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Basic Introduction of the Types and Structure of Bolts

Ⅰ. Overview of bolt connection of steel structure


Bolt connecting for steel structure is a method of connecting two or more steel structure parts or components with bolts into one body. Bolt connection is the simplest connection method in component pre-assembly and structural installation.


Ⅱ. The specifications of bolts


1. Commonly used bolt specifications in steel structure are M12, M16, M20, M24, M30, M is the bolt symbol, and the number is the nominal diameter.


2. Bolts are divided into ten grades according to performance grades 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9, among which bolts above grade 8.8 are made of low-carbon alloy steel or medium-carbon steel and have been heat-treated (Quenching and tempering), commonly known as high-strength bolts, and below 8.8, not including 8.8, refined ordinary bolts also include 8.8, are commonly known as ordinary bolts.


Ⅲ. Different types of bolts


1. There are many names for bolts, such as screws, bolts, standard parts, and fasteners.


2. In a broad sense, bolts include ordinary bolts, high-strength bolts, anchor bolts, expansion bolts, chemical anchors, screws, studs and so on.


3. In a narrow sense, bolts are divided into ordinary bolt connections and high-strength bolts.


Ⅳ. What is a high-strength bolt connection


1. Fasteners that are made of high-strength steel or require a larger pre-tightening force can be called high-strength. High-strength bolts exert pre-tension and transmit external force by friction.


2. Ordinary bolt connections rely on the shear resistance of the bolt and the pressure of the hole wall to transmit the shear force. The pretension generated when the nut is tightened is very small, and its effect can be ignored. In addition to its high material strength, high-strength bolts also apply to the bolts. A large pretension force produces an extrusion force between the connecting members, thereby causing a large friction force perpendicular to the screw direction. Moreover, the pretension force, the anti-slip coefficient and the type of steel directly affect the bearing capacity of the high-strength bolt.


3. High-strength bolts are mainly divided into two types according to their stress conditions: friction type and pressure-bearing type.


4. According to the construction process, high-strength bolts are divided into two types: torsion shear type high-strength bolts and high-strength hexagonal bolts.


5. Friction type high-strength bolt connection, the friction force generated on the contact surface of the steel plate after the bolt tightening pressure makes the connecting plate layer close to transmit the external force. After sandblasting the surface of the component, a red rust surface is formed, which can obtain a larger coefficient of friction and reduce the number of connecting bolts. The diameter of the friction-type high-strength bolt should be 1.5-2.0mm larger than the nominal diameter d of the bolt.


6. Pressure-bearing high-strength bolt connection is to make the friction force generated between the components and the shear force of the bolt center axis and the bearing pressure of the component act simultaneously to transfer the stress, and the hole diameter should be 1.0~1.5mm larger than the nominal diameter d of the bolt. Hole making uses such methods as CNC drilling machine drilling and drilling die drilling.


In short, friction-type high-strength bolts and pressure-bearing high-strength bolts are actually the same kind of bolts. The difference is whether slippage is considered in the design. In design, the friction surface of the friction type high-strength bolt cannot slide, and the screw does not bear the shear force. Once the friction surface slips, it is considered to have reached the design failure state, which is technically mature and reliable. The friction surface of the pressure-bearing high-strength bolt can slide, the screw also bears shear force, and the final damage is the same as that of ordinary bolts like bolt shearing or steel plate crushing.

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