Procedure For Bolt Tensioning

This procedure covers the tightening and inspection procedures to be adopted in tightening High-Strength steel bolts using The 'turn-of-nut method' in accordance with AISC shall be employed for bolt tightening of high strength bolts. Nut rotation control wrench should preferably be used in accordance with manufacturers instructions. 

The 'turn-of-nut method' tightening shall be adopted, and shall be carried out in accordance with the following procedure. 

Assembly Procedure

1 Ensure nuts be placed so that the mark specified to identify a high strength nut is visible after tightening. 

2 Ensure contact surfaces are clean and free from defects. 

3 Packing shall be provided wherever necessary to ensure that the load transmitting plies are in effective contact when the joint is tightened to the snug tight condition. All packing shall be steel with a surface condition similar to that of the adjacent plies. 

4 Line up holes with enough drift pins to maintain dimensions and plumbness of the structure. Pins should not be removed until bolts in balance of holes have received final tightening. 

5 Install bolts in the remaining holes, one washer only required to be placed under the rotating component. Taper washers should be used where slope on any surface exceeds 3° and where possible shall be placed under the non-rotating component. The nut shall be placed so that the mark specified to identify a high strength nut, is visible after tightening.

Pre-Installation Verification Procedure For Turn-Of-Nut Installation Method

1 Take 3 bolts of each diameter, length, grade and production lot; 3 washers of each diameter and production lot; and 3 nuts of each diameter, grade and production lot as they will be assembled.

2 Assemble the first set of bolt-washer-nut. 

3 Snug the assembly using the same technique to be used in the structure.

4 Match mark the nut, bolt.

5 Apply the required rotation as listed in the above table for the assembly being tested.

6 Verify that the tension on the dial gage is at least 5% more than the min. bolt pretension as listed in the table 2 below.

7 Record the tension achieved in a log book.

8 Remove the assembly and repeat steps 2 through 7 until three assemblies have been tested.

Turn Of Nut Installation

1 Turn-of-Nut Installation is one of four installation methods recognized by the Research Council on Structural Connections, RCSC, for installing Structural Bolts. When installations require pre-tensioned or slip-critical joints and hex head structural bolts are being utilized, this installation method results in more uniform bolt pretensions than is generally obtained with other torque-control methods. Combined with the Pre-installation Verification Procedure listed below, the “Turn-of-Nut” installation method will give your installation a proven method to achieve all of your installation requirements without torque wrenches and calibration concerns. Installation is as easy as 1-2-3 after Pre-installation Verification Testing;

• Snug-tighten the bolts in the joint;
• Match-mark the nut (see figures 1) and protruding end of the bolt; and 
• Rotate the nut by the proper amount listed in the table below.

Table 1 Turn-of-nut Method Required Turns for Pre-tensioning from Snug-tight

2 Rotations Tolerance:

• For 1/3 Turn (120 degrees) ± 30 degrees
• For 1/2 Turn (180 degrees) ± 30 degrees
• For 2/3 Turn (240 degrees) ± 45 degrees

3 Snug Tight = The tightness that is required to bring the plies into firm contact (Typically obtained with a few impacts of an impact wrench or the full effort of an Iron-worker using an ordinary spud wrench).

4 Bolts used in the Snug-Tightened Condition do not require Pre-Installation Verification Testing.

5 Always turn the nut during installation if you have the choice.

6 Always turn the same element during testing as you plan to turn during installation.

Figures 1 Match marking and Required Turns

Table 2 Minimum Bolt Pretension for Pretensioned and Slip-Critical Joints

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