What is the purpose of torque-controlled bolts and how are they installed?

• A. To ensure the specified pretension; installed using a torque wrench or direct tension indicators (DTIs) depending on bolt type.
• B. To minimize bolt material usage by using lighter fasteners.
• C. To improve corrosion resistance of the connection.
• D. To eliminate the need for any field tightening.

Answer: (A)

The main idea is that torque-controlled bolts are used to achieve a specific pretension in the bolt, which clamps the connected members together and helps the joint resist slip and carry the design loads. By reaching a defined clamping force, the connection behaves as intended under service conditions.

They are installed either by tightening with a torque wrench to a prescribed torque, which, when combined with the bolt type and lubrication, yields the target bolt tension, or by using direct tension indicators (DTIs) that show the bolt has reached the required tension. DTIs provide a direct verification of pretension rather than relying solely on torque, which can vary with friction and other factors.

This approach ensures consistent, predictable performance of the connection. The other options don’t fit because simply using lighter fasteners doesn’t control pretension, corrosion resistance isn’t the goal of torque-controlled bolts, and field tightening isn’t eliminated—these bolts still require proper tightening to meet the specified pretension, whether by torque or DTIs.