How is everyone measuring slab psi?

[wssix99]

It's posts like yours make me love this forum.

Thanks. The comfort you seek lies in the stress cone.

It's safe and possible to put a lift on lesser concrete than 3000 psi. All that is needed is more depth. A larger stress cone in lesser concrete can be equivalent to a smaller stress cone in regular concrete.

If you want really good holding power, the best investment is probably a quality drill and bit to get a clean hole for the anchor to bite into at a low depth, and get the maximum size in your cones.

 

[curiousB]

wssix99,

Good insight on the stress cone aspects to the fasteners. Regarding the tip of the cone. can it be near the bottom side of the slab or should it be located near the center of the slab?

For a 4" slab that could be the difference between 2" deep or 3.5" deep which would dramatically change the size (volume) of the cone.

 

[rlitman]

wssix99,

Good insight on the stress cone aspects to the fasteners. Regarding the tip of the cone. can it be near the bottom side of the slab or should it be located near the center of the slab?

For a 4" slab that could be the difference between 2" deep or 3.5" deep which would dramatically change the size (volume) of the cone.

The deeper the bottom tip of the cone, the larger in surface area the cone is. And the larger the cone's surface, the stronger it is.

 

[GMCGarage]

It SHOULD be, but I've seen crumbly old **** too.

OP didnt mention that. I would think after 30 years it would be toast.

 

[GMCGarage]

You know...the slab psi is only an important hair to split if you are using wedge anchors.

If you drill for epoxy anchors, prep the hole properly and make sure to cover the fastener completely, place the epoxy in proper temp range; I'd use B7 threaded rod at the same time, you're going to a place that a wedge anchor can't in terms of pullout strength.

I did not know that. If that's the only real concern, I'd gladly pony up for the cost of epoxy anchors if that's all it is.

Uhh, what do you think holds the epoxy anchor in? Its the strength of the concrete.

 

[GMCGarage]

All things aside, what tension capacity do your bolts need to have? Oh, thats right, no lift manufacture tells you what the load actually is.

 

[curiousB]

The deeper the bottom tip of the cone, the larger in surface area the cone is. And the larger the cone's surface, the stronger it is.

Yes that is clear but as you approach the bottom side of the slab and "set" the fastener don't you risk fracturing the slab (due to compression forces of the expanded wedge) on the underside and not even know it?

 

[GMCGarage]

Yes that is clear but as you approach the bottom side of the slab and "set" the fastener don't you risk fracturing the slab (due to compression forces of the expanded wedge) on the underside and not even know it?

Those forces are minimal to 'set' the wedge. If your concrete fails from that force, you need a new slab. After that, the tension load will be pulling upward, and creating the cone of failure.

 

[sberry]

Here is what I know. Thousands and thousands are installed every year without a test. The op said it was in good shape and that's about all the installers look at and the only ones seem to have concern is the diy types.
If this was a problem it would be designed differently so it could be used on common concrete and if it was much an issue there would be more inspection. They leave it to installers to self appraise and some of them barely got a drivers license.

 

[wssix99]

The deeper the bottom tip of the cone, the larger in surface area the cone is. And the larger the cone's surface, the stronger it is.

+1

All things aside, what tension capacity do your bolts need to have? Oh, thats right, no lift manufacture tells you what the load actually is.

It changes. If the load is perfectly balanced, then the tension is just the pre-load of the anchor or a Net of zero. If the load is off balance, then it starts growing...

 

[wssix99]

Yes that is clear but as you approach the bottom side of the slab and "set" the fastener don't you risk fracturing the slab (due to compression forces of the expanded wedge) on the underside and not even know it?

If the concrete were glass, yes - but it's not that brittle. Even if it was, it wouldn't matter as long as the stuff up top was good and the anchor couldn't pull up and out.

 

[brownbagg]

drill all the way through, that way if you need to move the lift later you can drive the anchors into the subsoil

 

[Flail]

If it's gonna keep you up at night, then just bite the bullet, rent a concrete saw and cut out the slab for a thickened reinforced slab. The rebar and concrete cost will be equivalent to a bunch of testing that will give you a probability of failure but assure you of nothing.

 

[GMCGarage]

+1




It changes. If the load is perfectly balanced, then the tension is just the pre-load of the anchor or a Net of zero. If the load is off balance, then it starts growing...

I understand that. But at some point something will fail, the upright, the baseplate, the anchor, the slab. It would be nice to make sure (as the installer) that your part is not the weakest link.

 

[GMCGarage]

Those forces are minimal to 'set' the wedge. If your concrete fails from that force, you need a new slab. After that, the tension load will be pulling upward, and creating the cone of failure.

EDIT: Based on a quick review, it seems about 2" is needed below the anchor to fully get the capacity, at least according to Hilti. More for bigger anchors.

 

[Partsguy57]

BTW, the slab is in very good condition overall. No surface flaking or cracks. Not missing major chunks or anything. Still measures very level (not more than 1.5 degrees of slope any of the 18 different places/ways I tested).

