Concrete anchor question: Setting/Installation torque versus the actual fastener torque

[HeavyG]

Hi All.
Electrical engineer here, got mechanical question. The concrete wedge anchors i'm using for my car lift (M16) specify 90 flt-lbs installation torque; which is required to "set" the anchor into the concrete. Per specs., for my concrete psi, this provides approximately 4200 lbs clamping force/tension. My question is should i use the same 90 ft-lb torque to install my fasteners that bolt the plate to the concrete? Am i right in thinking that i wouldn't want to place torque tension on the anchor system beyond its clamp rating?

Seems like the ad hoc convention is, after setting the anchor, to use the same 90 ft-lbs to actually fasten the base plate. Seem like i wouldn't want to install the plate at the same torque as I used to set the anchor. Thx for any feedback.

 

[SwinnyGG]

Are you talking about drop-in threaded anchors?

The setting torque is just the minimum required to ensure that the anchor is fully set. It doesn't mean you actually applied 4200lb of tension to the anchor.

It's not clear what specific anchor you're using (if you could tell us that you'll get better information) but 4200 lb is likely an ultimate load that has been factored.

Best practice is to follow the install instructions for your lift and not change things on the fly.

 

[HeavyG]

This is the anchor:

j

Wej-It, PD58. Yes, drop in type threaded anchor. The instruction for the lift are poor at best and only specify "setting" torque of 90 ftlbs.
So, for my concrete the allowable tensions is 4120 lbs. Safety factor of 4. So, if I fasten my lift plate also w/ 90 lb-ft of torque, won't the resultant tension on the bolt exceed the allowable load of 4120?

This chart shows approx 20,000 lbs for torque of 170 ft-lbs on my bolt, just guesstimating, a fastening torque of 90 ft-lbs would result in tension in neighborhood of 10,000 lbs.
Still digging for exact charts. But is there a legitimate concern here? I don't want to over torque the fastening bolt to apply a clamp load that exceeds pullout force(allowable tension). Thx for your feedback.

 

[SwinnyGG]

Well, if you install the lift base plates with less than the specified torque, you run the risk of the lift tipping.

I see your point about the applied tension exceeding the limit of the anchors.

The answer to me appears to be that you have the wrong anchors selected.

 

[desertfox]

Hi HeavyG

Can you post a picture of the scenario or a sketch which might help me and others to see your problem clearer.

Thanks

Desertfox

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

 

[TugboatEng]

Consider installing studs with leveling nuts below the plate. Install the plate and tighten the top nut to spec. This will prevent excess tension from being applied to the anchor.

 

[HeavyG]

THx for feedback. per request i can provide addtional info.
The anchoring system looks like this:

 

[HeavyG]

I like the idea of a 2nd nut but i'd have to buy all new bolts. Maybe my concern is over-blown. But according to this Fastenal chart.

With a M16,8.8 bolt, no lube (K factor = .2) and a torque of 90 lbs, i'd get about 8500 lbs of tension (clamp load). I think the relationship is linear so
15320/161*90 ~8500lbs. This is twice the Wej-It nominal retention force of about 4200 lbs. Keeping in mind that the 4200 lbs is spec'd by Wej-It with a safety factor of 4X. So in reality, the ultimate retention force of the anchor system is 16800 lbs. So i guess if I torque to 90, I'm at a safety factor of roughly 2X. Don't know what industry usually calls for but I guess i'll live with it.

Thx all for comments and correct me pls if it looks like i screwed up some assumption.

Best Regards
Greg

 

[WinelandV]

Don't pretension your wedge anchors beyond what the install info says. You're anchoring to concrete, not bolting 2 steel parts together. When you use 90ft*lbs of torque, you're not pre-tensioning your wedge anchor, you're causing the little flaps at the bottom of the anchor to push against the side of the concrete hole, causing the anchor to lock into place and let the anchor transfer tensile loads into the concrete.

Please note that is a "v" (as in Violin) not a "y".

