High strength bolts torque values

[ASFI]

Hi all ,

In Australia, to follow the standards AS3990 and AS4100, what should be the torque values for HSS bolts (grade 8.8) using a torque wrench?
for example I think it should be 580Nm for M20 galvanised bolts .
However everybody seems to use 370Nm
I have calculated as follows for M20:
1- T = K x Fi x D
2- Fi = 87 / 0.6 = 145 Kn. Minimum bolt pretension as per appendix F in AS3990. This is to drive the bolt to proof stress as i can see
3- K = 0.2 for galavanised
4- D = 20

therefrore T=580 Nm
this way all the threads are engaged and the bolt can actually surpass it s yeild strength and not be subjected much for fatigue, this is why the code prohibits using already torqued bolts.

 

[ajh1]

RCSC provides the guidance in North America for bolt tensioning methods. Their spec can be downloaded for free from

http://www.boltcouncil.org.

The guidelines in AS4100 read very similarly. The two permitted methods that are listed are "part-turn" (equivalent to US turn-of-the-nut) and the use of a direct-tension indication device. Appendix K does list a way to inspect tensioned bolts using a torque wrench but that is not a acceptable installation method. As indicated in the commentary of RCSC, torque is not a good way to evaluate tension in the bolt due to a high variability of values in the correlation. For bolts that are well lubricated torque may be as little as 50% of the torque required on a dirty or unlubricated bolt to achieve the same tension value.
The US methods do permit a calibrated wrench installation method, similar to what is described in Appendix K of AS4100, however the target torque must be established daily by running a series of tests in a calibration device.
There are no prepublished values available for torque installation that would be precise enough for general use.

 

[connectegr]

RCSC does not recognize torque as an accurate measure of adequate pretension in structural bolts. Several variables can impact the "torque" required for proper pretension; lubrication, surface friction, etc. These are listed in the RCSC commentary. These factors can vary by connection type and bolt lot. Thus, the requirement to determine calibrated wrench settings by preliminary testing. As ajh1 mentioned, RCSC references torque as a arbitration method and not as an approved installation method.

http://www.FerrellEngineering.com

 

[sdz]

Torque correlates poorly with bolt pretension as connectegr says. Preferred methods are "turn of nut"; AS4100 15.2.5.2 or load indicating washers.

Pretensioning to design values causes yielding of the bolt which is why they can not be reused.

 

[ricklts]

Have you considered using an Ultrasonic Extensometer - measure the elongation of the bolt? This practice is accepted by ASME and used extensively in PetroChem, Power (even Nuclear)and structural applications.

 

[Duwe6]

"Pretensioning to design values causes yielding of the bolt which is why they can not be reused"

Not entirely correct. If you stop at Design, you are still in the elastic range, deep into the elastic range. Torquing/tensioning slightly past Design will usually cause yielding, plus some coatings [galvanizing, yellow zinc, etc] increase the probability of cracking. Thus the general prohibition for reusing fasteners.

RCSC and AISC allow reuse of A325 non-galvanized bolting, if there is assurance that they were not overtorqued/overtensioned.