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From the perspective of raw materials:

High strength bolts are made of high strength materials. The screw, nut and washer of high-strength bolt are made of high-strength steel, commonly used 45 steel, 40 boron steel and 20 manganese titanium boron steel. Ordinary bolts are usually made of Q235 steel.

From the strength grade:

High strength bolts are increasingly used. Two strength grades of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. The strength grade of ordinary bolts shall be low, generally 4.4, 4.8, 5.6 and 8.8.

From the stress characteristics:

High strength bolts exert pretension and transmit external force by friction. Ordinary bolt connections transfer shear force by the shear resistance of the bolt rod and the pressure on the hole wall. The pretension generated when tightening the nut is very small, and its influence can be ignored. In addition to its high material strength, the high-strength bolt also applies a large pretension to the bolt, causing extrusion between the connecting members, so that there is a great friction perpendicular to the screw direction. Moreover, the pretension The anti slip coefficient and the type of steel directly affect the bearing capacity of high-strength bolts. According to the stress characteristics, it can be divided into pressure bearing type and friction type The two methods are different. The minimum specification of high-strength bolt is M12, commonly used is m16~m30, and the performance of super large bolts is unstable, so they should be used carefully in design.

Difference between friction type and pressure bearing type connection of high-strength bolts:

High strength bolt connection clamps the plates of the connecting plate through a large tightening pretension in the bolt rod, which is enough to generate a large friction force, so as to improve the integrity and stiffness of the connection. When under shear, according to the different design and stress requirements, it can be divided into high-strength bolt friction type connection and high-strength bolt pressure bearing type connection. The essential difference between the two is that the limit state is different. Although they are the same bolt, in the calculation method There are great differences in requirements and scope of application.

In the shear design, the high-strength bolt friction type connection is the limit state in which 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 internal and external shear forces of the connection do not exceed the maximum friction force during the whole service period. The plate will not be subject to relative sliding deformation (the original gap between the screw and the hole wall will always be maintained), and the connected plate will be stressed as an elastic whole. In the shear design, the allowable external shear force in the high-strength bolt pressure bearing connection exceeds the maximum friction force. At this time, the relative sliding deformation occurs between the connected plates until the bolt rod contacts the hole wall. After that, the connection relies on the joint force transmission of the bolt rod body shear, the hole wall pressure and the friction force between the plate contact surfaces. Finally, the rod body shear or the hole wall pressure bearing failure is taken as the shear limit state of the connection.

In a word, friction high-strength bolt and pressure bearing high-strength bolt are actually the same kind of bolt, but whether to consider sliding in design. Friction type high-strength bolts must not slide. The bolts do not bear shear. Once they slide, the design considers that they reach the failure state, which is technically mature; The pressure bearing high-strength bolt can slide, and the bolt also bears the shear force. The final failure is equivalent to the failure of the ordinary bolt (the bolt is sheared or the steel plate is crushed).

From the perspective of use method:

The bolt connection of the main components of the building structure generally adopts high-strength bolt connection. Ordinary bolts can be reused, while high-strength bolts cannot be reused. High strength bolts are generally used for permanent connections.

The high-strength bolt is a prestressed bolt. The friction type uses a torque wrench to apply the specified prestress, and the pressure bearing type screws off the Torx head. Ordinary bolts have poor shear resistance and can be used in secondary structures. Ordinary bolts only need to be tightened.

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