DTI Tensioning and Thread Location in a Slip-Critical Connection
A slip-critical connection uses 7/8-in A325 galvanized bolts, eight per side, with direct tension indicators (DTIs). The project specification requires pretensioned bolts and threads excluded from the shear plane. The ironworker calibrated the tension wrench that morning on a Skidmore gauge using one bolt and then tensioned all eight bolts with the same wrench setting. You observe the following: on six bolts, a 0.005-in feeler gauge will not enter the DTI gap; on two bolts, the gauge enters about 0.015 in; on three bolts, the threads are visibly within the shear plane. The erector says the two large gaps are 'close enough' and that the threads do not matter because the connection is slip-critical. You are the bolting inspector.
1. Evaluate the DTI observations. Which bolts are acceptable, and which require action?
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The DTI gap is an indicator of whether the bolt has been tensioned. For the six bolts where the 0.005-in feeler gauge will not enter, the gap is at or below the acceptance limit and those bolts are acceptable, provided the DTI was installed correctly and the correct gauge is used. The two bolts where the gauge enters about 0.015 in have a gap larger than the acceptance limit, which means the DTI has not been compressed enough and the bolt is under-tensioned. The erector's 'close enough' statement is not acceptable. Those two bolts must be re-tensioned or replaced and rechecked. If re-tensioning does not close the gap to the acceptance limit, replace the bolt and DTI. The inspector should mark the two bolts, document the readings, and not accept the connection until they are corrected.
2. Does the thread location matter in a slip-critical connection? What is your disposition on the three bolts with threads in the shear plane?
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The project specification expressly requires threads excluded from the shear plane, so the three bolts do not conform regardless of whether the connection is slip-critical. The inspector should enforce the project requirement and the approved drawings. The erector's argument that slip-critical behavior makes thread location irrelevant is not a basis for accepting a specified requirement. The three bolts should be replaced with longer bolts, or the connection should be corrected so that the threads are outside the shear plane. After replacement, the bolts must be tensioned and the DTI checked again. The inspector should document the nonconformance and require correction before final acceptance. If the contractor believes the specification should be waived, that request must go to the EOR in writing, not be decided in the field.
3. What about the calibration of the tension wrench on a single bolt? What should you require?
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A calibration using only one bolt is not sufficient evidence that the wrench setting is valid for the production lot. The inspector should require calibration per the RCSC requirements and the project specification, including the required number of test bolts and verification that the Skidmore gauge and wrench are functioning correctly. The calibration must represent the bolt assembly, lubrication, and DTI being installed. A single calibration bolt can be affected by thread conditions, washer seating, or gauge error. Require a documented calibration record, verify the wrench setting, and confirm that the production bolts are from the same lot and condition as the calibration. If the calibration is inadequate, the tensioning of all eight bolts is suspect and may require re-verification. Document the calibration deficiency and do not accept the connection until it is resolved.