Where Is The Height Of High-strength Bolt? Analyze The Difference Between High-strength Bolts And Ordinary Bolts.

Having been in the fastener industry for a long time, it is inevitable to encounter some problems. Some customers ask, what is high-strength bolt? When I listen to it, it is useless to say that it is hard and strong enough, it is high-strength bolt, but when I think about it carefully, it seems that it is not very strict. So Xiao Rui checked some data privately and gave us a wave of analysis.

High-strength bolts, that is, high-strength friction pre-tensioned bolts, need to apply a large pre-tightening force. Generally, many people will think that high-strength bolts use only high-strength materials, but in fact, compared with ordinary bolts, the core difference is the form of force, and the essence is whether to apply the pre-tightening force.

Where is the height of high-strength bolt?

Material strength is not the key to distinguish ordinary bolts from high-strength bolts. In the British and American standards, there are only two types of high-strength bolts: grade 8.8 and grade 10.9, while ordinary bolts include 4.6, 5.6, 8.8, 10.9 and 12.9.

Speaking of this, some friends wonder that high-strength bolts are not made of higher strength materials, so where is the "high"? We take the calculation of a Grade 8.8 M22 bolt as an example: the design tensile strength of ordinary bolt is 400kN, the general shear strength is 320kN, and the preload of high-strength bolt is 150kN.

It can be seen from the above calculation that the design values of tensile strength and shear strength of ordinary bolts are higher than those of high-strength bolts under the same grade.

Stress-strain curve of ordinary bolt and high-strength bolt under working condition

High-strength bolts are under stress. According to the calculation of individual bolts, the design strength of tensile and shear strength of high-strength bolts are lower than that of ordinary bolts. The essence of its high strength is that during normal operation, no relative slip is allowed at the joint, that is, small elastic-plastic deformation and large joint stiffness.

It can be seen that under the given design node load, the nodes designed with high-strength bolts may not necessarily save the number of bolts, but their deformation is small, stiffness is large, and safety reserves are high. It is suitable for the main beam, and other places where the node stiffness is required to be large, which conforms to the basic seismic design principle of "strong node, weak member".

The strength of high-strength bolts lies not in the design value of their own bearing capacity, but in the large rigidity, high safety performance and strong resistance to damage of their design nodes.

Comparison between high-strength bolts and ordinary bolts

There are great differences between high-strength bolts and ordinary bolts in standards, working principles and construction tools.

Due to the different design stress principle between high-strength bolts and ordinary bolts, there are great differences in construction inspection methods. From the figure below, we can see that the mechanical properties of ordinary bolts of the same grade are slightly higher than those of high-strength bolts, but high-strength bolts have one more acceptance requirement for impact energy than ordinary bolts.

The identification of ordinary bolts and high-strength bolts is the basic method for on-site identification of bolts of the same grade. Since the torque values of high-strength bolts in British and American standards are not the same, it is also necessary to identify the bolts in the two standards.

Finally, let's take a look at the price comparison between ordinary bolts and high-strength bolts. They are also M24, L60, and 8.8 high-strength bolts and ordinary bolts. Ordinary bolts account for about 70% of high-strength bolts.

In terms of use method, ordinary bolts can be reused, and high-strength bolts cannot be reused. High-strength bolts are generally used for permanent connection. The bolted connection of the main components of the building structure generally adopts high-strength bolts.