High strength bolts are a type of bolt commonly used in steel structural engineering, which have high strength. Unlike other bolts, high-strength bolts can only be used once and are commonly used for permanent connections. Reuse is strictly prohibited. This article will provide a detailed introduction to high-strength bolts and their inspection requirements before and after installation, for readers' reference.
The principle of high-strength bolts is to use a specially designed wrench to tighten the nut, generating a huge pre tension force. The connectors also generate the same amount of preload through nuts and pads. Under the action of preloading, a significant frictional force will be generated along the surface of the connected component. As long as the axial force is less than this frictional force, the component will not slip and the connection will not be damaged. This is the principle of high-strength bolt connection.
High strength bolt connections have the advantages of simple structure, good stress performance, detachable, and fatigue resistance. The method of using high-strength bolts for connection has broad prospects. High strength bolt connections rely on the frictional force between the contact surfaces of the connectors to prevent their mutual sliding. To ensure sufficient friction on the contact surface, it is necessary to increase the clamping force of the component and increase the friction coefficient of the component contact surface.
The clamping force between components is achieved by applying pre tension to the bolts, so the bolts must be made of high-strength steel, which is why they are called high-strength bolt connections. The magnitude of the friction coefficient has a significant impact on the bearing capacity in high-strength bolted connections. Experiments have shown that the friction coefficient is mainly affected by the form of the contact surface and the material of the components. In order to increase the friction coefficient of the contact surface, construction often uses methods such as sandblasting and cleaning with steel wire brushes to treat the contact surface of components within the connection range.
There are two main types of high-strength bolts, one is friction type and the other is pressure bearing type. The criterion for frictional high-strength bolts to withstand shear force is that the shear force caused by the design load does not exceed the frictional force. Pressure bearing high-strength bolts are designed based on the principle that the rod body is not sheared or the plate is not compressed.
For the inspection requirements of high-strength bolts before and after installation, it is generally necessary to pay attention to the following points:
Firstly, before construction, the high-strength large hexagonal head bolt connection pair should be retested for torque coefficient according to the factory batch number, and its uniform value and standard deviation should comply with the current national standard "Design, Construction and Acceptance Specification for High Strength Bolt Connection of Steel Structures"; The torsion shear high-strength bolt connection pair should be retested for pre-stress according to the factory batch number, and its uniform value and standard deviation should comply with the current national standard "Design, Construction and Acceptance Specification for High Strength Bolt Connection of Steel Structures".
Secondly, for steel grid structures with bolted ball joints and a span of more than 40m and a safety level of Level 1, surface hardness tests should be conducted on the high-strength bolts connecting them.
Once again, the manufacturing and installation units should conduct anti slip coefficient tests on steel structure manufacturing batches separately.
After the installation of high-strength bolt connections, torque inspection of the connecting pair should be carried out, which can be divided into torque method inspection and angle method inspection. The torque inspection should be completed within 48 hours after 1 hour of tightening. For the inspection of high-strength bolts after final tightening, the torsion shear type high-strength bolts can be visually inspected to see if the plum blossom head at the end of the bolt has been loosened; High strength large hexagonal head bolts can be inspected one by one using the "small hammer tapping method".
In addition, attention should be paid to the issue of over tightening bolts. Excessive tightening behavior may cause plastic deformation of bolts, and if not replaced in a timely manner, delayed fracture may occur. Therefore, for over tightened bolts, timely replacement is necessary to avoid the risk of delayed bolt fracture.
In summary, high-strength bolts play a very important role in steel structure engineering. The inspection requirements before and after installation are very strict and must be implemented according to regulations to ensure the safety and reliability of the connection.
