What is the strength of high-strength bolts?

High Strength Friction Grip Bolt, also known as HSFG. It can be seen that the high-strength bolts we refer to in Chinese construction are the abbreviation for high-strength friction pre tensioned bolts. In daily communication, simply abbreviating the words "friction" and "Grip" has caused many engineering and technical personnel to misunderstand the basic definition of high-strength bolts.

Myth 1:

Is a bolt with a material grade exceeding 8.8 a "high-strength bolt"?

The core difference between high-strength bolts and ordinary bolts is not the strength of the materials used, but the form of force applied. The essence is whether to apply preload and use static friction to resist shear.

High Strength Friction Grip Bolt, also known as HSFG. It can be seen that the high-strength bolts we refer to in Chinese construction are the abbreviation for high-strength friction pre tensioned bolts. In daily communication, simply abbreviating the words "friction" and "Grip" has caused many engineering and technical personnel to misunderstand the basic definition of high-strength bolts.

Myth 1:

Is a bolt with a material grade exceeding 8.8 a "high-strength bolt"?

The core difference between high-strength bolts and ordinary bolts is not the strength of the materials used, but the form of force applied. The essence is whether to apply preload and use static friction to resist shear.

To answer this question, it is necessary to start with the design working states of the two types of bolts, study their elastic-plastic deformation laws, and understand the limit state at the time of design failure.

Stress-strain curves of ordinary bolts and high-strength bolts under working conditions

Ultimate state at design failure

Ordinary bolt: The screw itself undergoes plastic deformation that exceeds the design tolerance, resulting in the screw being sheared out.

For ordinary bolted connections, relative slip occurs between the connecting plates before they begin to bear shear force, and then the bolt rod contacts the connecting plate, causing elastic-plastic deformation and bearing shear force.

High strength bolts: The static friction force between the effective friction surfaces is overcome, and the two steel plates undergo relative displacement, which is considered failure in design considerations.

In high-strength bolt connections, the friction force first bears the shear force. When the load increases to a point where the friction force is insufficient to resist the shear force, the static friction force is overcome, and the connecting plate undergoes relative sliding (ultimate state). But although it is damaged at this time, the bolt rod can still use its own elastic-plastic deformation to withstand shear force when in contact with the connecting plate.