Anchor bolt sleeves

[DropAnchor]

Hi Guys & Gals,

Long time reader - first time poster.

I have read numerous sources regarding the proper installation of anchor bolts for equipment baseplates and have yet to find the answer to my question.

It seems to be generally accepted good practice to install sleeves around the anchor bolts for 2 reasons -

1) to allow for minor flexibility when placing baseplate
2) for some misunderstand reason, at least on my part, to protect against failure (typically stated as fatigue failure)

I have yet to come across the technical reasoning behind the installation of sleeves (besides the minor fliexibility in adjustment). But have narrowed it down to three pretty vague reasons:

1) to allow the anchor bolt to stretch
2) so the anchor bolt can act like a spring
3) to protect against fatigue failure.

Now, I am having a hard time understanding how all three of these are related, at least assuming they are, and how the practice of installing a properly torqued anchor bolt protects against failure on vibrating equipment.

Can some one please explain this phenomenon to me in TECHNICAL terms. By all means, please use equations, for example stress, strain, moments, yield strengths, failure theories, (sy/st)^2 + (ss/sy)^2 < 1, etc ...

I can almost imagine it MIGHT be similar to a bolted joints fatigue interaction. But I cannot perfectly relate all the factors. It almost seems odd to me because of the moment arm that the sleeve provides to the bolt; if there were any movement at the baseplate - would this longer moment not lead to a bigger stress at the base of the sleeve and be worse for fatigue?!?!

Thanks for reading and replying.

And for additional credit, please comment on the practice/belief that the sleeves should be grouted in after the anchor is torqued.

 

[desertfox]

hi DropAnchor

Have a look at this site it might help:-

http://www.civildesignhelp.info/ab.html

you can just scroll down to the part about sleeves or read it all.

desertfox

 

[DropAnchor]

That is a good site, thanks. However, it also states:

"The sleeve permits elongation along the entire length of the bolt, and the bearing plate transfers the tension force from the anchor bolt to the concrete."

which I have read in other places, yet I cannot understand why or for what reason you would want the, almost, entire length of the bolt to stretch, leading to my request of potentially stating equations for backup (stress, strain, etc...)

The way I see it, and I must be missing something, is this:

Wouldn't an anchor bolt that is fully grouted to the bottom of the baseplate, that is torqued, also transfer "tension force" ... "to the concrete"?

Forgetting everything about bolt loading and its (un)relation to torque. Wouldn't an unsleeved anchor, torqued to 300 ft-lbs, provide the same clamping force to the baseplate as a sleeved anchor, torqued to 300 ft-lbs.

I hope I am missing something basic, because it seems that anchor sleeves are the obvious solution to everyone, except me. Yet no one can seem to fully satisfy my question, in my mind anyways. What does the stretch of the entire length of the bold have to do with anything?

However, much to my embarrassment, at least I did put my nickname to mechanical, that website referenced ACI 318, which I never thought to look at until now. It is quite thorough, so I hope it directly answers my question, if not - I will be back.

Thanks again.

 

[DropAnchor]

AHHHHHH!!!

Getting closer

From that website:

"An anchor is subjected to a large tensile force by intentionally elongating the entire length of the bolt. A nut is generally advanced along the bolt threads until the desired tensile force is achieved. In general, there are no standard criteria for preloading anchor bolts. The level of the required preload generally depends on the specific application of the anchor bolt. In general, preloaded bolts will require a sleeve or bond breaker to permit elongation to occur along the entire length of the bolt. Without a sleeve or bond breaker, the concrete would bond to the shank of the bolt during construction. The bond may not be completely broken during the preloading process. Over time, the bond may become completely broken and may cause a significantly reduced preload in the bolt.
Anchor bolts used for equipment supports should be preloaded to the equipment manufacturer's recommendations when specified. This is especially true for bolts anchoring rotating or vibrating equipment."

It seems the connection is between sleeves and preloaded anchors.

