A slip-critical joint is a type of bolted connection where the shear force is transferred between connected elements through the friction developed between the bolted surfaces. The design of a slip-critical joint ensures that the connection remains slip-free under the applied loads. Here is a step-by-step procedure for designing a slip-critical joint:
Step 1: Identify the Connection Requirements Determine the specific requirements of the connection, including the type and size of the connected elements, the desired level of slip resistance, and the applicable design code provisions.
Step 2: Determine the Applied Loads Identify the loads that will be applied to the connection, such as shear forces and, if applicable, tension or compression forces. Determine the magnitude, direction, and distribution of these loads.
Step 3: Determine the Friction Coefficient Calculate the required coefficient of friction between the connected surfaces. The friction coefficient depends on factors such as the material properties of the connected elements, surface conditions, and design code provisions. Typical values for the friction coefficient range from 0.3 to 0.5.
Step 4: Determine the Required Bolt Tension Calculate the required tension in the bolts to achieve the desired slip resistance. The bolt tension is determined based on the applied shear force and the coefficient of friction. The formula to calculate the required bolt tension is:
T = F / (µ x n)
Where: T is the required bolt tension F is the applied shear force µ is the coefficient of friction n is the number of bolts transferring the shear force
Step 5: Select Bolt Type and Size Select the appropriate bolt type and size based on the calculated bolt tension and considering other factors such as the material properties, desired level of slip resistance, and design code provisions.
Step 6: Check Bolt Shear Strength Check the shear strength of the selected bolts to ensure they can safely transfer the applied shear force. The bolt shear strength should be greater than or equal to the calculated shear force.
Step 7: Verify Slip Resistance Verify that the selected bolts and the connected surfaces provide the desired slip resistance. This can be done by checking the applied tension in the bolts against the required bolt tension and ensuring that the friction force exceeds the applied shear force.
Step 8: Design Verification Perform a design verification to confirm that the slip-critical joint meets the desired safety and performance requirements. This may involve performing structural analysis or calculations to confirm the adequacy of the selected bolts, the slip resistance, and the overall connection strength.
Example: Consider a slip-critical joint connecting two steel plates subjected to a shear force of 100 kN. The desired level of slip resistance requires a coefficient of friction of 0.4. The joint will use four bolts to transfer the shear force.
Step 1: Identify the Connection Requirements The connection requires slip resistance to transfer the shear force between the plates.
Step 2: Determine the Applied Loads The applied load is a shear force of 100 kN.
Step 3: Determine the Friction Coefficient The desired coefficient of friction is 0.4.
Step 4: Determine the Required Bolt Tension T = F / (µ x n) = 100 kN / (0.4 x 4) = 62.5 kN
Step 5: Select Bolt Type and Size Select an appropriate bolt type and size that can accommodate a tension of 62.5 kN.
Step 6: Check Bolt Shear Strength Check that the selected bolts have a shear strength greater than or equal to 100 kN.
Step 7: Verify Slip Resistance Verify that the applied tension in the bolts is 62.5 kN and that the friction force between the surfaces exceeds the applied shear force of 100 kN.
Step 8: Design Verification Perform a design verification to confirm that the slip-critical joint meets the desired safety and performance requirements. This may involve additional calculations or structural analysis.
It’s important to note that the example provided is simplified, and actual design calculations for a slip-critical joint can be more complex, considering various factors such as load combinations, bolt preload, surface conditions, and design code provisions. It is essential to consult the relevant design code or consult with a qualified structural engineer to ensure accurate and compliant connection design.