Prefered 2-post lift location

[Rigpig]

Hi, i'm getting ready to pour the slab for the shop in a few days and wanted some feedback from those that have 2-post lifts in their shops. I've decided on the right side, but was wondering if should stuff it near the back with a couple feet from the front to get around a vehicle, or near the front of the shop for ease of access and it leaves room at the back for other stuff?? Post up what you've done, or if you'd change what you have. I'm also thinking of having about a 2-foot wide footprint, 8"thick concrete where the hoist will stand.
What are your thoughts?
Thanks.
PS:this is a shot of the front with forms removed etc... dimensions are 30'X36'deep, 2x 10' doors, 14'walls.

 

[wssix99]

I'm also thinking of having about a 2-foot wide footprint, 8"thick concrete where the hoist will stand.

You don't want to do this... You should follow the instructions that come with the lift and go with a slab of uniform thickness.

Its a natural expectation that making the concrete thicker around the post or reinforcing the concrete will make it "better," but in reality it will overstress the transition area between the thicknesses, could cause cracking, and the whole thing could fall over.

If you look at the lift from the side and a cross section of the slab underneath, you'll see that the two form an upside down "T," which counteracts the tipping forces when the car is not evenly loaded on the lift. Small "footers" under the posts don't counteract the tipping forces unless they are very very deep.

 

[sberry]

Up front install with adequate space in front of it, 12 ft or so to posts at minimum.

 

[wedge40]

You don't want to do this... You should follow the instructions that come with the lift and go with a slab of uniform thickness.

Its a natural expectation that making the concrete thicker around the post or reinforcing the concrete will make it "better," but in reality it will overstress the transition area between the thicknesses, could cause cracking, and the whole thing could fall over.

If you look at the lift from the side and a cross section of the slab underneath, you'll see that the two form an upside down "T," which counteracts the tipping forces when the car is not evenly loaded on the lift. Small "footers" under the posts don't counteract the tipping forces unless they are very very deep.

Hmm, never heard of this before. Is this still true even with rebar? My slab will 5" thick with 3/8" rebar on 1' centers so the extra thickness is kind of over kill.

Wedge

 

[Rigpig]

Thanks for the info guys, i'm pouring this saturday and decided with a 5" slab, 32MPA, 1/2" rebar on 16" centers, the cement will also be mixed with fiber. I'm also keeping it simple with a uniform pour throughout.
I've got a couple dually trucks for work, they are quite long so i'll set the lift closer to the front, making sure i've got enough room to get around.
Cheers!

 

[wssix99]

Hmm, never heard of this before. Is this still true even with rebar? My slab will 5" thick with 3/8" rebar on 1' centers so the extra thickness is kind of over kill.

The natural expectation is that the punch-through forces of the lift are so great that beefed-up structure under the posts is helpful. In reality, the stresses are similar to the car just sitting on the parking pad without a lift. Tipping of the posts, and hence bending of the slab is typically where the greatest forces will be.

When the slab bends, one half is in compression and the other half is in tension. That compressive strength is where the concrete does its thing. In a monolithic slab, that compression and tension will be balanced. When there is a deeper "footer" under the posts, the transition can get loaded up with more tension than compression (they fall out of balance) which will propagate cracking. Rebar could help this, but it would require hard-core engineering, commercial style rebar arrangements, and just won't pay off. (Because the "footer" isn't needed to counteract any of the forces in play.)

The best way to get more strength is to make the entire slab just a little bit thicker overall.

To combat shrinkage cracking, reinforcement in the middle of the slab is necessary. (Putting the reinforcement in the middle will do most to counteract the shrinkage cracking.) That can be done with fiber, steel mesh, or rebar. At some point there are equivalencies between mesh and larger rebar at certain spacings, but I'm not sure what those are off hand.

 

[Haggar]

Here are some examples from a few lift manufacturers:

From Rotary Lift's website:

*On all Two post models; If anchors do not tighten to specified torque, replace concrete under each column base with a 4’ X 4’ X 6” thick 3,000psi minimum concrete pad keyed under and flush with top of existing floor.

From Atlas:

If you mount the lift and the "anchors" begin to lift out of the concrete over a period of time, you will need to relocate your lift or pour "new concrete pads" under the columns. There is also many types of epoxy concrete adhesives that help anchor your lift in marginal concrete. The new pad should be at least 4' by 4' and 10" thick. These new concrete pads should have "rebar" installed into the new pour and into the sides of the existing concrete slab.

 

[wssix99]

Here are some examples from a few lift manufacturers:

That's the most important thing - following the lift manufacturer's recommendations. There are a lot of little devils in the details...

From Rotary Lift's website:

*On all Two post models; If anchors do not tighten to specified torque, replace concrete under each column base with a 4’ X 4’ X 6” thick 3,000psi minimum concrete pad keyed under and flush with top of existing floor.

