6.2 Torque Arbitration Procedure

Torque Arbitration Procedure

Sometimes, after the required inspection has been done, someone still suspects that bolts in a pretensioned or slip-critical joint do not have the required pretension. RCSC Section 10 provides an arbitration procedure for that situation. It is a dispute-resolution tool, not an installation method and not a substitute for the inspection steps in Section 9.

When Arbitration Applies

RCSC Section 10 applies "when it is suspected after inspection in accordance with Section 9.2 or Section 9.3 that bolts in pretensioned or slip-critical joints do not have the proper pretension." Key limits:

• Snug-tightened joints are never arbitrated. RCSC Table 4.1 and the Section 9.1 commentary say their pretension should not be inspected, because it does not affect their performance.
• Pretensioned and slip-critical joints are arbitrated only if required to resolve a dispute (Table 4.1).
• If verification is needed after time and exposure of the completed joints, RCSC requires an alternative arbitration procedure suited to the situation.

Arbitration also does not cure a missed inspection. If calibrated wrench or unmarked turn-of-nut installation was not observed, report the uninspected work as a discrepancy; the engineer of record and the building official decide how it will be verified.

Step 1: Determine the Arbitration Torque

1. Select a representative sample of five bolt and nut assemblies of each combination of diameter, length, grade and lot in question.
2. Install each assembly in a tension calibrator with the same material under the turned element as in the work (structural steel or a hardened washer).
3. Pretension the bolt to about 15% of the Table 8.1 value, then to the minimum Table 8.1 pretension. (The 1.05 multiplier belongs to pre-installation verification, not arbitration.)
4. Apply a manual torque wrench, either a dial wrench or one that signals when a set torque is reached, and determine the torque needed to rotate the nut or head 5 degrees (about 1 in. at a 12 in. radius) relative to its mating component in the tightening direction.
5. Reject the high and low values and average the remaining three. That average is the arbitration torque.

Step 2: Test the Joint

1. Select at random 10% of the bolts, but no fewer than two, in each joint in question.
2. Apply the arbitration torque in the tightening direction.
3. If no nut or bolt head turns relative to its mating component, accept the joint as properly pretensioned.

Step 3: When a Bolt Turns

If any nut or bolt head turns under the arbitration torque, all bolts in the joint must be tested. Every bolt whose nut or head turns must be re-pretensioned by the fabricator or erector and reinspected. Alternatively, the fabricator or erector may re-pretension all bolts in the joint and resubmit the joint for inspection.

Worked Example

A disputed slip-critical splice has 30 bolts, all 7/8 in. A325 from one lot. Five assemblies from the lot are pretensioned in the calibrator to 39 kips (Table 8.1). The torques needed to turn the nuts another 5° are 520, 545, 560, 575 and 610 ft-lb. Dropping 520 and 610 leaves 545, 560 and 575, which average 560 ft-lb: the arbitration torque. Ten percent of 30 bolts is 3 bolts, so three bolts chosen at random are tested at 560 ft-lb. If none of the three nuts turns, the joint is accepted. If one turns, all 30 bolts are tested, and each bolt that turns is re-pretensioned and reinspected. In a small 8-bolt joint, 10% would be less than one bolt, so the two-bolt minimum governs.

Why the Commentary Urges Caution

The RCSC commentary warns that after-the-fact torque arbitration carries all the uncertainties of torque-controlled installation, with fewer controls: hardened washers may be absent, lubrication is unknown, and the joint may have weathered. Such arbitration can wrongly reject bolts that were properly installed. The commentary notes that measuring bolt elongation with an ultrasonic or mechanical extensometer on a representative sample can give a very accurate measurement of actual pretension.