Nov 2012

Super High Tension Bolts

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Super high tension bolts (designated as XTB) were developed in Japan and are currently being used in construction in Japan.  The minimum specified tensile strength is 200 ksi as compared to 150 ksi for ASTM A490 Bolts.  There is a desire to make super high tension bolts available for use in the United States to facilitate more compact connections using less bolts.  However, to use super high tension bolts in the United States, it is necessary to develop installation procedures that are similar in approach to current U.S. practice as described in the Research Council on Structural Connections (RCSC) specifications.  Japanese bolt installation procedures are generally more restrictive than U.S. procedures in that the torque at the snug-tight condition and the subsequent angle of nut turn are controlled even for twist-off type bolts.

A research project was conducted to develop installation procedures for twist-off super high tension bolts that match the general approach of the RCSC specifications.  Over 300 pretension tests were conducted with many variations including bolt diameter, bolt length, temperature, number of threads in the grip, variations in lubrication from as-received to rusted or relubricated, and bolt hole geometry.  The pretension tests were conducted using an instrumented Skidmore Wilhelm machine to record bolt tension during the installation procedure including snug tight condition, twist-off of the spline, and in some cases tightening past yield with a hydraulic wrench.  The nut angle of turn was also recorded at key times during each test.  Based on this research, recommendations were made about the applicability of the RCSC specifications for use with twist-off super high tension bolts.

XTB Twist-Off Bolt

Installation with Special Wrench

View After Twist-Off of the Spline

Eatherton, M.R., Galitz, C. (2013) Pretension Testing of Super High Tension Twist-Off Bolts (XTB), Presentation at the North American Steel Construction Conference, April 19, 2013, St. Louis, MO

Eatherton, M.R., Galitz, C.L., Wood, M. (2013) Pretension Testing of XTB Twist-Off Type Tension Control Bolt Assemblies, Virginia Tech Structural Engineering and Materials Report No. CE/VPI-ST-13/07