2 post lift n cracking concrete

[xxBotelloxx]

Also contractor said lift was supposed to be on center beam which I agree which would be thick enough. Not sure why it's not in some spots.

He said he will need to look at it before deciding anything. Lift may need to be moved to be mounted on the beam. He did say 4 inch not good enough for lift mounting n if that's case cutting it out is option along with moving lift to beam which I believe is 12x16

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[brownbagg]

concrete was pour wet, over troweled on top. that not a structural crack but a stress crack from the paste on top being weaken by extra water. the chip under the plate i would classifiy as spalling, which is weaken top coat. the crack is just too tight to be a structural failure. and it goes from bolt to lead head, whick means its in the top 1/8 inch. now if it open up then yes it could turn into structural, now i have no dog in the anchor bolt not tighten up

 

[xxBotelloxx]

Well like I said before I didn't try with much force with fear of damaging it further.

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[INTMD8]

You may not be understanding how the wedge anchors work. With the AB in tension the embeded wedge applies a force to the concrete below the surface. The strength of the connection (for a single AB) is based upon the depth of this fixity to concrete. The failure mode is typically thought of as an inverted cone with the apex at the wedge and the larger dia at the surface. The capacity of the connection is based upon the cone surface area. In your minds eye can you picture the wedge close to the surface and "see" a small inverted cone?

The capacity of the AB is not based upon the dimension you suggested, but the depth of the wedge fixity below the concrete surface.

Really? The way they function seems rather intuitive just by looking at it.

So the failure mode would be an inverted cone shaped piece breaking away from the entirety of the slab x however many fasteners there are in the base plate?

Not saying that's impossible of course but seems rather unlikely. (at least nothing I've ever seen a photo of but I am not in the trade).

In my limited experience most of the failures I've read about have been anchors pulling out of the concrete rather than the slab itself breaking away.

And would the dimension I suggested not be exactly relative to the depth of the wedge fixity below the concrete surface if anchor length is (of course) known? One cannot be independent of the other.

I guess I would understand better if I could see a picture of failures. I would think a tension failure would be most common on a 2 post lift with no top beam as inward moment would be more than fore/aft (unless the vehicle placement was extremely unbalanced).

Either way, I find the discussion interesting, just wondering how many people cut out 4" concrete to replace it with a deeper section when it would have worked fine as-is.

 

[wssix99]

Are you saying I am a troll because I don't agree with you?

I am saying you are a troll because you are showing up on this site and spouting off nonsense by claiming that the world is wrong and you are right. (Although, most web trolls I've seen know how to properly format replies on forums so they don't loose the formatting tags.)

The moderation on this forum is weak, at times, so unfortunately threads like this one are allowed to spiral out of control.

 

[wssix99]

Also contractor said lift was supposed to be on center beam which I agree which would be thick enough. Not sure why it's not in some spots.

He said he will need to look at it before deciding anything. Lift may need to be moved to be mounted on the beam. He did say 4 inch not good enough for lift mounting n if that's case cutting it out is option along with moving lift to beam which I believe is 12x16

What lift do you have? If you look at the instructions, you will find that it says nothing about installing a beam under the slab. It will just direct you to have a normal flat slab.

(Again, assuming this isn't a post-tensioned slab.) This appears to be something that your contractor improvised. I'd suggest you call them out on that and confirm if that's the case - because they likely aren't qualified to do such a thing. This will give you additional leverage for whatever comes next.

 

[Denwood]

With regard to the anchor hole depths, the minimum distance for the 5/8 thread inserts I used was 6 1/4". Many have installed these into 4" holes which must seriously de-rate tensile to failure for several reasons.

