Pretensioning Different Grades of Bolts

[vStevev]

Hi There,

I'm wondering if anyone knows how to relate the turn-of-nut method for pretensioning bolts, to weaker and stronger grades than the standard A325. If a bolt has a dramatically lower, or higher yield stress than typical - shouldn't the rotation beyond snug tight be modified to attain 0.7Fu? Does the bolt pitch account for this?

Any thoughts would be greatly appreciated.

Thanks!

 

[rb1957]

if you know the strain for 0.7Ftu, and you know the pitch of the thread, then you can calculate the rotation required.

another day in paradise, or is paradise one day closer ?

 

[Shotzie]

Would page 59 (page 73 of the pdf) of the following document Link be of any help?

 

[ajh1]

In general you would be correct, each grade of bolt potentially has its own turn-of-nut rules to follow. Similarly, two bolt standards with the same tensile strength but different thread pitches would require different turn-of-nut rules. For example, you can't use the RCSC rules for imperial-definition bolts for the equivalent bolt with a metric definition as the thread pitch standards are different. RCSC Table 8.2 with the turn-of-nut rules has been tweaked to work for both A325 and A490 bolts (conservatively in some cases) for simplicity. This is explained in the Guide to Bolted Joints that is available on the RCSC website as a free download.

 

[vStevev]

Thanks for sharing your thoughts. For bolts with the same diameter and pitch (UNC), would a ratio of the yield-stress be appropriate for other grades of anchors (For instance F1554 Gr 36, 55, 105 and B7/B7M studs)?

 

[thaidavid40]

Be very careful about extrapolating bolt pre-tension for other than A325 and A490 bolts. The methods tested by the RCSC are for those types only, and I am not aware of any testing that supports any other types or grades of connecting bolts.
Dave

Thaidavid

 

[ajh1]

The most straight-forward way to approach this is to use the RCSC preinstallation procedure. Put a series of bolts in a Skidmore testing rig, snug the bolt up, start doing a turn-of-nut operation until you reach the desired tension (70% of ultimate capacity for the RCSC process). Note the amount of turn you've done. Repeat with each bolt and take an average or minimum as you desire. That will work for any bolt strength and thread pitch. The only question that leaves is what is your desired tension stress.