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Pretension in Bolts

Pretension in Bolts

Pretension in Bolts

(OP)
Does anyone know what value of pretension is typically acheived by "turn of nut" method using 3/4"ø A325 bolts?

I am designing a very lightly loaded clamped connection, and instead of specifying a value for the pre-tension, I would rather (if it is suitable) just note that the turn of nut method should be used.

The load per bolt is about 36kN (8kip).

RE: Pretension in Bolts

This is a common question with no easy answer due to all the variations in friction coefficients and material properties. I suggest three turns of the nut. Check out www.boltscience.com. Hope this helps. Bill B.

William H. Bernhart, P.E.


I work as a development engineer for AMP - Tyco Electronics located near Harrisburg, PA. I help design interconnections and electrical connectors for aerospace applications.

RE: Pretension in Bolts

See www.steelstructures.com for the "Structural Bolting Handbook" - the best reference for "Turn of the nut" that I have found.

Pre-Installation Testing Required Tension (kips):
 3/4" A325 = 29 kip

Steel Structures Technology Center, Inc
42400 W. Nine Mile Road
Novi, MI  48375-4132

Phone (248) 244-2910
fax (248) 344-2911

RE: Pretension in Bolts

The "Turn-of-the-Nut" method, if properly executed, should provide a minimum pre-tension load of approximately 28 kips for a 3/4 inch diameter A325 bolt (reference AISC Table J3.7).   The "as-installed" or actual bolt tension should be a bit higher than 28 kips.   The theory behind the use of the turn-of-the-nut method is that the bolt will likely never fail if the bolt does not fail during installation.

The difficulty is always in the execution.   The turn-of-the-nut method is an acceptable high-strength bolt installation procedure.   Properly trained personnel should be able to use this method of bolt installation.   It takes a fair amount of training under the supervision of experienced iron-workers to gain the experience necessary for the use of the turn-of-the-nut method.

It seems this response may be too late based on the date of your post, but this response was posted for future reference.

RE: Pretension in Bolts

can anyone help me with info on Brinell hardness testing and help on calculations on 10mm ball using a 300kg load if the indentation was found to be 3.9mm

RE: Pretension in Bolts

There is a general formula for converting the Brinell indention to hardness,


Hardness Brinell = Load / (Pi /2 x D x (D - Sqrt(D^2 - d^2)))

where Load is Kilograms
    D = Ball diameter in mm
    d = impression diameter in mm

Thus for your test, 10 mm ball and 300 kg load and 3.9 mm indentation,

HB = 300 / (3.142/2 x 10 x (10-sqrt(10^2-3.9^2)))

   =  300/(15.71 x (10-9.208))

   =  24 (a very low value for Brinell)

If you meant 3000 kg instead, the hardness would be

   =  241 Brinell (roughly equivalent to RC 23 for steel)

Hope this helps.

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