The Differences And Misunderstandings Between High-Strength Bolts And Ordinary Bolts

Mar. 14, 2024

Please translate the following article into fluent and easy-to-understand English.

High-strength bolts are the most common construction content in steel structure construction, and all steel structure engineers will feel extremely familiar with them.

However, this may not be the case. Today we will start with the most basic concepts and take you to re-understand high-strength bolts, which may subvert your most basic understanding.

01What are high-strength bolts?

High-Strength Friction Grip Bolt, the English literal translation is: high-strength friction preload bolt, English abbreviation: HSFG. The full name of high-strength bolts is called high-strength bolt connection pair in production, and it is generally not referred to as high-strength bolts for short.

It can be seen that the high-strength bolts we refer to in Chinese construction are the abbreviation of high-strength friction pretensioned bolts. In daily communication, the words "friction" and "grip" are simply used, which has caused many engineering and technical personnel to understand the basic definition of high-strength bolts and lead to misunderstandings.

Misunderstanding 1: Bolts with a material grade exceeding 8.8 are "high-strength bolts"?

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

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

So where is the "strong" in high-strength bolts?

Ordinary bolts: The screw itself undergoes plastic deformation exceeding the design allowable, and the screw is sheared.

In ordinary bolt connections, relative slip will occur between the connecting plates before they begin to bear shear force. Then the bolt rod and the connecting plate come into contact, undergo elastic-plastic deformation, and bear the shear force.

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High-strength bolts: The static friction between the effective friction surfaces is overcome, and the relative displacement of the two steel plates occurs, which is considered damage in terms of design.

In high-strength bolt connections, the friction force first bears the shear force. When the load increases to the point where the friction force is not enough to resist the shear force, the static friction force is overcome and the connecting plates slip relative to each other (limit state). However, although it is damaged at this time, the bolt rod is in contact with the connecting plate and can still use its own elastic-plastic deformation to withstand shear force.

Misunderstanding 2: The load-bearing capacity of high-strength bolts is higher than that of ordinary bolts. Are they considered "high-strength"?

It can be seen from the calculation of a single bolt that the design strength of high-strength bolts in tension and shear is lower than that of ordinary bolts. The essence of its high strength is that during normal operation, the nodes do not allow any relative slip, that is, the elastic-plastic deformation is small and the node stiffness is large.

It can be seen that under the given design node load, nodes designed with high-strength bolts may not necessarily save the number of bolts used, but they have small deformation, high stiffness, and high safety reserve. It is suitable for main beams and other locations that require greater joint stiffness, and conforms to the basic seismic design principle of "strong nodes, weak members".

The strength of high-strength bolts does not lie in the design value of its own load-bearing capacity, but in the stiffness of its design nodes, high safety performance, and strong resistance to damage.

02The difference between ordinary bolts and high-strength bolts

Ordinary bolts can be reused,

High strength bolts are not reusable.

High-strength bolts are generally made of high-strength steel (No. 45 steel (8.8s), 20MmTiB (10.9S)). They 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 are generally made of ordinary steel (Q235) and only need to be tightened.

Ordinary bolts are generally grade 4.4, grade 4.8, grade 5.6 and grade 8.8.

High-strength bolts are generally grade 8.8 and grade 10.9, with grade 10.9 being the most common.

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