Good grief... bolt in and go..

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[wssix99]

I understand that. But at some point something will fail, the upright, the baseplate, the anchor, the slab. It would be nice to make sure (as the installer) that your part is not the weakest link.

The anchor bolt isn't close to its breaking point, so I would expect everyone omits it because its so far from being critical.

My back of the napkin math:
- Concrete tensile strength is roughly 1/10 that of the compressive strength. (say 300 psi)
- A 4" bolt will grab at 3.5" down giving a stress cone surface area of around 30 sq in.
- That is around 9000 lbs of force before the bolt pulls out of the concrete
- A 3/4" anchor bolt has .45 sq. in. of cross section, which would be just over 20ksi of tensile force before the thing pulls out of the concrete...

A cheap, mild steel will withstand over 50ksi of force, so the concrete slab should fail/pull-out way before any 4", 3/4" wide anchor bolt does.

^ This would not hold true for longer and/or thinner bolts. The lift manufacturers usually spec the 3/4" bolts, which makes us good as long as we follow that guideline.

 

[brownbagg]

go out there an hit the slab close to the edge with a sledge hammer, one good wack, and if it doesnt break, Its 3000

 

[darkbuddha]

go out there an hit the slab close to the edge with a sledge hammer, one good wack, and if it doesnt break, Its 3000

I like that idea, but talk about "destructive" testing!

 

[darkbuddha]

go out there an hit the slab close to the edge with a sledge hammer, one good wack, and if it doesnt break, Its 3000

In all seriousness for a minute, how big a sledge hammer? Short handle, long handle? I gave it a shot with the 3 lbs. short handle sledge I have and it made a divot in the surface, maybe 1/3" deep, chipping off just the surface.

 

[darkbuddha]

Looks like this...

https://photos.app.goo.gl/4N5RDXNsimpofba86

 

[Partsguy57]

Looks like this...

https://photos.app.goo.gl/4N5RDXNsimpofba86

Good grief bolt it in and go.. I am going to say what many are thinking.. if you have this much concern you really don't belong working under a lift. There are thousands of lifts installed per year with none of the **** you are so concerned with. Common sense is king. Inspect the slab, Drill your holes, take a good look, set your anchors if they torque properly good to go.... moving on. ( your post of measuring for level 18 different ways speaks volumes of your mentality on this issue) and yes I have have a lift, three of them to be exact on all three different slabs, garage, small shop, large shop... I installed all three in less time then you have thinking about installing a lift. Cheers

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[darkbuddha]

Good grief bolt it in and go.. I am going to say what many are thinking.. if you have this much concern you really don't belong working under a lift. There are thousands of lifts installed per year with none of the **** you are so concerned with. Common sense is king. Inspect the slab, Drill your holes, take a good look, set your anchors if they torque properly good to go.... moving on. ( your post of measuring for level 18 different ways speaks volumes of your mentality on this issue) and yes I have have a lift, three of them to be exact on all three different slabs, garage, small shop, large shop... I installed all three in less time then you have thinking about installing a lift. Cheers

I think I'm getting there with the bolt-in-and-go mentality. Thing is, I have worked under lots of lifts in unknown situations without any real thought on its worthiness, mainly because all of them had been installed and used for some amount of time prior to me getting under them. My main fear is that this is going in my HOME garage, where my wife or kids might walk out there, and that's added a level of concern I've not encountered before. My relative flagrant macho reckless stupid disregard for my own safety is trumped by my concern for the well-being of my kids and missus. And with that statement, I realized I can almost completely mitigate that concern by changing the location of the lift install several feet. 2 more feet that way >>>, 3 more feet the that way ^^^, and the only time they'd be in any danger is if they were under it. And once it's up and been up a while, I'm sure I'll have nearly no concern at all.

So yeah, bolt in and go is sounding more and more likely.

 

[Partsguy57]

I think I'm getting there with the bolt-in-and-go mentality. Thing is, I have worked under lots of lifts in unknown situations without any real thought on its worthiness, mainly because all of them had been installed and used for some amount of time prior to me getting under them. My main fear is that this is going in my HOME garage, where my wife or kids might walk out there, and that's added a level of concern I've not encountered before. My relative flagrant macho reckless stupid disregard for my own safety is trumped by my concern for the well-being of my kids and missus. And with that statement, I realized I can almost completely mitigate that concern by changing the location of the lift install several feet. 2 more feet that way >>>, 3 more feet the that way ^^^, and the only time they'd be in any danger is if they were under it. And once it's up and been up a while, I'm sure I'll have nearly no concern at all.



So yeah, bolt in and go is sounding more and more likely.

Like I said. Common sense is king here. Slab looks good.. check... holes look good when drilled. Check... anchors torque properly... check ... enjoy... you will only have one regret.... why did I not install a lift way earlier in life... they are so nice to work under Cheers

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