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BOLTING METHODS

TURN-OF-NUT BOLTING METHOD

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Turn-of-Nut (Part Turn)

Illustration of turn-of-nut(part turn) installation with nut and bolt

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 AISC/RCSC sections with the caveat that Turn-Of-Nut verification may not verified by Direct Tension Indicator verification (AISC/RCSC 59).

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.

Bolting Methodology

  1. Determine the ratio between the fastener's length and diameter as well as the slope disposition of the outer steel plies.
  2. Snug the steel plies, in as many steps as necessary, to bring the steel into firm contact as required by AISC/RCSC section 8.1.
  3. Using a marker, or similar, draw a mark from the center of the end of the bolt, across the nut, and onto the steel. This will indicate the starting position of the nut.
  4. Apply the specified turn from the appropriate table, or from tension calibrator testing, while the unturned element is restrained from rotation (usually requiring a second installer).
  5. Rotation exceeding the table can be ignored and may not be corrected or reworked except by replacing the fastener assembly.

Marked nut, bolt & steel, before and after 2/3 turn, in turn-of-nut(part turn) installation method
Figure 1: Marked nut, bolt & steel, before and after 2/3 turn.

For reference, values from table 8.1 of the Research Council on Structural Connection's (AISC/RCSC) Specification for Structural Joints Using High Strength Bolts, 2020

Table for nut rotations from Snug-tight condition, when using turn-of-nut(part turn) installation method

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 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 5.2.

The inspector MUST NOT:

  1. Use a torque wrench as an inspection tool. Inspection is only as described above, verifying initial position at snug and final position after nut is turned.
  2. Assume marking was applied prior to turning. Tightening and THEN marking the nut, after the turn is a very popular way to perform Turn-Of-Nut and is an undetectable deception, if initial position marks are not observed by the inspector.
  3. Instruct bolting crews to back off any turn that is greater than what appears in the table. Over rotation is not a cause for rejection or rework. Compensating for over rotation, by backing the nut off, will result in less than the required pretension.

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. A pretension that is greater than the [required] value shall not be cause for rejection.5

Minimum pretension for ASTM F3125 structural bolts per AISC

Minimum pretension for ASTM F3125M structural bolts per AISC

Advisories:

Ineffective for some bolts, The Guide:

Users of large diameter high-strength bolts, especially A490 bolts, should be aware that the AISC/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 pretensioned], calibration tests in a load-indicating device are advisable. (Kulak 59)6

Hot Dipped Galvanizing, AISC/RCSC:

Some problems with the turn-of-nut pretensioning method have been encountered with hot-dip galvanized bolts. In some cases, the problems have been attributed to an especially effective lubricant applied by the manufacturer to ensure that bolts and nuts from stock will meet the ASTM Specification requirements for minimum turns testing of galvanized fasteners. (AISC/RCSC 61-62).

Bolt Lubrication too effective or absent, AISC/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. (AISC/RCSC 61).

AISC/RCSC section 9.1 snug-tightened inspection criteria flawed:

AISC/RCSC section 9.2.1 requires the inspector to witness one element being rotated relative to the other. If not, improper pretension 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 AISC/RCSC does.
  6. When both surfaces sloped 1:20 max, all bolts rotated 3/4 turn (AISC/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.

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.

Turn-of-Nut marking clearly indicates 1/3rd turn; bolts on the left are ready for initial inspection; bolts on the right appear turned but its impossible to know
Figure 1: Ideal marking technique clearly indicating 1/3rd turn, Hwever, 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.
Nuts appear to have been turned but the exact angle is not obvious since the lines extend through the bolts' diameters and induced pretension cannot be verified
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.
The gap along the upper flange of the structural connection implies incomplete snug-tightening
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.

 

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5. AISC/RCSC 9.2.1 "A pretension that is greater than specified in table 5.2 shall not be cause for rejection."

6. Guide. The Guide to Design Criteria for Bolted and Riveted Joints, 2nd Edition (Kulak et al., 1987).