1. The epoxy insert itself is 5 3/4" or so.
2. The insert installation is quite specific on how much epoxy to add to the hole, so that when the anchor is tapped in, it is fully enveloped in epoxy. If the hole is bottomless, the epoxy will just be partly pushed into the aggregate under the slab, you won't have the entire anchor enveloped in epoxy, and you therefore won't see rated tensile performance. Maxjax has shown with their small lift that around 3300lbs of tension is exerted each of the three outside bolts, their mechanical anchors are rated at 12800lbs, and pull to fail load actual was 20000lbs. From the looks of the pics though, these were in full depth concrete...not a 4" residential slab.

The epoxy anchor properly installed is rated 26000lbs tensile...in 4000psi concrete, and 6 1/4" hole depth. That is a safety margin that allows for a lot of deviation from "correct" anchor installation. The reason lifts are not dropping left and right on marginal installs is that bolt loads even on a 10K lift are likely under 6000 lbs tensile. The minute you start going to town on a lifted car though, I'd sleep easier knowing that the much higher dynamic loads are taken care of by a large safety margin.

On the wedge type anchors, the bigger danger in a thin slab is the spalling that would occur on the underside of the slab, unseen. Concrete can take a lot more compressive force than expansive..which is what the wedge type anchor is exerting, potentially quite close to the underside surface of a slab if it is too thin, and further weakened by drilling through it. That mode of failure might give the illusion that the bolt is tensioned properly, but result in pull out under load. From Confast: "It is important to avoid having the working end of the concrete anchor too close to the bottom of the concrete. This will create an unsupported edge which will reduce the holding values of the concrete anchors."

I'm no expert here, but I did read the technical documents from Wej-it ...I also set aside the wedge anchors included with my lift and spent the $$ on the epoxy versions.

 

[INTMD8]

From Confast: "It is important to avoid having the working end of the concrete anchor too close to the bottom of the concrete. This will create an unsupported edge which will reduce the holding values of the concrete anchors."

Just to add from that website-

"The best holding values is achieved when the concrete is hard and the embedment into the concrete is deep. The larger the diameter- the higher holding values the anchor will have. For example, a 3/4" diameter anchor has a minimum embedment of 3-1/4"

 

[xxBotelloxx]

What lift do you have? If you look at the instructions, you will find that it says nothing about installing a beam under the slab. It will just direct you to have a normal flat slab.

(Again, assuming this isn't a post-tensioned slab.) This appears to be something that your contractor improvised. I'd suggest you call them out on that and confirm if that's the case - because they likely aren't qualified to do such a thing. This will give you additional leverage for whatever comes next.

Nothing sure what u mean post tensioned slab.
Lift is auto lift
http://www.bestbuyautoequipment.com/2-post-car-lift-al2-9k-ac-asymmetric-9000-lb-p/al2-9k-ac.htm

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[xxBotelloxx]

With regard to the anchor hole depths, the minimum distance for the 5/8 thread inserts I used was 6 1/4". Many have installed these into 4" holes which must seriously de-rate tensile to failure for several reasons.

1. The epoxy insert itself is 5 3/4" or so.
2. The insert installation is quite specific on how much epoxy to add to the hole, so that when the anchor is tapped in, it is fully enveloped in epoxy. If the hole is bottomless, the epoxy will just be partly pushed into the aggregate under the slab, you won't have the entire anchor enveloped in epoxy, and you therefore won't see rated tensile performance. Maxjax has shown with their small lift that around 3300lbs of tension is exerted each of the three outside bolts, their mechanical anchors are rated at 12800lbs, and pull to fail load actual was 20000lbs. From the looks of the pics though, these were in full depth concrete...not a 4" residential slab.

The epoxy anchor properly installed is rated 26000lbs tensile...in 4000psi concrete, and 6 1/4" hole depth. That is a safety margin that allows for a lot of deviation from "correct" anchor installation. The reason lifts are not dropping left and right on marginal installs is that bolt loads even on a 10K lift are likely under 6000 lbs tensile. The minute you start going to town on a lifted car though, I'd sleep easier knowing that the much higher dynamic loads are taken care of by a large safety margin.

