Turn-of-Nut on a Pretensioned Brace Connection Without Verification
A special inspector is assigned to a new four-story braced-frame building. The structural drawings designate the brace connections as pretensioned. In the field the inspector watches two ironworkers installing 7/8 in. diameter, 6 in. long, hot-dip galvanized ASTM F3125 Grade A325 heavy-hex bolts in standard holes. They are using the turn-of-nut method, match-marking with a paint pen, and applying a quarter turn past snug-tight; the foreman calls this 'the standard quarter turn, it's what we always do.' The erector's quality file contains no pre-installation verification records, and the tension calibrator on site is still in its crate. Bolt certificates of conformance are on file and current. The contractor asks the inspector to sign off the connection so the deck can be placed.
1. Compute the turn required by the turn-of-nut method for these bolts and state whether the observed installation complies.
Read the worked answer
The required turn is keyed to bolt length in diameters, not to the diameter alone:
- Diameter D = 7/8 in. = 0.875 in.; bolt length L = 6 in.
- 4D = 3.5 in.; 8D = 7.0 in.
- L falls between 4D and 8D, so the required turn is 1/2 turn past snug-tight, not 1/4 turn.
The observed 1/4 turn is half the required rotation. A quarter turn may still bring the bolt into the required pretension on some assemblies, but the code does not accept a single 'standard' turn - the turn is a function of length, and it must be verified. The installation as observed does not comply. Note also that 'snug-tight' has a defined meaning (plies in firm contact, installers as tight as they can get them with an ordinary spud wrench, with several hits of the impact wrench), so the inspector must confirm the turn was measured from that condition, not from finger-tight.
2. What is the missing element of the method, and what should the inspector require before the joint can be accepted?
Read the worked answer
The missing element is the pre-installation verification test, which is required for every pretensioning method, including turn-of-nut, before production installation. It must be run on a tension calibrator using representative assemblies - same diameter, grade, length, coating, washer type and installation condition - three assemblies per set, with the turn that reproducibly develops the required pretension recorded and used for production.
Inspector's actions:
- Do not sign off. Issue a nonconformance describing the observed 1/4 turn versus the 1/2 turn requirement and the absence of verification records.
- Require the verification test to be witnessed and documented; confirm that the verified turn matches the 1/2 turn used, or that the crews adopt the verified value.
- Because the bolts are galvanized, confirm the assemblies, lubrication and washers match those tested.
- Only after verification, require the crews to complete the additional turn on the affected bolts (total rotation measured from snug-tight). Bolts that have been fully pretensioned and then released must be replaced, not reused.
- Document all of it and route through the EOR/AHJ.
3. The foreman argues that pretensioning is not required at all because 'bolts in bearing don't need to be tightened beyond snug.' How would you resolve this?
Read the worked answer
The inspector does not decide this by argument; the inspector decides by the contract documents. Verify on the erection drawings and specifications which connections are designated as pretensioned versus snug-tight (some drawings mark pretensioned or slip-critical joints with a symbol; a general note may make all connections snug-tight unless noted).
- If the drawings and specification require pretensioning - which is typical for bracing and for joints subject to fatigue, load reversal, or slip-critical design - then the full method applies: verification test, specified turn or calibrated wrench/DTI, and inspection of all bolts.
- If the joint is truly a snug-tight bearing connection under static load, pretensioning is not required and the 1/2 turn issue is moot; the acceptance criterion becomes firm contact and full nut engagement.
Any ambiguity between the general notes and the connection designation is a documentation conflict to be resolved in writing by the EOR before the inspector accepts the work. Until resolved in writing, the conservative reading (pretensioned) governs.