The effects of torque-tension relationships on roof bolt systems

CIM Bulletin, Vol. 84, No. 951, 1991

Stephen C. Tadolini, Department of the Interior, Bureau of Mines Denver Research Center, Denver, Colorado

The use of a torque wrench to establish the amount of actual load developed on an expansion anchor bolting system dates back to work completed in the early 1950s. The advent of ultrasonic measurement systems and hydraulic torque wrenches has made it possible for the U.S. Bureau of Mines to perform torque-tension measurements in the laboratory and the field much more accurately. The results indicate that the amount of initial bolt preload may be overestimated by amounts in excess of 45% of the expected load using torque wrenches. This inaccuracy is primarily due to the friction that occurs between the bearing plate and the head of the bolt. These parameters become extremely important when the support system is being used to prevent joint members from slipping relative to each other or when the bolt acts as a heavy spring to clamp two or more sections together. Methods to overcome the frictional losses are presented in conjunction with a laboratory method to determine the actual torque-tension ratios for expansion anchor and combination bolting systems.
Keywords: Rock mechanics, Roof bolting, Bolting systems.