A friction grip connection, also known as a slip-critical connection, is a type of connection where the shear force is transferred between connected elements through the frictional resistance between the surfaces in contact. The design of a friction grip connection ensures that the connection can safely transfer the applied loads without slippage or failure. Here is a step-by-step procedure for designing a friction grip connection:
Step 1: Identify the Connection Requirements Determine the specific requirements of the connection, including the type and size of the connected elements, the applied loads, and the desired level of performance and safety.
Step 2: Determine the Applied Loads Identify the loads that will be applied to the connection, such as shear forces, tension forces, and, if applicable, compression forces. Determine the magnitude, direction, and distribution of these loads.
Step 3: Calculate the Friction Force Calculate the required friction force to resist the applied loads. The friction force is determined based on the shear force and the coefficient of friction between the surfaces in contact. The formula to calculate the friction force is:
Ff = μ x N
Where: Ff is the friction force μ is the coefficient of friction N is the normal force (perpendicular to the surfaces in contact)
Step 4: Select the Surface Preparation and Material Properties Select the appropriate surface preparation for the connected elements to achieve the required coefficient of friction. Common surface preparation methods include abrasive cleaning, power tool cleaning, or profiled surfaces. Ensure that the material properties, such as surface roughness and hardness, meet the specified requirements.
Step 5: Verify the Coefficient of Friction Verify that the selected surface preparation and material properties provide the desired coefficient of friction. The coefficient of friction depends on the surface conditions, such as roughness and cleanliness. Design codes or guidelines provide recommended values for different surface preparations and materials.
Step 6: Check Bolt Tension Capacity Check the tension capacity of the bolts to ensure that they can provide the required clamping force. The clamping force is necessary to achieve the required friction force. The tension capacity depends on the bolt size, material properties, and design code provisions.
Step 7: Verify Connection Strength and Safety Perform a comprehensive design verification to ensure that the friction grip connection meets the desired strength and safety requirements. This may involve additional checks for bolt shear strength, bearing strength, bolt spacing, edge distances, and other connection details as per the design code provisions.
Example: Consider a friction grip connection joining two steel plates subjected to a shear force of 150 kN. The coefficient of friction between the surfaces in contact is 0.45, and the normal force between the surfaces is 100 kN.
Step 1: Identify the Connection Requirements The connection requires a friction grip joint to transfer a shear force of 150 kN.
Step 2: Determine the Applied Loads The applied load is a shear force of 150 kN.
Step 3: Calculate the Friction Force Calculate the friction force using the formula: Ff = μ x N
Ff = 0.45 x 100 kN = 45 kN
Step 4: Select the Surface Preparation and Material Properties Select a suitable surface preparation method, such as abrasive cleaning or power tool cleaning, to achieve the desired coefficient of friction. Ensure that the material properties, such as surface roughness and hardness, meet the specified requirements.
Step 5: Verify the Coefficient of Friction Verify that the selected surface preparation and material properties provide the desired coefficient of friction. In this case, the coefficient of friction is assumed to be 0.45.
Step 6: Check Bolt Tension Capacity Check the tension capacity of the bolts to ensure that they can provide the required clamping force. This step involves calculating the required bolt tension based on the friction force and verifying that the selected bolts can provide this tension.
Step 7: Verify Connection Strength and Safety Perform a comprehensive design verification considering other factors such as bolt shear strength, bolt bearing strength, bolt spacing, edge distances, and other connection details as per the design code provisions.
It is important to note that the example provided is simplified, and actual design calculations for a friction grip connection 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.
