Question on torque limits when tightening nuts on epoxy set anchor bolts

[VitalSigns]

Hopefully this type of question is acceptable here as it's not work related....if it's against forum rules then I apologize.

I am installing a diving board base at my home. There was an existing diving board but the old cast in place anchor bolts were rusted past mere surface rust (sections of thread had corroded away) so my plan was to drill new holes and epoxy set anchors. The base plate of the diving board base is a 1/4" thick steel square plate with four corner holes at 12" OC. The diving board manufacturer sells an epoxy kit to do exactly what I am planning but rather than buy the kit (as it had a rather hefty markup) I just separately purchased what was in the kit for less than half the cost. I purchased 6" grade B8M 1/2" SS bolt studs and Simpson Strong-Tie SetXP epoxy. I offset the new drilled holes by 5" (3" back, 2" over) from the old ones (Simpson Strong-tie lists a 3" min spacing) and epoxy set the anchors to 4" depth.

When looking at the base install instructions from the diving board manufacturers website I see that the nuts on the anchor bolts are called out to be torqued to the 40 to 50 ft-lbs range. However, Simpson Strong-Tie SetXP epoxy specs list a max torque for 1/2" anchors at 20 ft-lbs. When epoxy manufacturers give max torque specs like that are they meaning the nut tightened to the anchor bolt can't exceed 20 ft-lbs? I guess I'm a little unsure on the physics of it on when you're tightening a nut on a bolt...how much of the torque on the nut (measured with a torque wrench) is transferring to the bolt...is it 1:1 once you get snug? Or is the torque applied to the nut transferring to axial force on the bolt via the threads and if so could I apply more than 20ft-lbs of torque to the nut and still be ok with the minimum torque specs on the epoxy set anchor? Sorry if this is a question that seems silly...I'm in project management and my college engineering classes were long ago. I just don't want to over tighten and compromise the epoxy anchors.

 

[desertfox]

Hi VitalSigns
Yes the epoxy manufacturer is saying the torque should not exceed 20ft-lbs.
The torque is merely a means of getting the axial load in the bolt, I wouldn’t recommend exceeding the epoxy manufacturer’s advice maybe you should give them a call and ask them can you overload the bolts.



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

 

[Enable]

If you exceed the max torque you are liable to shear the bond between the epoxy and the parent material. I would recommend observing the max allowable of the epoxy manufacturer.

Hilti HY200 provides a max allowable of 30ft-lbs for a 1/2" diameter anchor, which might get you a bit closer. Alternatively, 5/8" anchors have allowable installation torques of 60ft-lbs if you have the room to install a slightly larger anchor.

I'd call the diving board supplier and let them know of the discrepancy with the materials in their kit and see what they say. If it's a hand waving answer (likely will be) I bet they just slapped some numbers together and what ends up happening in practice is home owners install as tight as they can by hand and let it be. That being the case, I'd be pretty comfortable just observing the epoxy manufacturer's literature.

 

[VitalSigns]

I've already installed the bolts so what's done is done unfortunately. I went with the 1/2" bolts since that's what they sell in their kit so I figured I'd just match them exactly (along with the epoxy type) assuming if they are selling it then it must be ok. Although in hindsight had I known about the torque limits prior to buying and installing things I would have gone with 5/8" bolts (and/or used Hilti HY200) since the holes in the base plate are 3/4".

I'm guessing the install instructions with the base were written assuming cast in place anchors so I'll definitely just go with Simpson's specs of 20 ft-lbs torque and then just check on it periodically. I mean worst case scenario is the base plate just loosens a little bit after a while and I retighten down to 20 again right? I'll be using marine grade anti-sieze so I shouldn't have any issues with the SS hardware seizing on me over time. I will contact the diving board manufacturer just because I'm curios what they'll say....I agree it will likely be a handwaving type of response.

 

[Enable]

You should be fine. Re-tightening is not an issue.

I'd either score the threads just above the nut or tack-weld the nuts in place so they cant back off. It's also just good practice for applications where kids or others might look at it and go "FUN...I can twist this so lets try it!"

 

[desertfox]

Hi VitalSigns

When you say you are using the Anti seize grade, do you mean you are torquing to the 20ft-lbs with Anti seize on the bolts? If so I wouldn’t because you will alter the friction factor on the threads and that can increase the axial load in the bolts by a factor of three or four in which case you might be exceeding not only the epoxy load for adhesion but also the axial stress for the bolt. Checking the bolts periodically might well be okay so long as no one gets hurt between checks.

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

 

[dik]

Loctite thread locker...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[VitalSigns]

Desertfox, yes putting anti-seize on the bolts before putting the nut on was going to be my intention as that was part of the installation instructions. I didn't realize that it could alter the force applied to the bolt when using my torque wrench. I've read that stainless steel fasteners have a nasty habit of seizing to each other which is why the instructions to use anti-seize made sense. Dang...that gives me another thing to ponder. I looked up the stuff that I bought (Loctite 8023 Marine grade Anti-seize) and in the tech data it gives a torque coefficient of 0.18. Is there an equation where I can use this coefficient to find what I should be torqueing to?