On the wedge type anchors, the bigger danger in a thin slab is the spalling that would occur on the underside of the slab, unseen. Concrete can take a lot more compressive force than expansive..which is what the wedge type anchor is exerting, potentially quite close to the underside surface of a slab if it is too thin, and further weakened by drilling through it. That mode of failure might give the illusion that the bolt is tensioned properly, but result in pull out under load. From Confast: "It is important to avoid having the working end of the concrete anchor too close to the bottom of the concrete. This will create an unsupported edge which will reduce the holding values of the concrete anchors."

I'm no expert here, but I did read the technical documents from Wej-it ...I also set aside the wedge anchors included with my lift and spent the $$ on the epoxy versions.

Never seen or heard of the epoxy anchors got a link?

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[lakeroadster]

.......

If it is just surface cracking I would suggest considering installation of threaded rods using an epoxy anchoring system.

Adhesive anchors are known to be more secure and reliable because the adhesive allows the stress/weight to be spread throughout the anchor instead of placing it all at the tip.

Never seen or heard of the epoxy anchors got a link?

Here's a link: http://anchsand.com/Portals/3/propoxy400info.pdf

There are literal hundreds of different folks making epoxy for anchors... the above link is just one example.

With regard to the anchor hole depths, the minimum distance for the 5/8 thread inserts I used was 6 1/4". Many have installed these into 4" holes which must seriously de-rate tensile

The embedded strength of the anchor is indeed based on the length expoxied into the concrete. Note the chart in the link I posted above shows two different embeddment depths for each anchor diameter, with two corresponding strengths.

 

[wssix99]

Nothing sure what u mean post tensioned slab.

A post-tensioned slab runs cables through the slab and then after its poured, the cables are jacked and stressed to squeeze the slab together. This gives it strength and resistance to cracking. It's only used in certain parts of the country and your contractor can confirm if you have one. If he says, "What???" then you don't have one.

http://www.bestbuyautoequipment.com/2-post-car-lift-al2-9k-ac-asymmetric-9000-lb-p/al2-9k-ac.htm

You can download the lift instructions from this site. They only call for a normal floor with a 4" thickness. They also warn specifically not to install the lift if there are cracks in the floor.

There is a note in the instructions that if you need specs on "pads" to call them. This is the same thing type of thing/process as Denwood has pointed out from the Mohawk instructions. This lift manufacturer will give you their spec for how a re-pour will need to be done, keyed to your old slab, and reinforced to fix your problem.


Regardless, I think a call to the lift manufacturer may be worthwhile here. You might also email them your pictures. I'm sure they will be interested to see what your contractor has done and can probably give you more definitive advice than you have received here. Nobody here will be sued if they give you advice that will cause your lift to fail. The manufacturer could - so I expect they will be very motivated to put their engineers to work to get you the right answer.

The manufacturer may also have a good opinion for you on epoxy anchors (not all epoxies are the same) but I think your problems go much deeper than the anchors.

 

[ltusler]

Brace it to the top of the building so it can't tip? Did you mean to suggest he should attach the top of the lift to the roof framing so that in the event the lift overturns the entire building comes down?

Yes, that's exactly what I mean.

Have you ever seen or used a lift?

 

[David C]

INTMD8



So the failure mode would be an inverted cone shaped piece breaking away from the entirety of the slab x however many fasteners there are in the base plate?


One of the possible failure modes. If the AB's are spaced close together the cone would be truncated where the surfaces intersect. You can see a sketch of the single cone failure in the PCI design handbook fig 6.5.3. You can see the sketch of a group of AB failing in tension in fig 6.5.4.

In my limited experience most of the failures I've read about have been anchors pulling out of the concrete rather than the slab itself breaking away.

This failure mode occurs when the wedge is not set correctly. This would be a construction error and it wasn't really relevant to the discussion on the mfg's attachment designs. The mfg's have to assume it is installed correctly.

You can see on the second page of ACI 318 App D attachment fig RD.4.1 where other failure modes are shown in sketches. All of these possible failures must be checked in calculations.

