Bolts made of high-strength steel or that require a large pre-tightening force can be called high-strength bolts. High-strength bolts apply pretension and transmit external forces by friction. Ordinary bolt connections rely on the shear resistance of the bolt rod and the pressure bearing of the hole wall to transmit shear force. The pretension force generated when tightening the nut is very small, and its impact is negligible. However, in addition to the high material strength of high-strength bolts, it also exerts a large force on the bolt. The pre-tension force creates an extrusion force between the connecting components, resulting in a large friction force perpendicular to the direction of the screw. The pre-tension force, anti-slip coefficient and steel type all directly affect the bearing capacity of high-strength bolts.

How high-strength bolts work

High-strength bolts are mainly divided into two types according to their stress conditions: friction type and pressure-bearing type.
High-strength bolts are divided into two types according to construction technology: large hexagonal high-strength bolts and torsion-shear high-strength bolts.

1.. Friction type high-strength bolts

Friction type high-strength bolt connection relies on the friction force generated on the contact surface of the steel plate after the bolt tightening pressure makes the connecting plate layers close to each other to transmit external force. After sandblasting the surface of the component to generate a red rust surface, a larger friction coefficient can be obtained and the number of connecting bolts can be reduced. The hole diameter of friction-type high-strength bolts should be 1.5 to 2.0 mm larger than the nominal diameter d of the bolt.

2. Pressure-bearing high-strength bolts
The pressure-bearing high-strength bolt connection transmits stress by making the friction force between components and the shear force of the bolt's central axis act simultaneously with the bearing pressure of the component. The hole diameter should be 1.0 to 1.5 mm larger than the nominal diameter d of the bolt. Holes are made using methods such as CNC drilling machine drilling and drill mold drilling.

In short, friction-type high-strength bolts and pressure-bearing high-strength bolts are actually the same type of bolts. The difference lies in whether slippage is considered in the design. In terms of design, the friction surface of friction-type high-strength bolts cannot slide, and the screw does not bear shear force. Once the friction surface slips, it is considered to have reached the design failure state, which is technically mature and reliable; the friction surface of pressure-bearing high-strength bolts can slide, and the screw It also bears shear force, and the final failure is the same as that of ordinary bolts (bolts are sheared or steel plates are crushed).

3. Large hexagonal high-strength bolts
A high-strength bolt connection pair can be composed of a high-strength bolt, a nut and two washers.

During construction, the structure is temporarily fixed with rough bolts. After the structure is installed and aligned, the rough bolts are replaced with high-strength bolts one by one starting from the middle of the bolt group and are initially tightened. After the initial tightening, re-tightening is performed sequentially. And finally twist.

4. Torsional shear type high strength bolts

A high-strength bolt, a nut and a washer form a torsion-shear high-strength bolt connection pair.

When installing a torsion-shear type high-strength bolt connection, only one washer should be added to one side of the nut. The calculation formula for the initial tightening torque value is: Tc=0.065*Pc*d. In the formula, Tc is the initial tightening torque value, in N·m; Pc is the construction pretension force, in kN; d is the thread diameter of the high-strength bolt, in mm. Finally, use a special wrench to twist off the torx head at the end. The focus of quality inspection should be on the supervision and inspection of the construction process.