High strength is mainly used in some outdoor projects, wind power or dynamic machinery. The tensile strength is relatively high.
General bolts can be used in some small machinery, such as turbines. Mainly It depends on the application environment and operation method
High-strength bolts are increasingly used. Two intensity levels, 8.8s and 10.9s, are commonly used, with 10.9 being the most common. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.
1. From the perspective of raw materials:
High-strength bolts are made of high-strength materials. The screws, nuts and washers of high-strength bolts are all made of high-strength steel. Commonly used are 45 steel, 40 boron steel, and 20 manganese titanium boron steel. Ordinary bolts are commonly made of Q235 steel.
2. From the strength level:
High-strength bolts are increasingly used. Two intensity levels, 8.8s and 10.9s, are commonly used, with 10.9 being the most common. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.
3. From the perspective of stress characteristics:
High-strength bolts exert pre-tension force and transmit external force by friction. Ordinary bolt connections rely on the shear resistance of the bolt rod and the pressure bearing of the hole wall to transmit shear force. The pretension force generated when tightening the nut is very small, and its impact is negligible. However, in addition to the high material strength of high-strength bolts, it also exerts a large force on the bolt. The pre-tension force creates an extrusion force between the connecting components, resulting in a large friction force perpendicular to the direction of the screw. The pre-tension force, anti-slip coefficient and steel type all directly affect the bearing capacity of high-strength bolts.
According to the force characteristics, it is divided into pressure type and friction type. The calculation methods of the two are different. The minimum size of high-strength bolts is M12, and M16~M30 are commonly used. Oversized bolts have unstable performance and should be used with caution in design.
The difference between high-strength bolt friction type and pressure-bearing type connection:
High-strength bolt connection uses a large tightening pre-tension force in the bolt rod to clamp the connecting plate parts, which is enough Generates a large friction force, thereby improving the integrity and stiffness of the connection. When subjected to shear force, according to the design and stress requirements, it can be divided into two types: high-strength bolt friction type connection and high-strength bolt pressure-bearing type connection. The essential difference between them is that the limit states are different. Although they are the same bolt, they are very different in terms of calculation methods, requirements, scope of application, etc.
In the shear design, the high-strength bolt friction type connection is the limit state when the external shear force reaches the maximum possible friction force provided by the bolt tightening force between the plate contact surfaces, that is, to ensure that the connection is in the entire During use, the internal and external shear forces do not exceed the maximum friction force. The plates will not undergo relative sliding deformation (the original gap between the screw and the hole wall is always maintained), and the connected plates will bear elastic force as a whole.
In the shear design, the external shear force allowed to exceed the maximum friction force in the high-strength bolt pressure-bearing connection will beRelative sliding deformation occurs between the connecting plates until the bolt rod contacts the hole wall. After that, the connection relies on the shearing of the bolt rod and the pressure of the hole wall as well as the friction between the contact surfaces of the plates to transmit force. Finally, the shearing of the rod Or the pressure failure of the hole wall serves as the ultimate shear state of the connection.
In short, friction-type high-strength bolts and pressure-bearing high-strength bolts are actually the same type of bolts, but the difference is whether slippage is considered in the design. Friction-type high-strength bolts must not slide. The bolts do not bear shear force. Once they slip, the design is considered to have reached a state of failure. Technical comparison
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Mature; pressure-bearing high-strength bolts can slide, and the bolts also bear shear force. The final damage is equivalent to the damage of ordinary bolts (bolt shearing or steel plate crushing).
4. From the perspective of use:
The bolt connections of the main components of the building structure generally use high-strength bolt connections. Ordinary bolts can be reused, but high-strength bolts cannot be reused. High-strength bolts are generally used for permanent connections.
High-strength bolts are prestressed bolts. For friction types, use a torque wrench to apply the specified prestress, and for pressure-bearing types, unscrew the torx head. Ordinary bolts have poor shear resistance and can be used in secondary structural parts. Ordinary bolts only need to be tightened.
Knowledge of high-strength bolts
The full name of high-strength bolts in production is high-strength bolt connection pairs, and is generally not referred to as high-strength bolts. According to the installation characteristics, they are divided into: large hexagonal head bolts and torsion shear bolts. Among them, the torsion shear type is only used in grade 10.9.
The full name of high-strength bolts is called high-strength bolt connection pair in production, and is generally not referred to as high-strength bolts. According to the installation characteristics, they are divided into: large hexagonal head bolts and torsion shear bolts. Among them, the torsion shear type is only used in level 10.9.