You have suggested two AB issues that weren't previously discussed. There are many more. Understand I have to reduce the information in my posts to something that can be quickly understood and is not so long that you all lose interest. There is considerably more to each of the issues and I already believe that my posts are too long.

AB design requires consideration of the following: AB loading (tension and shear, not all of a group of AB's are similarly loaded), AB steel strength, concrete strength and depth of AB embedment, proximity of concrete reinforcing to AB's, edge and end distances to concrete edges, discontinuities, and construction joints, number of AB's C to C AB spacing, base plate thickness and strength. There is quite a lot of engineering required to design concrete anchor bolts. 20 years ago we all referred to a 1/2 page table.

I think you have realized the concern with the two post lift in that with the vehicle support arms cantilevered from the column the vehicle load puts a moment on the column and subsequently on the base plate. In a 4 AB column base plate, two AB's are unloaded and the other two are in tension. This is why a single AB failure could be significant.

Either way, I find the discussion interesting, just wondering how many people cut out 4" concrete to replace it with a deeper section when it would have worked fine as-is.

I suspect that very few of these lifts are placed on anything but an existing 4" slab on grade unless it is otherwise required by the mfg. and even then maybe not. Dennis shows the Mowhawk design that requires the slab be cut out and replaced with a footing. But many of the other mfg designs show 4" slabs. There probably are not many failures either. However what I have been saying here, in this thread, is that I don't think the installations on 4" slabs would comply with current codes. And while these 4" slab installations maybe don't fail is this something that you would want to work under.

Some years ago I was asked by a school district to design the installation of two of these lifts each at several high schools. I provided a design per the code applicable at that time and it required a large pad footing with deep anchor bolts. The lift mfg's chief engineer provided me with about 30 sheets of well thought out calculations and I referred to those when it was appropriate. His was an excellent effort however he made a couple of serious errors. I wrote him a letter with my calculations and narrative describing his mistakes. He wrote back to thank me. That lift mfg may be one of those that require a footing.

 

[lakeroadster]

I think you have realized the concern with the two post lift in that with the vehicle support arms cantilevered from the column the vehicle load puts a moment on the column and subsequently on the base plate. In a 4 AB column base plate, two AB's are unloaded and the other two are in tension. This is why a single AB failure could be significant.

True, but it really depends on the lift design...

The anchor bolt loading David is discussing is applicable to 2 post lifts that have no upper structural crossbeam that span the columns, or they merely have a support for some cables and hoses, a non structural member.

Other 2 post lifts have a substantial structural overhead crossbeam that spans between the columns. With this design type the "side to side" anchor bolt loading is minimal.

If the lift has the upper structural crossbeam, the greatest loading on the anchor bolts would be "front to back" as a result of the vehicle center of gravity not being directly between the posts.

That's why following the manufacturer's requirements is so important. Different lift designs produce different anchoring specifications.



_____
John

 

[wssix99]

Dennis shows the Mowhawk design that requires the slab be cut out and replaced with a footing.

FYI - Mowhawk's guide does not give a "footing" spec. It gives a retrofit slab spec, only if the per-existing slab doesn't have the required thickness, strength, or quality. They, also, just require a simple 4"-ish flat slab. (They do call for thicker slabs for some of their larger models.)

However what I have been saying here, in this thread, is that I don't think the installations on 4" slabs would comply with current codes.

Please don't say this any more. With hundreds of thousands of lifts installed, as specified, and no actual crisis of lift failures about in the world - the message comes across as the "Sky is Falling."

The mechanics of this type of equipment, on a slab-on-grade, are very different than a structure sitting on actual "footers." Yet, the equipment is safe. This site, forum, and this thread are probably not the best places to have a good discussion on such things. If you aren't comfortable with this type of installation, you might discuss the problem with a colleague who designs pavements. The way the (thin) 4" slabs withstand the bending moments of the 2 post lift is very similar to how pavements withstand the bending they see when heavy loads roll across them.