How is bolt pretension verified on a slip-critical joint?

• A. By torque-tension checks or direct tension indicators (DTIs) and/or tension-control bolts installed to specified tension.
• B. By visual inspection alone.
• C. By measuring bolt color.
• D. By using a hammer test.

Answer: (A)

In a slip-critical joint, the resistance to sliding comes from the friction created by bolt pretension, so you must confirm the bolts are tightened to the specified tension. That's the only way to ensure the clamping force is enough to transfer shear through friction rather than bearing.

Verification is done with methods that directly relate to tension. Torque-tension checks estimate bolt tension from the applied torque, but actual tension varies with lubrication, bolt condition, and fit, so it’s an indirect method that requires proper calibration and conditions. Direct tension indicators (DTIs) provide a visible, real-time indication that a bolt has reached the target tension. Tension-control bolts are designed to be installed to a specified tension and often include a built-in mechanism or indicator to confirm proper pretension.

Relying on visual inspection, bolt color, or a hammer test does not reliably confirm pretension and cannot guarantee the necessary friction to resist slip. Therefore, the correct approach is to verify pretension using torque-tension checks, DTIs, or tension-control bolts installed to the specified tension.