The key thing here is "keyed under." I'm not sure how one could do this without some very specialized concrete cutting equipment or pinning the slabs. Either would require specialty tools or engineering. Pouring this kind of footer with a thinner floor around (as the OP describes) it is not the same thing as Rotary is talking about here.

From Atlas:

If you mount the lift and the "anchors" begin to lift out of the concrete over a period of time, you will need to relocate your lift or pour "new concrete pads" under the columns. There is also many types of epoxy concrete adhesives that help anchor your lift in marginal concrete. The new pad should be at least 4' by 4' and 10" thick. These new concrete pads should have "rebar" installed into the new pour and into the sides of the existing concrete slab.

Atlas is talking about pinning the slab here. It's different than putting rebar in the footer for regular "reinforcement." Like keying the slab as Rotary suggests, pinning will keep the lift from tipping, however I don't think I'd do this without and Engineer or Atlas giving me a certified plan for the pins and their placement.


Both of the above methods are for installing lifts on marginal slabs and require specialized tie-ins to the old slab. For a 100% new slab, its most economical (and perfectly stable) to pour as the manufacturers recommend with a normal flat, monolithic slab.

 

[Charles (in GA)]

I've got a couple dually trucks for work, they are quite long so i'll set the lift closer to the front, making sure i've got enough room to get around.
Cheers!

Here is a friends Dodge 3500 on my Challenger 9000 lb lift. The posts are 15 ft (center of the post) to the door. For me, this was too much and I wished I had set them at about 13 ft or so. I don't own anything big and the lift intrudes way too far into the building where it is. The back end of the truck could have stuck out for all it matters, I just needed it where the cars would clear with room to walk behind them.

Lift installer (who moved this used lift for me) talked me into the 15 ft.

Charles

 

[brownbagg]

i got mine two foot off the wall, cant get my fat belly bewteen the wall and lift so have to walk all the way around

 

[sands35]

When the slab bends, one half is in compression and the other half is in tension. That compressive strength is where the concrete does its thing. In a monolithic slab, that compression and tension will be balanced. When there is a deeper "footer" under the posts, the transition can get loaded up with more tension than compression (they fall out of balance) which will propagate cracking. Rebar could help this, but it would require hard-core engineering, commercial style rebar arrangements, and just won't pay off. (Because the "footer" isn't needed to counteract any of the forces in play.)

The best way to get more strength is to make the entire slab just a little bit thicker overall.

(I'm a ME, not a civi) 1st engineering principals suggests that it's the transition from the "thick" part to the "standard" part of the footer that creates a stress concentration. So bevel the edge of the transition so it's a soft change, not a step. Or make the "pad" under the lift big enough to carry all the anticipated loads and put crack control cuts around it's perimeter so when it does crack, it does so in a controlled way.

But yeah, follow the MFGs recommendations. Probably simpler to just add 1/2" in overall thickness if it makes you sleep better - or just pay for a commercial quality poor. Over-thinking it is bad.

 

[KPSquared]

I have my work area in front of my 30x36 shop. I'm set up to have my hoist 20 feet from the doors. That way I can get my crew cab 1 ton up without issue.

I drive in on the 30' end. Bays are 36' long.

 

[camarotoolman]

3' feet all the way around, using your biggest truck for a guide. position rebars so you don't hit then when you drill your anchor bolt holes. I would just dig out 3-4" inches of dirt about 3x3 ft. to beef up the floor. imo its not that big a deal. Me, I would put the anchor bolts in beforew the pour, just make sure they are positioned correctly. Thats how they do steel columns on a slab. Makes finishing harder though.

 

[wnstwolf]

At first I had mine centered in the bay that is 40' deep and 22' wide. The entry door is 10'wide. In the end I kept it centered at 20' back from the garage door but offset it so the right post was in ligh with the edge of the garare door. this gave me a lot of walk by space to the left of the lift. Still plenty of room to drive in and have vehicle centered

 

[wssix99]

(I'm a ME, not a civi) 1st engineering principals suggests that it's the transition from the "thick" part to the "standard" part of the footer that creates a stress concentration. So bevel the edge of the transition so it's a soft change, not a step.

Because concrete is assumed to hold up to zero tension, even a transition will cause problems. The cracks come from its inability to resist tension vs. repetitive stress.

Or make the "pad" under the lift big enough to carry all the anticipated loads and put crack control cuts around it's perimeter so when it does crack, it does so in a controlled way.

This is really it. The pad needs to be large enough to handle the loads on its own or smaller footers need to be separate from the slab and have "pin" joints by using rebar pins or the other keying method mentioned above. (The pinning uses the larger slab to resist tipping of the posts with the smaller footers.)

 

[Blk88GT]

I centered mine in the bay (40ft). That way I have plenty of room. There's nothing worse than being cramped for space!