BOLTING METHODS

TURN-OF-NUT BOLTING METHOD

AISC & RCSC Turn-of-Nut (Part Turn)

Note: As with every bolt pretensioning method, Turn-of-Nut may only be performed after all steel plies in a connection have been drawn into firm contact, i.e. snug-tightened. Failure to do so will result in inadequate bolt pretension and loose connections. Also, The Pre-installation Verification fastener testing must be conducted per applicable RCSC sections with the caveat that Turn-of-Nut verification may not verified by Direct Tension Indicator verification (RCSC 48).

Turn-of-Nut is performed by rotating the nut or bolt of a fastener assembly a specific turn angle based on the fastener's length, and diameter while restraining the unturned element from rotating.

Installation - How to perform Turn-of-Nut on a single fastener assembly:

  • Determine the ratio between the fastener's length and diameter as well as the slope disposition of the outer steel plies.
  • Apply the specified turn from the appropriate table, while the unturned element is restrained from rotation (usually requiring a second installer).
  • Rotation exceeding the table below may not be corrected or reworked except by replacing the fastener assembly.

For reference, table 8.2 from the Research Council on Structural Connection's (RCSC) Specification for Structural Joints Using High Strength Bolts, 2014, appears below:

Inspection

The inspector must:

  1. Verify pre-installation verification has been performed.
  2. Verify the joint has been snug-tightened.
  3. Verify the original position of all elements.
  4. Routinely witness the installation crew actually turning the nuts/bolts.
  5. Verify the final rotated position of all elements.
  6. Prohibit excessive rotation discovered or witnessed from being adjusted to reflect turn values from table 8.2.

While it is recommended that the bolt, nut, and steel be marked and inspected prior to Turn-of-Nut implementation, this step is not mandatory. In any case, failure to inspect nuts in their original (snug), un-turned position will result in unverifiable Turn-of-Nut execution. Nut marking does not relieve an inspector's responsibility to inspect the connection twice, before and after the turn.

Advisories:

Ineffective for some bolts, The Guide:
Users of large diameter high-strength bolts, especially A490 bolts, should be aware that the RCSC specification requirement for installation of short grip bolts may not produce the required preload [pretension]. If such bolts are to be used in a slip-resistant joint [i.e., slip critical and pre-tensioned], calibration tests in a load-indicating device are advisable. (Kulak 59)
Hot Dipped Galvanizing, RCSC:
Some problems with the turn-of-nut pretensioning method have been encountered with hot-dip galvanized bolts. (RCSC 60)
Bolt Lubrication too effective or absent, RCSC:
Specification requirements for minimum turns testing of galvanized fasteners. Job-site testing in the tension calibrator demonstrated that the lubricant reduced the coefficient of friction between the bolt and nut to the degree that "the full effort of an ironworker using an ordinary spud wrench" to snug-tighten the joint actually induced the full required pretension. Also, because the nuts could be removed with an ordinary spud wrench, they were erroneously judged by the inspector to be improperly pretensioned. Excessively lubricated high-strength bolts may require significantly less torque to induce the specified pretension. The required pre-installation verification will reveal this potential problem.

Conversely, the absence of lubrication or lack of proper over-tapping can cause seizing of the nut and bolt threads, which will result in a twist failure of the bolt at less than the specified pretension. (RCSC 61)
RCSC section 9.1 snug-tightened inspection criteria flawed:

RCSC section 9.2.1 requires the inspector to witness one element being rotated relative to the other. If not, improper tension may result if an installation crew only applies the minimum amount of snug-tightening per section 9.1, Snug Tightened Joints:

9.1 ...It shall be determined that all of the bolts in the joint have been tightened sufficiently to prevent the turning of the nuts without the use of a wrench. No further evidence of conformity is required for snug-tightened joints.

If only hand tightening is used as a snug inspection criterion, it is possible for Turn-of-Nut’s initial starting point, (i.e. adequate snug) will not be achieved and the method will fail. Such failures will not depend on whether fasteners were marked or not.

Canadian Institute of Steel Construction, i.e. CISC, Turn-of-Nut

Installation:

The CISC Turn-of-Nut installation method is similar to AISC/RCSC Turn-of-Nut with a few exceptions.

  1. Required turn values are the same when both faces normal as well as when one face is sloped 1:20.
  2. 3/4 turn to be applied to all instances, when both faces sloped 1:20.
  3. The acceptable rotation tolerance is ±30°.
  4. Beveled washers are required when either steel ply is sloped 1:20 Max when using ASTM A490 and A490M bolts.
  5. CISC Turn Table 8 does not limit itself to 12” maximum bolt length, as the RCSC does.
  6. When both surfaces sloped 1:20 max, all bolts rotated 3/4 turn (RCSC requires length dependent turn values).

For a complete turn details, please refer to the Nut Rotation from Snug-Tight Condition table within the latest CISC Handbook of Steel Construction.2

Inspection:

The inspector must:

  1. Verify pre-installation verification of all assemblies has been performed.
  2. Verify the joint has been snug-tightened.
  3. Verify the original position of all elements.
  4. Verify the final rotated position of all elements.
  5. Routinely witness the installation crew actually turning the nuts/bolts.
  6. Prohibit excessive rotation discovered or witnessed from being adjusted to reflect turn values from the applicable table within the CISC handbook.

While it is recommended that the bolt, nut, and steel be marked and inspected prior to Turn-of-Nut implementation, this step is not mandatory. In any case, failure to inspect nuts in their original (snug), un-turned position will result in unverifiable Turn-of-Nut execution. Nut marking does not relieve an inspector's responsibility to inspect the connection twice, before and after the turn.

CISC Advisories:

See advisories for AISC & RCSC.

Additional Advisories:

While the CISC does not specifically require Pre-installation Verification testing of all fasteners when using Turn-of-Nut per se, testing is recommended to assure hardware compliance with the appropriate ASTM Specification. Thread failure (stripping) will not be discernible otherwise.

Figure 1: Ideal marking technique clearly indicating 1/3rd turn, However, the three bolts on the left are ready for initial inspection; the three on the right appear turned but it is impossible to know if they were turned or marked to appear turned.
Figure 2: These four nuts appear to have been turned although the exact angle is not obvious since the lines extend completely through the bolts' diameters. Nevertheless, without inspection of their initial position, turn or whether the joint was properly snug-tightened, induced pretension cannot be verified.
Figure 3: Obvious deception attempt. While markings suggest each nut or bolt has been rotated, the nut appears to be in its initial position suggesting bolt rotation. It is unlikely the bolts were rotated relative to the nuts since the washers are under the nuts, meaning the hex heads of the bolts would need to be rotated directly against the painted steel surface. A difficult endeavor, that would be exaggerated by the additional effort required to perform proper Turn-of-Nut in the first place. The gap along the upper flange also implies incomplete snug-tightening.