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

 

[racookpe1978]

A few things.

1. What diameter is the A325 anchor bolt you are trying to tighten?

2. Are you sure - absolutely positive!!!!! - that you are trying to get to 25,000 lbs force on that bolt, or did you really want 25,000 psi pre-stress on the bolt?

3. Did you double check (again) where you found the answer to nbr 2? Why do you thik 25,000 lbs force is correct?

4. How long is the anchor bolt penetrating into the slab?

5, What quality concrete have you assumed, and how long has that concrete/rock/gravel been curing?

6. How far apart are the anchor bolts?

7. How far is the closest anchor bolt to an unsupported edge, curb, or expansion joint?

8. How thick is the slab you are drilling into?

9. How are you fastening the anchor bolt into the concrete? (expansion collar, adhesive or epoxy, grout, plate on the end of the anchor bolt, nut on the anchor bolt, bent anchor, smooth anchor bolt or deformed?)

10. Where is your force? Sideways to the surface of concrete, vertical, or a combination of the two?

11. Are the loads on every anchor bolt the same?

 

[Ron]

First of all, I doubt your anchor bolts are A325....more likely A307.

Unless you have a really strange spec, there is generally no reason to pre-tension anchor bolts. Anchor bolts are generally tightened to a "snug tight" condition.

Give more info please.

 

[jdgengineer]

I was under the impression most anchor bolts are F1554?

Assuming you meant you are installing a A325 high strength bolt that is required to be pretensioned, there are 3 methods available to pretension: turn of the nut, calibrated torque wrench, and direct tension indicator.

A non-calibrated torque wrench can not be used. It's my understanding that the wrench needs to be calibrated to prove that a certain torque on the wrench provides the required pretensioning value.

 

[ANaja]

Ron and Racookpe,

Thanks for trying to answer. I need a very simple answer. The minimum tension to verify at the anchor bolts are 20kips and 30 kips. Which values in ft-lbs at click torque wrench makes me feel confident that I have reached the correct minimum tension of tightening.
The bolt are A325 - 5/8" and 3/4" diameter. They are used on prefabricated steel buildings.
Thanks

 

[jdgengineer]

If you are using A325 bolts you'll most likely also need special inspection for installation to verify correct procedures were used...

 

[dik]

Most steel codes for A325 don't permit the use of a calibrated torque wrench for torquing.

Why the high preload?

You can get A325 bolts in a longer format, and they may be specially fabricated. For that sort of load, you should be looking at maybe 16" min embedment to develop the headed bolt.

Dik

 

[paddingtongreen]

Anaja, there is no simple answer, it depends on the surface smoothness and lubrication. I have tested a smooth lubricated bolt in a Skidmore and then followed with a somewhat weathered, un-lubricated bolt of the same size and found that it took nearly twice the torque. If no preparation was made for inspection on your job, I see no way to perform it now.

Like the others, I haven't seen A325s used for anchor bolts, nor can I think of a reason to pre-torque to those levels.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[bridgebuster]

Put an A325 bolt in a Skidmore and tighten it to 20k, put a torque wrench on it and measure the foot-pounds. Do at least three bolts and use the average torque.

 

[bridgebuster]

Disk - what is the prohibition against calibrated torque wrench and A325 bolts? In NY state we've been checking bolts forever using this method on bridge construction.

 

[desertfox]

Hi

Doing purely the maths the torque figures according to me are 7363.3lbft and 4090.66 lbft. These calculations of torque are based on dry fasteners for which I have assumed .2 is the friction factor.
I would add that using torque to preload bolts is highly inaccurate with errors of upto 40%, also the clamping of component parts need to be considered which you give no details of.

 

[bridgebuster]

7300 ft-pounds seems quite high. I recall 7/8" dia A325 SC bolts being torqued to about 550 ft-lbs.

 

[racookpe1978]

I'm going to (again) challenge the original poster about assuming he needs to place 25,000 lbs preload on an anchor bolt...... That is, that load will will simply pull an ordinary 5/8 diameter anchor bolt out of the concrete. Unless, as asked, the anchor bolt were fastened into the concrete with a pad or plate, and EVERY anchor bolt were far enough from its neighbors and deep enough so the stress cone of the loaded concrete will not intersect.



Dia. nth Ab Abe Abr Abe/Ab Abr/Ab

5/8" 11 0.3068 0.2260 0.2017 0.7367 0.6576

3.7.1.1 LRFD
An alternative method for the determination of the tensile strength of bolts is
used by the LRFD (AISC, 1994). The capacity is computed as
EQ 3-7
where
Bn = the nominal tensile resistance of the bolt
Ft = the effective tensile strength of the bolt material,
Ab = the nominal area of the bolt’s shank
Ft is taken as 90 ksi for A325 bolts and 113 ksi for A490 bolts. Table 3-4
shows the ratios of the effective tensile area of a bolt to its gross area. Since Abe
/ Ab is approximately equal to 0.75, Equation 3-7 provides a reasonable estimate
of the bolt strength for the range of bolt sizes typically used for bolted connections.
For design, the nominal tensile resistance of the bolt, Bn, is multiplied by a resistance
factor, fbf, of 0.75.

 

[dik]

BB...

I'll dig up a source. It could be that the various friction surfaced... washer to base metal, nut to washer and thread to thread make it difficult to establish a uniform value for all instances...

Dik

 

[desertfox]

Hi ANaja
I think I made a mistake in doing the maths, when I rechecked my figures I now get 64 lbFT and 165 lbFT sorry!

 

[OHIOMatt]

Not to pile on, but something just does not add up. The allowable tension on a 5/8" dia. A325 bolt is 13.8 kips. Pre-tensioning in excess of the allowable does not add up to me.

http://www.engineersedge.com/calculators/torque_calc.htm

The above link will help you figure this out, but once again, I would really question the values that you are trying to achieve. As you will note on the site, it makes certain assumptions about friction, so calibrating the wrench would be required.

 

[JedClampett]

I once specified a torque for an anchor bolt. I think it was about 100 ft. lb. They tried it and the bolt pulled clean out. I said they did a bad job in constructing it and told them to reinstall it and not torque it.
I think the 100 ft. lb. developed about 20 kips tension. The bolt deserved to pull out. My lesson was not to torque anchor bolts. There is no reason.
And there's no reason to use A325 bolts. These are for high strength steel to steel connections. You torque them to develop frictional resistance or to allow energy to be used during repetitive loadings (like a seismic event).
I'd snug them up. Torqueing them adds no advantage.

 

[gwynn]

Jed,
Just out of curiousity, was this with an anchor plate on the anchor rod or just an embedded rod?

 

[JedClampett]

I think it was a headed anchor (no plate). It was actually a retrofit anchor set in grout. We didn't use epoxy (it was at a nuclear power plant) because no one how it acts long term vs. radiation.
I think the torqueing loosened the bond at the top of the anchor and it progressed from there.

 

[Ron]

JedClampett...exactly. My experience as well. No reason to pre-tension an anchor bolt. You are not trying to clamp the baseplate to the concrete, just hold it down in the event of uplift.