A slip-critical joint is a type of connection used in steel structure construction where the transfer of shear forces between structural members relies on the frictional resistance generated at the interface of the connected surfaces. These joints are designed to prevent relative movement or slippage between the connected members, ensuring the integrity and stability of the structure.
The key characteristic of slip-critical joints is that they require the bolts to be tightened with sufficient preload to create a clamping force, which generates the necessary frictional resistance to prevent slippage. The bolted connections in slip-critical joints are typically subjected to shear forces.
Here are some different types of slip-critical joints commonly used in steel structure construction:
- High-Strength Bolted Connection: This type of slip-critical joint uses high-strength bolts to connect the structural members. The bolts are tightened with high preload using calibrated torque wrenches or hydraulic tensioning devices. The frictional resistance at the bolted interface prevents slippage and ensures the transfer of shear forces. High-strength bolted connections are widely used in steel bridges and high-rise buildings.
- Friction Grip Connection: In a friction grip connection, the surfaces of the connected members are prepared to provide a rough texture, such as by shot blasting or grinding. The bolts are then tightened with sufficient preload to create friction between the surfaces, preventing slippage. Friction grip connections are commonly used in situations where disassembly or future maintenance may be required.
- Turn-of-Nut Connection: The turn-of-nut method is a simple and widely used technique for achieving slip-critical joints. In this method, the bolt is initially tightened snugly and then rotated a predetermined number of degrees (specified by design standards) to achieve the desired preload. The rotation ensures that the required clamping force and frictional resistance are achieved to prevent slippage.
- Direct Tension Indicator (DTI) Connection: DTI connections use special washers with protrusions or marks that indicate the proper preload in a bolted joint. The washers are placed under the bolt head or nut, and as the bolt is tightened, the protrusions deform or move, providing visual indication of the achieved preload. DTI connections offer a reliable method for achieving slip-critical joints with accurate preload.
- Calibrated Wrench Connection: In this method, the torque required to achieve the desired preload in the bolts is determined based on the specific bolt size and grade. The torque is then applied using a calibrated torque wrench, ensuring that the proper clamping force is achieved to prevent slippage. Calibrated wrench connections are commonly used in slip-critical joints where torque control is critical.
It’s important to note that slip-critical joints require careful design, proper installation, and quality control to ensure their effectiveness. The design of these joints considers factors such as the magnitude of the shear forces, the surface conditions of the connected members, the bolt grade and size, and the required preload. Adherence to industry standards and specifications is crucial to ensure the safety and performance of slip-critical joints in steel structures.
