Structural Bolt Inspection

Bolted connections are the most common type of connection in building structures. These connections are well researched and their design is governed by the structural standards CSA or AISC. Although the inspection of bolted connections is straightforward, it is critical. Every connection must be visually inspected to verify that all bolts are properly installed and tightened; it is critically important that the connections be complete. The following guidelines will aid in the proper inspection of bolted connections.

Truss Clamp with Bent Bolt
Truss Chord Bolted Splice
Typical Double Angle Bolted Shear Connection

In general, it is important to observe closely during installation for:

  • workmanship
  • technique and results
  • fit up and alignment
  • cyclic snugging up of heavy connections
  • turn of nuts relative to bolts after snugging

If pretension is not required, the inspection will be a simple matter of verifying that the bolting is complete and that the bolts are spud wrench tight. It is not critical that an impact wrench be used to tighten the bolts (see article "Tightening of Structural Bolts"). However, it is normal practice on construction sites to use impact wrenches and a simple verification of deformations of the tightened part, the nut or the bolt head, is all that is required to verify that the bolts are tight.

If pretension is required and "Turn of Nut" is used, visual inspection of the connection will be necessary to verify that a washer is in place, if required by standard or specified by the designer. Witnessing the visual evidence of impacting (see the prior paragraph) will also be necessary.

If the connection is slip critical, please note that there is no such thing as a "friction" connection. This terminology has been replaced by the more precise term "slip critical." Inspection of slip critical connections is identical to an inspection when pre-tension is required. The connection will bevisually inspected for fit up and the bolts will be inspected for evidence of impacting.

In the event of unacceptable issues with slip critical bolted connections, S16-01 Appendix I outlines the "Arbitration Procedure for Pretensioning Connections." This process should be used to verify the job site torque that is then used to verify the tightness of bolts. The text of the appendix follows:

Appendix I: Arbitration Procedure for Pretensioning Connections

For pretensioned connections, when there is disagreement concerning the results of inspection of bolt pretensioning procedures, the following arbitration procedure shall be used unless an alternative has been specified:

(a) The inspector shall use a manual or power torque inspection wrench capable of indicating a selected torque value.

(b) Three bolts of the same grade and diameter as those under inspection and representative of the lengths and conditions of those in the structure shall be placed individually in a calibration device that indicates bolt tension. There shall be a washer under the part turned if washers are so used in the structure, or, if no washer is used, the material abutting the part turned shall be of the same specification and condition as that in the structure.

(c) When the inspection wrench is a manual wrench, each bolt specified in item (b) shall be pretensioned in the calibration device by any convenient means to an initial tension of approximately15% of the required bolt tension and then to the minimum tension specified for its size in Table 7. Tightening beyond the initial condition shall not produce greater nut rotation than that permitted in Table 8. The inspection wrench shall then be applied to the tightened bolt, and the torque necessary to turn the nut or head an additional shall be determined. The average torque measured in the tests of three bolts shall be taken as the job inspection torque to be used in the manner specified in Item (e).

(d) When the inspection wrench is a power wrench, it shall first be applied to produce an initial tension of approximately 15% of the required fastener tension and then adjusted so that it will tighten each bolt specified in item (b) to a tension of 5% to 10% greater than the minimum tension specified for its size in Table 7. This setting of the wrench shall be taken as the inspection torque to be used in the manner specified in Item (e). Tightening beyond the initial condition shall not produce greater nut rotation than that permitted in Table 8.

(e) Bolts represented by the sample prescribed in Item (b), which have been tightened in the structure, shall be inspected by applying, in the tightening direction, the inspection wrench and its job inspection torque to 10% of the bolts, but not less than two bolts, selected at random in each connection. If no nut or bolt head is turned by this application of the job inspection torque, the connection shall be accepted as properly tightened. If any nut or bolt head is turned by the application of the job inspection torque, this torque shall be applied to all bolts in the connection and all bolts whose nut or head is turned by the job inspection torque shall be tightened and reinspected. Alternatively, the fabricator or erector may choose to retighten all the bolts in the connection and then resubmit the connection for the specified inspection.

Practical guidelines for the inspection of bolted joints

1. If the bolt does not break upon installation it is okay.

2. There is no such thing as a "friction" connection or "friction-type bolt." There are only slip critical connections.

3. Bolt threads do not need to "stick out" beyond the face of the nut.

4. To avoid threads in shear plane, some stick through may be required.

5. Burrs that prevent the solid seating of parts need to be removed.

6. Paint that is 25mm, but not less than the bolt diameter, from the hole edge does not reduce slip resistance.

7. Wire brushed, galvanized surface is best inspected by hand to roughen the surface. Do not polish or remove galvanizing.

8. Only steel is permitted in the plys of connections where bolts are pretensioned.