Building Department

High Strength Bolting in Metal Framed Buildings

Industrial metal building frame components are fastened together using high-strength bolts and nuts at critical joints. The proper tensioning, or tightening, of high strength bolts at these slip-critical (or slip-resistant) joints is necessary for metal frames to achieve engineered strength. If engineered strength is not attained, resistance to live, dead, wind, snow, and seismic loads may cause the building frame to distort and/or fail. Additionally, insufficiently tightened bolt/nut pairs may simply loosen over time and cease to contribute to frame stability.

Approved Procedures for Tensioning Bolts

There are two generally accepted methods for tensioning high strength bolts on metal framed buildings. These are:

"Turn-of-the-nut" method.

Bolt is tightened as snug as possible with a 14"-16" spud wrench. A mark is made on a point of the nut and on the frame at the same point. Using a 4'-6' cheater bar or using an impact wrench, the nut is tightened two points past the mark on the frame.

Calibrated wrench method.

The bolt/nut pair is tightened using a calibrated wrench, (not a common torque wrench), and is tightened to the manufacturer's specifications.

Other methods approved by the manufacturer or project engineer may be used with prior approval of the Building Official.

After final tightening, we recommend the bolt/nut pair be distinctively marked with paint or light colored indelible marker, to indicate the bolt/nut pair has been fully tensioned.

Building Department Inspection Procedures

All metal-framed buildings, like wood-framed buildings, must pass a framing inspection by the Building Department before further construction can proceed.

The first step in the Building Department's inspection procedure is to examine the frame to see that all bolts have been marked as properly tensioned. The next step in the testing procedure is for the Building Inspector to attempt to tighten a sample of bolt/nut pairs using a 14" wrench. If the nut or bolt can be moved with a 14" wrench using reasonable effort, it is considered to be insufficiently tensioned, and to have failed the test.

Buildings which have an unacceptable number of failed bolt/nut pairs will not be approved for further construction or for occupancy. Reinspection will be necessary. A fee of $94 is charged for reinspections.

 

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