The turn-of-nut method is a simple and commonly used method for preloading high-strength bolts in steel connections. It involves rotating the nut a specified number of degrees beyond the snug-tight condition to achieve the desired bolt tension. The design criteria for a turn-of-nut connection include the determination of the bolt preload, the calculation of the required rotation angle, and the verification of the connection strength and safety. Here is a step-by-step procedure for designing a turn-of-nut 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: Select the Bolt Size and Material Select the appropriate bolt size and material based on the design requirements and specifications. Consider factors such as load capacity, corrosion resistance, and compatibility with the connected elements.
Step 4: Determine the Bolt Preload Calculate the required bolt preload based on the applied loads and the desired safety factor. The bolt preload provides the clamping force necessary to resist the applied loads. The formula to calculate the bolt preload is:
P = T / Fp
Where: P is the bolt preload T is the applied tension force Fp is the desired preload factor (typically specified in the design code or guidelines)
Step 5: Determine the Rotation Angle Determine the rotation angle required to achieve the desired bolt preload using the turn-of-nut method. The rotation angle is typically specified as a certain number of degrees beyond the snug-tight condition. The specific rotation angle depends on the bolt diameter, bolt length, and the desired bolt tension. The design code or guidelines provide recommended rotation angle values based on these parameters.
Step 6: Verify Connection Strength and Safety Perform a comprehensive design verification to ensure that the turn-of-nut 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 turn-of-nut connection joining two steel members subjected to a tension force of 100 kN. The bolt diameter is 20 mm, and a preload factor of 0.70 is specified.
Step 1: Identify the Connection Requirements The connection requires a turn-of-nut joint to transfer a tension force of 100 kN.
Step 2: Determine the Applied Loads The applied load is a tension force of 100 kN.
Step 3: Select the Bolt Size and Material Assume a suitable bolt size and material based on the design requirements.
Step 4: Determine the Bolt Preload Calculate the bolt preload using the formula: P = T / Fp
P = 100 kN / 0.70 = 142.86 kN (approximately)
Step 5: Determine the Rotation Angle Consult the design code or guidelines to determine the recommended rotation angle for the specific bolt diameter, length, and desired bolt tension. Assume a recommended rotation angle of 180 degrees.
Step 6: 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 turn-of-nut connection can be more complex, considering various factors such as load combinations, bolt material properties, 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.
