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If this is a real job site question then you have been given good advise in that relying on torque to achieve a given tension in a bolt is highly unreliable.  If this is the case then I would go to the AISC website, download the Research Council's bolt spec, and send it to whoever is asking the question.  In the spec you will find the required bolt tension and several approved methods for achieving this tension.  As mentioned previously, no matter which method is used they need to verify the bolt assembly tension in a pre-installation test which is normally done with a Skidmore-Wilhelm device.  All of this is clearly explained in the spec and further detailed in the commentary.

If this is more of a research or general question then there are several formula for determining torque vs tension values in bolts.  Some formula, especially from the machine handbooks, can be quite involved and would take a spreadsheet to solve.  Below is a more basic formula that has been around for many years.  I have had to use this in certain anchor bolt pretension cases but again I would caution against using any such formula for actual structural steel applications.

T = KDW/12

T = Torque (ft lb)
K = Torque coefficient (similar to but different from coefficient of friction)
D = Nominal bolt diameter (inch)
W = Bolt tension (lb)

K Values

0.35  Dirty or rusty threads
0.30  Heavy galvanized coatings
0.25  Dry plated fasteners
0.20  Normal as "received" fasteners
0.15  Plated lubricated fasteners
0.12  High lubricity using grease, oil, or wax

Thomas Hunt, S.E.
ABS Consulting

savksa savksa <savksa(--nospam--at)>

06/15/2003 08:22 AM
Please respond to seaint

        To:        seaint(--nospam--at)

1.  I need to tension an ASTM A490 bolt to a certain value (somewhat of a hypothetical question).  I need to calculate the torque required to achieve this tension.  Could someone tell me how to calculate the torque?
2.  Do I need to worry about pre-tension?  If yes, will pre-tension need to be considered in the equation in item 1 above for calculating the torque?
Thanks for your help!

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