BOLTING METHODS

TWIST-OFF BOLTS

As every method, the RCSC/AISC specification requires Preinstallation Verification testing that must be conducted onsite with hardware delivered to site. Test results, by the distributor or manufacturer, DO NOT satisfy the specification’s requirements. However, test results appearing on supplier Material Test Reports (MTRs) MUST represent all details of bolts being tested. It is especially important that the MTRs represent a coating when bolts are coated.

This method usually defaults to "twist-off" bolts, sometimes called "tension-control" bolts. These assemblies function by calibrating the torque needed to twist off a splined extension manufactured into the bolt shank. Made correctly, the "twist-off" will occur at a bolt tension above the minimum required.

The main advantage of "twist-off" bolts is that they can be tightened from one side by one person, although bolt installers now realize that regular hex bolts and DTI's can also be installed one-side, one-man, too.

View/Download Twist-Off Bolt Fact Sheet

There are several disadvantages to the "twist-off" system:

  1. Special wrenches are needed.
  2. Special connection clearances must be detailed for wrench access.
  3. Frequently more expensive than hex bolts and DTI's.
  4. Galvanized twist-off assemblies are in VERY limited supply.
  5. Field relubrication is prohibited by code.
  6. Compacting plies must be accomplished prior to twist-off.
  7. Deterioration of the thread condition for any reason will change the torque-tension relationship, and Kulak has shown in an ASCE paper that in as little as three days out of protected storage, one-third of all the twist-off bolts he tested did not develop the required tension at break-off.
TC Bolt Advisories

Studies conducted by the University of Toronto in 20041 and 20072 proved:

  1. TC bolts behave differently in a Hydraulic Tension Calibrators (Skidmore) than they do when installed in steelwork. Therefore, TC bolt testing, with hydraulic load cells, will show results 10% higher than what will be realized in the steelwork.
  2. Wetting a TC bolt, by rain or otherwise, reduces effective tension by 10%.
  3. Changes in temperature negatively affect tension by 10%.
  4. These issues are cumulative, i.e. a production lot of ¾” A325 TC bolts tested in a skidmore whose dial shows 29 kips will only provide ~ 26 kips when installed in steelwork. If rain wets installed TC bolts, of the same lot, effective installed tension will be ~23.5 kips. Falling temperature ambient air will result in an additional reduction of effective tension of 21 kips.

Since the 2005 study was funded by the RCSC, the information described above was presented at the 2007 RCSC Annual Meeting, in Cleveland Ohio by Professor Emeritus of U of T, Peter Birkemoe. Professor Birkemoe made the following recommendations that were not adopted into the RCSC Specification:

  • Revise the Specification to have bolts removed from the steelwork for verification.
  • Revise the Specification for TC bolts to be required to reach 1.1? times the minimum required tension in the as-received condition and modify the current preinstallation verification requirements.
  • Add cautionary notes in the commentary about the requirements for cold weather verification and installation.
  • Include the “wet” condition as part of the verification requirements.

The above bullet points have been copied from Professor Birkemoe’s RCSC presentation, verbatim.

To read the complete 2007 report and meeting minutes, proceed to the RCSC website. The report can be found at:
http://boltcouncil.org/files/FinalReportonTCBoltsPhase1.pdf

The Professor Birkemoe’s presentation to the RCSC committee in 2007 can be found by following the link below and reading pages 36 to 50 of the PDF file:
http://boltcouncil.org/files/2009RCSCMainMinutesPackage.pdf

Additionally, ASTM 3125, the standard that governs manufacturing and testing of TC bolts (formerly standards F1852 and F2280), requires assembly testing in a “tension measuring device”. For most manufacturers, a hydraulic load cell, such as a Skidmore Wilhelm, is used. Therefore, the tension values listed on a Material Test Report (MTR) for a TC bolt Lot, should be viewed through the lens of the U of T findings described in number 1 above. Achieved results in steelwork may be 10% less than what is listed on the MTR.

1Installation behaviour of ASTM F1852 twist-off type tension control bolts. W. Tan, University of Toronto, 2005.

2INSTALLATION CHARACTERISTICS OF F1852 TWIST-OFF TYPE TENSION CONTROL STRUCTURAL BOLT/NUT/WASHER ASSEMBLIES. V. Maleev, University of Toronto, 2007.