Another weird thing is that I found two data sheets for seemingly the same product and material but one has 20ft-lbs for 1/2" anchor and the other says 40ft-lbs for the same thing.

https://www.buildsite.com/pdf/simpsonanchors/SET-XP-Product-Data-1849358.pdf

https://www.buildsite.com/pdf/simpsonanchors/SET-XP-Installation-Instructions-592257.pdf

Guess I need to contact Simpson Strong-Tie to clarify.

Also as far as ensuring the nut doesn't back off I was thinking of just using a jam nut as it seemed less permanent if i ever did need to remove the thing for some reason.

 

[dik]

Catch my notes for jam nuts...
-INSTALLATION OF ‘JAM’ NUTS:
THE OUTER JAM NUT SHALL HAVE A THICKNESS OF NOT MORE THAN 75% OF THE INNER NUT.
THE INNER NUT SHALL BE INSTALLED FIRST AND SNUGGED TIGHT.
THE OUTER JAM NUT SHOULD BE INSTALLED SNUG TO THE INNER JAM NUT.
THE INNER NUT SHOULD THEN BE SNUGGED TIGHTLY TO THE OUTER JAM NUT WITH THE OUTER JAM NUT RESTRAINED FROM ROTATING.
WRECK THE THREADS BELOW THE NUT.
in this case, above the nut...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[desertfox]

Hi VitalSigns

The reason I mention it is that I had some failures of threads during my working life because the fitters had not intentionally but managed to get grease on the threads and when they tightened them the threads failed.
I noticed in the epoxy manufacturer’s information it doesn’t mention any lubricant alongside the maximum torque, so that to me suggests the torque figures are without lubricant and furthermore does the torque figure refer to stainless steel bolts as that’s another factor?
I looked up your studs and they are 316 stainless steel and the epoxy only mentions 303 stainless and a 400 series stainless and you will notice they give the strength for those materials in psi on the page below the torque figure.

Have a look at the link below that shows a formula you can use.

https://www.fastfixtechnology.com/offshore/bolted-joint-consistency-with-stainless-steel-fasteners/

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

 

[desertfox]

Hi VitalSigns

If you want to use a jam nut bolt science recommends this method

https://www.boltscience.com/pages/twonuts.htm

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

 

[Tmoose]

Were all four anchor bolts more or less equally rust damaged?
I'm thinking the two farthest from the launching end of the board are subjected to primarily tension load.
But If the base plate is very flexible at all then the prying loads on the nuts could be pretty substantial.
I guess the fact they have lasted this long //may// indicate the design is OK. Unless the jumpers are always lightweight subteen kids.


1/4" thick steel can be a mighty wimpy base plate in my world, which admittedly has not include any diving boards, yet.
For base plates in general I want the fasteners to be close to ribs, gussets, stand-offs etc. Anything that is going to be providing the load path from what is above, to the base plate, and the anchor bolts. This is to provide stiffness, and reduce unplanned prying loads.

https://www.bestbuypoolsupply.com/media/interfab-duro-spring-stand.PNG

https://cdn.shopify.com/s/files/1/0...-4eeb-a67e-329068bce649_360x.jpg?v=1620047754

https://i.ebayimg.com/images/g/I~IAAOSwMapgY1-V/s-l1600.jpg

A 1/2 inch bolt/nut tightened to 20 lb-ft is likely exerting 3000 lbs tension ± .
Depending in

 

[dik]

thanks, desertfox... will fix.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Revised:
-INSTALLATION OF ‘JAM’ NUTS:
THE INNER JAM NUT SHALL HAVE A THICKNESS OF NOT MORE THAN 75% OF THE OUTER NUT.
THE INNER NUT SHALL BE INSTALLED FIRST AND SNUGGED TO 50% OF THE TORQUE VALUE.
THE OUTER JAM NUT SHOULD BE INSTALLED SNUG TO THE INNER JAM NUT.
THE INNER NUT SHOULD BE RESTRAINED TO KEEP FROM ROTATING AND THE OUTER JAM NUT SHOULD BE FULLY TORQUED TO IT.
WRECK THE THREADS BELOW THE NUT.

thanks... fortunately I don't use them often and wrecking the threads will prevent removal/loosening... I generally use Locktite Red...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[VitalSigns]

Tmoose,

it was one of the rear bolts that was worse than the other (actual sections of thread corroded away)...the others were corroded pretty bad...fine where the existing nut was but the threads below that were deformed enough to not let the nut go down any farther but not as bad as that one. When I cut the old anchors (I left one for grounding purposes since the originals are supposed to have been attached to the grounding loop) the cross section looked ok so maybe they would have been fine to just reuse but it just seemed wrong to reuse them given their appearance.

The base I have is actually the exact same one as the middle and last link you posted (probably off in my 1/4" thickness estimate, might be 3/8"), it's an SR Smith Cantilever base....the tan version that is shown with the anchor bolt/rebar setup is what I'm assuming was cast in place at my house when the pool builders originally installed everything since that is what SR Smith sells for installing with new pool deck pours. Their epoxy anchor kit for installation on concrete decks that have already been poured consists of what I matched (6" bolts and Set XP epoxy).

The weight limit for the setup is supposed to be 250lbs.