What are direct tension indicators (DTIs), and how are they used?

• A. DTIs are devices embedded in bolts to show that the bolt has achieved the required pretension when the indicator shows the specified color or reading.
• B. DTIs are used to monitor humidity.
• C. DTIs measure the temperature of the bolt.
• D. DTIs are decorative.

Answer: (A)

Direct tension indicators show whether a bolt has reached the required pretension in a connection. They are built into the bolt so that, as the bolt is tightened to the specified preload, a part of the indicator moves or changes color to reveal that the target tension has been achieved. This provides a clear visual or readable confirmation that the joint is clamped with the intended force, which is crucial for the performance of slip-critical or tension-critical connections. Torque alone can be misleading due to friction and fit, and the DTI gives a direct verification of pretension. These devices are not used for humidity or temperature sensing and are not decorative.