Tightening Anchor Bolts with torque wrench

[ANaja]

Please, I need urgently check the tension of A325 anchor bolts. The bolts are supposed to be tensioned to 20 and 30 kips. The torque wrench to check the bolts shows the values in ft-lbs.
What value in ft-lbs at the torque wrench corresponds to 20 kips and 30 kips? Thanks any objective and straight forward reply

 

[JLNJ]

I have used pre-tensioined anchor bolts in process towers, but not in building foundations. And not with A325 bolts.

If this is a process tower and bolt is properly embedded and sleeved you can determine the bolt pretension by the bolt elongation.

There are specialty contractors who will do this type of work without asking "what tork do ya want?

 

[desertfox]

Hi
Okay I realise I must be getting old as I read kips as being pounds per square inch, now I realise that its just pounds, using the approximate formula for bolt torque below:-

F= T/(0.2*d)

transpose for T therefore T= 0.2*0.625/12 * 20000 = 208 lbft

for the 3/4" bolt I get 375 lbft.

Having looked into this more, I have to agree with the other posters regarding the snug tight comments and these high preloads not being required.

 

[Ron]

desertfox....I've used that relationship many times and it is fairly accurate within a tight range of bolt diameters. For the 3/4" bolt you gave it is almost spot on when compared to a Skidmore-Wilhelm device. The relationship seems to hold for up to about 1" in diameter then it starts to get less accurate.

Still the bottom line is....no need to pre-tension an anchor bolt in a building.

 

[desertfox]

Hi Ron

Thanks for your comment,its amazing what one can learn on here, I never realised that a base plate wasn't clamped down till I looked on here for more information, I just assumed that bolted joints always clamped stuff down, but it does mean that any uplift on the base plate is directly transferred to the anchor bolts and that theoretically the base plate may separate from the concrete, that's something we would avoid in mechanical joints.

 

[Ron]

desertfox....we try to avoid the baseplate lifting as well! The difficulty with pre-tensioning in concrete is that enough tensioning to develop a significant clamping force might actually fail the concrete in tension...not good!

 

[racookpe1978]

Only way to develope that much force through an anchor bolt is to weld the other end of the bolt to a steel member inside the concrete, or to a plate on the other side of the concrete.

But a 5/8 dia bolt (even A325) isn't strong enough (as I see the references) to stand that much preload.

 

[zmazim]

Generally, for any material, the torque needed is directly related to the bolt size. The torque function is exponential. For 5/8" and 3/4" diameter, your torque would need to be:

18 and 36 ft-lb for 20 ksi
27 and 56 ft-lb for 30 ksi

ASSUMING:

-A coefficient of friction of around 0.12
-Tightening of new, lubricated bolts

 

[JStephen]

Except the question wasn't for ksi, it was for kips, and hopefully, he hasn't been standing there with a torque wrench for six months waiting for the answer.

 

[Ron]

zmazim....36 ft-lbs will not develop 20ksi in a 3/4", A325 bolt. You are off by an order of magnitude.
The relationship desertfox gave above is reasonably accurate and comports with direct tension testing in a Skidmore-Wilhelm device.

 

[darthsoilsguy2]

what a strange question. i wonder if they were epoxy-anchored rods and he forgot to mention.

on a side topic.... i'm pretty sure the 'calibrated torque wrench' is an installation wrench calibrated to stop turning gears at the torque that a bolt qualifies in skidmore, and the inspection occurs at the time of bolting to confirm successful procedures used... the clicking torque wrench as an inspection tool is for arbitration when there is no better alternative left, which occurs a lot. i suppose if that wrench were used to install that would work, but once the bolts are in-place, it's arbitration.

 

[bridgebuster]

"...hopefully, he hasn't been standing there with a torque wrench for six months..." No unless it's IW Local 40

 

[zmazim]

Ron my bad I thought it was from pressure to torque. Yes the equation you reffered to holds assuming the same conditions I have given. This is however an aproximation. The full equation is given in ASME PCC-1, and takes into account much more parameters.

 

[Ron]

zmazim....agree. Thanks for the full reference.