Correct understanding of high-strength bolts and ordinary bolts

Correct understanding of high-strength bolts and ordinary bolts

01. What is high-strength bolt

High Strength Friction Grip Bolt, also known as HSFG. It can be seen that the high-strength bolt we refer to in Chinese construction is the abbreviation for high-strength friction pre tensioned bolt. 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.

Disruption 1:

Misunderstanding: Bolts with a material grade exceeding 8.8 are considered "high-strength bolts".

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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. （1）*

In fact, in the British standard specifications, the American standard specifications only mention two types of high-strength bolts (HSFG BOLT): grade 8.8 and grade 10.9 (BS EN 14399/ASTM-A325&ASTM-490), while ordinary bolts include 4.6, 5.6, 8.8, 10.9, 12.9, etc. (Table 2 of BS 3692 Clause 11); It can be seen that the strength of the material is not the key to distinguishing high-strength bolts from ordinary bolts.

02. Correct understanding of "high strength" and where it lies

According to GB50017, calculate the tensile and shear strength of individual ordinary bolts (Class B) with grade 8.8 and high-strength bolts with grade 8.8.

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Through calculation, we can see that under the same grade, the design values of tensile strength and shear strength of ordinary bolts are higher than those of high-strength bolts.  （2）*

So where is the strength of high-strength bolts?

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.

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

In high-strength bolt connections, the frictional force first bears the shear force. When the load increases to a point where the frictional force is insufficient to resist the shear force, the static frictional force is overcome, and the connecting plate undergoes relative slip (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