So now my intrigue is shifting towards - "Why you would use a preloaded anchor, and not simply set in a "relaxed" anchor and torque it?"

And, as I typed that question, it has occurred to me that the answer is likely due to the (un)relation between load and torque. One would want ton install a sleeved, loaded anchor, that has been stretched for a good portion of its length to be able to ensure the amount of anchor load.

I wonder, because this answer has been so hard to come by for me - if people are blindly installing "relaxed", sleeved anchors, and then torquing them. Which goes against all of these new revelations (for me at least).

Or have I just been so ignorant to this practice and not asked the right people?

 

[desertfox]

Hi DropAnchor

Yes the load is between the sleeve and anchor bolt in some situations, have a look at this catalogue attached below.
Possibly one of the reasons to use a preloaded anchor is to guard against fatigue, for example if you have a joint subject to cycling forces the higher the bolt preload the less the fluctuating stress.
Also bolts that are preloaded are probably based on bolt elongation which is more accurate than relying on torque.
Finally have a look at this site, whilst its mainly concerned with mechanical bolted joints it explains the theory of joint stiffness, preload etc:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html#Bolt_loading

hope this helps

desertfox

 

[DropAnchor]

Thanks Desertfox

However, your attachment does not seem to work.

I appreciate/understand (now) the added benefit of a longer "stretch" length in order to determine anchor loading as opposed to relying on torque-to-anchor load "guesses".

I do understand the phenomena between metallic bolted joints and bolt load to protect against fatigue by lowering the stress fluctuationntion, HOWEVER I am still struggling to understand the difference between sleeved and unsleeved anchor bolts and this phenomena.

For example, would the same fatigue resistant effect not also be present in an unsleeved anchor that is fully grouted in to the bottom of the baseplate and subsequently loaded (that is, put the entire "joint" in compression). If this is indeed the case, it is safe to say that the only benefit of a sleeve is to more accurately measure bolt stretch to calculate the REAL anchor load (and not have to guess with torque)?

 

[desertfox]

hi DropAnchor

Hopefully the file will work this time.

Right I believe that an unsleeved anchor bolt embedded in
concrete would have a lower elastic resilience than one in a sleeve free of grout, because the bolt embedded in concrete can only elongate above the concrete level when being tightened or when under service loads.
Now a bolt in a sleeve can elongate
along its entire length and therefore as an increased elastic resilience.
Increasing bolt elastic resilience improves the bolt fatigue life.

http://books.google.co.uk/books?id=f5Eit2FZe_cC&pg=PA244&lpg=PA244&dq=how+to+

increase+elastic+bolt+resilience&source=bl&o
ts=WNzxbASKzX&sig=BesUbNngbYs
ln9OSFNoeFbmlrRo&hl=en&ei=es9gS8
mpJZSI0wTBq-XJCA&sa=X&oi=book_result&ct=result&
resnum=2&ved=
0CAsQ6AEwAQ#v=onepage&q=how%20to%20increase%20elastic%20bolt%20resilience&f=false

 

[desertfox]

Hi DropAnchor

Well it didn't work again, but go here and open the second pdf down titled:-

Anchor Bolts and Plates


desertfox

 

[Compositepro]

Torquing the anchor bolt is preloading it. The length of the bolt controls the load stain curve. The load on a longer bolt is less sensitive to changes in stain. Strain changes could be caused by slip of the anchor or shrink in the grout. Anchor bolts are never intended to operate where a gap opens in the clamped joint. That means the load must always stay under the preload. A longer bolt can more reliably maintain a constant preload.

 

[desertfox]

hi DropAnchor

Sorry I meant to put this link here:-

http://www.maineport.com/downloads.aspx

Then go to Anchor Bolts and Plates

desertfox

 

[DropAnchor]

Thanks guys, from your inspiration I am starting to get a better understanding.

I have referred to Shigley for some formulas.

The stiffness of the bolt kb = (AE)/l
So if you increase l, you decrease the stiffness - which is what happens when you have a sleeve.

