High-strength bolts are special fasteners commonly used in steel structure engineering with high mechanical strength. Unlike ordinary bolts, high-strength bolts are designed for one-time use only. They are mostly applied to permanent connections of steel structures and are strictly prohibited from repeated use after disassembly. This paper introduces the principle, classification and inspection requirements of high-strength bolts before and after installation in detail for reference.
Working principle of high-strength bolts: A special wrench is used to tighten the nut to generate a huge pre-tension force on the bolt. Transmitted by the nut and washer, an equal pre-compression force is formed between the connected components. Under the action of pre-compression force, a large static friction force is generated on the contact surface of the connected parts. As long as the external axial force is less than this friction force, relative sliding of components will not occur, and the connection joint will not be damaged. This is the working principle of high-strength bolt connections.
High-strength bolt connection has the advantages of simple structure, excellent mechanical performance, detachability for maintenance and good fatigue resistance, enjoying broad application prospects. The connection relies on the friction between the contact surfaces of connected parts to prevent relative sliding. To ensure sufficient friction on the contact surface, it is necessary to improve the clamping force of components and increase the friction coefficient of contact surfaces. The clamping force between components is realized by applying pre-tension to the bolt, so the bolt must be manufactured of high-strength steel, hence the name high-strength bolt.
In high-strength bolt connections, the friction coefficient has a great impact on bearing capacity. Tests show that the friction coefficient is mainly affected by the contact surface treatment method and component material. To increase the friction coefficient, construction methods such as sand blasting and steel wire brush cleaning are commonly adopted to treat the joint contact surfaces.
High-strength bolts are mainly divided into two types: friction type and bearing type.The shear design criterion of friction-type high-strength bolts is that the shear force caused by design load shall not exceed the maximum friction force on the contact surface.For bearing-type high-strength bolts, the design criterion is to ensure neither shear failure of the bolt shank nor extrusion failure of the plate members.
The following inspection requirements shall be strictly followed for high-strength bolt connections before and after installation:
First, before construction, the torque coefficient of large hexagon high-strength bolt assemblies shall be re-inspected by production batch, and their average value and standard deviation shall comply with the provisions of the current national standard Specification for Design, Construction and Acceptance of High-Strength Bolt Connections of Steel Structures. The pre-tension force of torsion-shear high-strength bolt assemblies shall also be re-inspected by production batch, with its average value and standard deviation meeting the same specification requirements.
Second, for bolted spherical grid steel structures with a safety grade of Class I and a span of more than 40 meters, surface hardness tests shall be conducted on the connecting high-strength bolts.
Third, both the steel structure manufacturer and the installation unit shall conduct anti-slip coefficient tests respectively based on each manufacturing batch of steel structures.
After installation, torque inspection shall be carried out on high-strength bolt assemblies, including the torque method and the rotation angle method. Torque re-inspection shall be completed 1 hour to 48 hours after final tightening.For quality inspection after final tightening: torsion-shear high-strength bolts can be checked visually to confirm whether the plum blossom tail is properly sheared off; large hexagon high-strength bolts can be inspected one by one by light hammer tapping.
In addition, attention shall be paid to over-tightened bolts. Over-tightening will cause plastic deformation of bolts, and delayed fracture may occur if not replaced in a timely manner. Therefore, any over-tightened bolts found during construction must be replaced immediately to avoid safety risks caused by delayed fracture.
In conclusion, high-strength bolts play a vital role in steel structure engineering. All inspection requirements before and after installation are stringent and must be implemented in accordance with specifications to ensure the safety and reliability of structural connections.