According to the performance level of high-strength bolts, they are divided into: level 8.8 and level 10.9. Among them, grade 8.8 only has large hexagonal high-strength bolts. In terms of marking method, the number before the decimal point indicates the tensile strength after heat treatment; the number after the decimal point indicates the yield strength ratio, which is the ratio of the measured value of yield strength to the measured value of ultimate tensile strength. . Grade 8.8 means that the tensile strength of the bolt rod is not less than 800MPa and the yield-to-strength ratio is 0.8; grade 10.9 means that the tensile strength of the bolt rod is not less than 1000MPa and the yield-to-strength ratio is 0.9.
The diameters of high-strength bolts in structural design generally include M16/M20/M22/M24/M27/M30, but M22/M27 is the second choice series. Under normal circumstances, M16/M20/M24/M30 is the main choice. .
High-strength bolts are divided into high-strength bolts pressure-bearing type and high-strength bolt friction type according to design requirements in terms of shear resistance design. The load-bearing capacity of the friction type depends on the anti-slip coefficient of the force transmission friction surface and the number of friction surfaces. The friction system that develops red rust after sandblasting (shot)The number is the highest, but from the actual operation point of view, it is greatly affected by the construction level. Many supervision units have asked whether the standard can be lowered to ensure the quality of the project. The load-bearing capacity of the pressure-bearing type depends on the minimum shear capacity of the bolt and the pressure-bearing capacity of the bolt. When there is only one connection surface, the shear bearing capacity of the M16 friction type is 21.6~45.0kN, while the shear bearing capacity of the M16 pressure bearing type is 39.2~48.6 kN, which is better than the friction type. In terms of installation, the pressure-bearing type process is simpler, and the connecting surface only needs to be cleaned of oil stains and floating rust.
The tensile bearing capacity along the shaft direction is very interestingly written in the steel structure specifications. The friction type design value is equal to 0.8 times the pretension force, and the pressure type design value is equal to the effective area of the screw multiplied by the material tensile strength. The strength design value seems to be very different. In fact, the two values are basically the same. I have never understood why the specification is written this way. Why should the same value be calculated using two expressions when the same material is used?
When bearing shear force and tensile force in the direction of the rod axis at the same time, the friction type requirement is the ratio of the shear force the bolt bears to the shear bearing capacity plus the sum of the stress ratio of the axial force the screw bears to the tensile bearing capacity. Less than 1.0, the pressure-bearing type requirement is that the sum of the square of the ratio of the shear force of the bolt to the shear bearing capacity plus the square of the stress ratio of the axial force of the screw to the tensile bearing capacity is less than 1.0, that is to say, under the same load combination In this case, the design safety reserve of pressure-bearing high-strength bolts of the same diameter is higher than that of friction-type high-strength bolts.
Considering that the friction surface of the connection may fail under repeated strong earthquakes, the shear bearing capacity at this time still depends on the shear resistance of the bolts and the pressure bearing capacity of the panels. Therefore, the seismic code stipulates Calculation formula for ultimate shear bearing capacity of high-strength bolts.
Although the pressure-bearing type has an advantage in design values, due to its shear compression failure type, the bolt holes are pore-type bolt holes similar to ordinary bolts, and the deformation when subjected to load is much greater than that of the friction type. , so the high-strength bolt pressure-bearing type is mainly used for the connection of non-seismic components, the connection of non-dynamic load-bearing components, and the connection of non-repetitive components. The normal service limit states of these two types are also different:
Friction-type connection refers to the relative slippage of the friction surface of the connection under the action of the basic combination of loads; Pressure-type connection refers to the relative slippage of the connection friction surface under the action of the basic combination of loads; Relative slippage occurs between the connecting parts under the action; Weld and bolt knowledge Weld grade
1. The weld grade is the construction acceptance grade, and there are three levels. Level 3 is the lowest and only requires appearance inspection and dimensional inspection. Ultrasonic flaw detection shall be carried out for the secondary required parts. The highest level requires all inspections to be carried out.
2. For the weld grade, the principle is that the tensile grade is higher than the compressive grade, and the dynamic grade is higher than the static grade.
3. Butt welds generally require non-destructive testing (or partially required). Therefore, the general welding grade of butt welds is Class II or Class I, not less than Class II.
4. There is no need for non-destructive testing of fillet welds, so if the fillet weld is level one, it doesn’t make much sense. Generally fillet welds are grade two or grade three. 5. For the weld grade, see Steel Regulation 7.1.1