 

[RMiller77]

Our lab placed strain gages inside similar concrete anchor bolts and measured the bolt tension after torqueing the nut. The initial tension in the bolt relaxed significantly over the first 15 minutes; postulated to be the failure of the concrete crushed under the wedges. Torque again and the bolt tension relaxes again. Do this enough, and the bolt pulls past the wedges and out of the hole.

As previously stated, set the wedges into the concrete using the recommended technique. Concrete strength is an important factor and here it affects the bolt tension.

 

[SwinnyGG]

His problem is not setting the anchors - his problem is that the amount of clamp load required by the lift exceeds the capacity of the anchors.

@HeavyG the torque specified by the lift manufacturer for the anchor bolts is effectively telling you how much preload the anchor bolts need to apply to the flanges for the lift to function safely.

Part of the reason anchor manufacturers derate anchors (allowable load being less than ultimate) is because of the creep behavior described above by RMiller.

I'll say again - you have the wrong anchors. You need anchors which have an allowable load less than the load you apply when you tighten the lift mounting flanges to their specified torque.

 

[desertfox]

Hi HeavyG

I agree totally with SwinneyGG you have the wrong anchors, what you need to determine is the actual clamp force on the plate the bolts have to withstand and you can lift the car safely, once you have that figure you can get the correct size and capacity of the drop in anchor, so if the bolt tension is in the order of 10000lbf then those drop in anchors need to be capable of withstanding say 20000lbf.
However why do you need 90lbft on the plate hold down bolts? The bolts holding down the lift legs shouldn’t see much tension unless of course the centre of gravity of the thing your lifting is offset from the bolted legs.
Is your car lift a two or four legged frame.?
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[WinelandV]

If HeavyG needs pre-tensioned anchors to produce a clamp force, then wedge anchors should not be used (because all the torque on the wedge anchor is doing is setting the wedges). In fact, post-installed anchors shouldn't be used at all. Cast in place anchors with a heavy hex nut at the bottom, and a sleeve that bears up against the baseplate so that the bolt has an actual length that can be stretched should be used if pre-tensioning is required.

Does the lift manufacturer require that the anchor be pre-tensioned, or does the anchor just need to resist a tensile load? None of the information presented as of yet has shown that pre-tensioning the anchors is required.

Please note that is a "v" (as in Violin) not a "y".

 

[HeavyG]

The dimensions of the 2 post lift are here:

Holes for anchors are about 3-4" apart.
Here's how manufacturer's anchor setting instructions: (looks to be copy and paste from Wej-It)

No where is there a recommended fastening torque. Although, interestingly enough there is a recommended fastening torque of 60 ft-lbs for the alternate Epoxy anchoring system (which i'm not using). Very interesting RMiller77, regarding the retorquing and creep behavior. Scary because this manufacturer advertises that lift can be unbolted and removed and then re-installed at your convenience. I'm not going to do that.
Also the sleeve does not contact the base plate, after setting the anchor, it's about 1/3" below concrete surface. I've torqued my bolts to 70 ft-lbs which should provide ~ 6660 lbs of tension. The bolts have split-lock washers so I'll be depending on those to retain the bolts.
Thx for all the insight folks. Definitely learned a few things here and I agree this manufacturer's anchoring system definitely is questionable and appears to not provide a safety factor of 4x.

Thx Again.

 

[desertfox]

Hi HeavyG

I wouldn’t rely on the split lock washers they are not reliable see link below

https://www.nord-lock.com/campaign/bolt-securing-options/methods/result/split-ring-washers/

If you can state the weight of vehicle and provide the horizontal distance looking on the top of the car lift from the bolt holes to the the lifting points on the arms I can estimate some bolt tensions for you.



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

 

[kjoiner]

Hello,

You might want to consider Hilti and talk to one of their application engineers.

Kyle

 

[dvd]

Here is a spreadsheet with some info from ACI 318, that I created. There is more info off to the right in the spreadsheet, so scroll across to see it all. Your anchor is governed by concrete breakout and anchor pullout (also fastener strength, but that is usually more than the concrete failure). If you can use a longer anchor with more embed, the breakout strength will increase, but the pullout strength stays the same. Note that the breakout strength is not a function of the anchor diameter.

 

[HeavyG]

dvd, Thank-you for this information.