmission is reliable, the plasticity and toughness are good, and the quality is easy to check and guarantee. It can be used for heavy-duty and direct dynamic load structures.

Disadvantages: The riveting process is complicated, the manufacturing costs labor and materials, and the labor intensity is high, so it has been basically replaced by welding and high-strength bolting.

Welding connection

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

The welding seam quality of automatic welding is stable, with fewer internal defects, good plasticity, and good impact toughness. It is suitable for welding long direct welds. Due to manual operation, semi-automatic welding is suitable for welding curves or welding seams of any shape. Automatic and semi-automatic welding should use welding wire and flux compatible with the main metal. The welding wire should meet the requirements 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) as the arc protection medium to isolate the molten metal from the air to keep the welding process stable. Gas shielded welding arc heating is concentrated, the welding speed is fast, and the penetration depth is large, so the weld strength is higher than that of manual welding. Moreover, it has good plasticity and corrosion resistance, which is suitable for welding thick steel plates.

According to the mutual position of the connected components, the weld connection can be divided into four forms: butt joint, overlap joint, T-shaped connection and corner joint. The welds used for these connections have two basic forms: butt welds and fillet welds. In the specific application, the selection should be made according to the stress of the connection, combined with the manufacturing, installation and welding conditions.

Screw Connection

The commonly used screw form of steel structure is large hexagon head type, and its code is represented by the letter M and the nominal name and diameter (mm). M18, M20, M22, M24 are commonly used in engineering. According to international standards, screws are uniformly represented by performance grades, such as "Class 4.6" and "Class 8.8". The number before the decimal point indicates the lowest tensile strength of the 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 material, that is, the ratio of the yield point to the lowest tensile strength.

According to the machining accuracy of screws, ordinary screws are divided into three levels: A, B, and C. Grade A and Grade B screws (refined screws) are made of grade 8.8 steel, processed by machine turning, with smooth surface and accurate size, and are equipped with type I holes. Because of its high processing accuracy and close contact with the hole wall, its connection deformation is small, and its stress performance is good, and it can be used for connections that can withstand greater shear and tension. However, manufacturing and installation are more labor-intensive and costly, so it is rarely used in steel structures.

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