The stiffness of the joint members km:
km = 1/k1 + 1/k2 + 1/k3 + ... + 1/ki

Then, the portion of the external load that the bolt carries (C), with the stiffness of the is:
C= kb/(kb+km)

Because I haven't quite figured out how to calculate the the stiffness of the sleeved joint members I just used some representative numbers to illustrate the point, using the fact that the modulus of elasticity in compression for concrete is significantly lower than steel.

Example 1:

Let kbolt = 100
Kconcrete= 25

km = 1/(1/100+1/25) = 7.143
C1 = kb/(kb+km) = 100/(104.143) = 93.3% of load taken by bolt

Then comes the magical part that I was looking for

Example 2 with sleeve:

ksleeve = 50 (just as a random lower number from AE/l)
kconcrete = 25

km = 1/(1/50+1/25) = 16.7
C2 = 50/(50+16.7) = 75% of external load taken by bolt

I believe I found my Holy Grail. Please comment, but go lightly if you must point out flaws/errors.

Thanks for the inspiration.
(now I just need to find more accurate numbers)

 

[desertfox]

Hi DropAnchor

I think your on the right track with the bolts elasticity as per your example.
So if you have a bolt with a reduced stiffness, any strain on the bolt either tensile or compressive, increases or decreases the force in it, so a bolt with a stiffness of 10N/mm if stretched or compressed by a millimetre gains or loses 10N, but a bolt with a stiffness of only 5N/mm gains or loses only 5N for the same strain.
Therefore having a longer bolt ie- reduced stiffness increases the bolts resilience (its ability to absorb and return energy below its elastic limit) and hence improves its fatigue life.
Have a look at this site:-

http://www.boltscience.com/pages/basics5.htm

desertfox

 

[EngineerTex]

The easiest analogy is to remember back to a Slinky. Lay it on the ground and stretch it out so that you have 1 pound of tension (preload) in it. Now pull it an additional 10 inches. No big deal, right?

Now do the same experiment with one single coil of the slinky. Put 1 pound of tension (preload) in the single coil -- no problem. Now pull it an additional 10 inches. Complete failure.

For some loads like heating/cooling cycles, vibrational and impacts, the load is is so large that the strength of the fastener is irrelevant -- the load *will* move the end of the fastener. In these cases, you would not calculate the value of the load, rather, you would only consider the resulting displacement.

So remembering the Slinky analogy, you simply make the member longer and then don't bother trying to determine the exact loads that these other forces apply, you simply plan for the fact that there are certain conditions under which an additional (small) displacement will be applied to the fastener and you plan to allow the member to stretch that additional amount. In doing so, you simply make the elastic section of the member longer and let everything work itself out.

Engineering is not the science behind building. It is the science behind not building.

 

[DropAnchor]

Ahh yes, it is coming together nicely. Thanks guys.

I guess I will just have to take faith in the notion that some loads *will* move the end of the fastener. But doesn't that sort of go against the notion of F=ma for vibration. I can't easily picture how a system/event would apply a strain/displacement against an anchor (as opposed to a force) - through heat expansion, yes - but not for the often stated "sleeves for vibration" concept.

 

[desertfox]

Hi DropAnchor

Have a look at this site I deals with the vibration bit.

http://books.google.co.uk/books?id=72MOFiuR-akC&pg=PT144&lpg=PT144&dq=anchor+bolt+sleeves+and

vibration&source=bl&ots=KsqiS_JR8V&sig=NZe3uX_P3oUhQZ
s3pDdNv9cgNJ4&hl=en&ei=gMxnS_GYDIai0gTgjtGzCA
sa=X&oi=book_result&ct=result&resnum=5&ved=0CBcQ6
AEwBDgK#v=onepage&q=a
nchor%20bolt%20sleeves%20and%20vibration&f=false

 

[DropAnchor]

That is a good reference. Thanks.

 

[desertfox]

hi DropAnchor

Your welcome

desertfox