Bolt Turn of the Nut Method - Bolt Force

[Rabbit12]

I'm trying to determine the number of turns required to get a predetermined prestress value for an anchor rod. Would I determine this per the following?:

Stress in Bolt: P/A
Strain: Stress/E (in/in)

Corresponding Anchor Change in Length: Stretch Length*Strain

I know the number of threads per inch so I know the amount of length the nut will move with each rotation and I can calculate the amount of pretension needed. Am I off base here? I can't seem to find anything in my references or online. Thanks.

 

[3DDave]

What you will find is that for critical applications you need to set up an instrumented test. Your calculations will be a good approximation, but the tension is very sensitive to the actual deformation and it is very hard to determine where the zero point for that deformation is. In the embedded anchor there is going to be non-uniform stretch of the rod and deformation of the material the rod is anchored in. So calculate to see if you are in the ball park, but test if it is important.

 

[desertfox]

Hi Rabbit

Your thinking is correct however the real difficult bit is knowing the actual length of the anchor bolt to consider in the stretch calculation, example if I were doing this for a stud in a tapped hole, I would consider half of the thread engagement to be part of the effective length of the stud in the stretch calculation.

https://books.google.co.uk/books?id...ve length of stud for bolt elongation&f=false

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[BUGGAR]

You can use a Skidmore gage to equate the torque vs tension in a bolt. You could additionally measure the rotation and equate that to the tension. You would likely be best to go to a test lab for the skidmore. They'll even come to your site with the instrument.