Hi everyone, I have an unfortunate and unique situation that I could use some input on. I already have an idea of what I may do to fix my issue, but additional insight is always welcome. Let me start off by saying I moved in October, one of the requirements for the place I bought was it had to have a decent shop that would allow me to put my 2 post lift in, this place had a 30x40 insulated steel building that fit my criteria. I specifically asked if the seller knew the thickness of the concrete and she said she "thought" it was 6" which was good enough for me. I recently drilled through the concrete to find 3 1/2" before I hit sand. Now, I know many people will say just put the lift on that and call it good, but I don't feel comfortable doing that at all after finding several areas where they cut corners trying to save money when they put this building up so I have reason to believe that this floor will not cut it. Most people put at least a 6" floor in a steel building as far as I know, and this one is 4" or less, so I imagine they probably put weak concrete in as well, and part of the floor has shifted which leads me to believe there's not a footing under the building either. Having said all that fun stuff, here's what I'm thinking to fix my problem, but I definitely appreciate some input from you guys as well. The posts are fairly close to where I want to install them permanently, they're just sitting there loose right now. They will be moved closer to the relief cut running the width of the posts, The loft in the bay where the lift is going will obviously be removed. Unfortunately there's only 2 doors in this building and they are both right next to walls so I don't have a choice but to install the lift right next to a wall, but that can be somewhat alleviated by removing the vertical interior metal and that would give me another 6" or so next to the lift on the wall side. Sorry that the shop is a mess, still unpacking from the move. I will definitely have to cut out concrete and replace with stronger, but since it's right next to the wall that will make this a difficult process. Here's what I'm currently thinking, since it's a steel building basically temporarily remove the entire wall next to the one post. Leave all the structural steel, but remove the exterior metal and obviously the insulation as well leaving just the structural skeleton. Then, cut the concrete to get it removed, I was thinking roughly a 7'x12' or 8'x12' removal, but that's just a rough estimate and can possibly change. I'd like at least 7' because the wider that portion is, the less stress it will make on the current weak concrete when it's tied in. The 12' width is just a rough guess as of now but am open to going wider if necessary. I'll have to find out for sure if there's not a footing under this building, but I highly doubt there is. Then the plan was to cut all the way to the perimeter, remove all that concrete and replace with 4,500psi concrete with fiber in it, at least 8" thick, maybe 10"? I figure if I have to go through all this, might as well make it nice and strong and go with more than 6" because I want to be able to put a 9,000lb truck on there and lift it high and not have to worry about the concrete, but opinions on concrete thickness would be appreciated. Obviously also tying it into the current slab and trying to make it as strong as possible. So there it is, there's my current headache along with some other highly unfortunate finds on the new place. Thanks for reading and thanks for anyone with any additional ideas!
Concrete for 2 post lift
- Thread starter CPete13
- Start date
mike93lx
ALLIANCE MEMBER
Paragraphs, please.
Lookup repair slab instructions from your lift manufacturer and follow them. No need for improvisation
Lookup repair slab instructions from your lift manufacturer and follow them. No need for improvisation
Agreed, your lift manual will tell you exactly what their minimum concrete requirements are, as well as minimum distances from edges, control joints, cracks in concrete, and any keying under existing slab required. They have taken all the guesswork out for you.
wssix99
Well-known member
BendPak may be able to help you out. Their old concrete spec called for 4" holes. That allowed for some factor of safety in the event that a few bolts pulled up a little bit. They may have worst-case scenario instructions for you to use epoxy and alternate bolts at 3.5" to maintain the needed strength.
The depth requirement is to allow the bolts to get deep enough so the concrete develops enough pull-out resistance. The old 4" guidelines covered all scenarios with all bolts. Over the past couple of years, they increased the fudge factor and changed the guide to 4.5" or 5". ... but its the same product.
Only the BendPak support team can tell you for sure. They have done the math. You could hire your own engineer to do the math, but that would cost you more than the lift or putting in a repair slab. (The internet will not give you mathematically proven information.)
This is not the case. 6" is a very thick floor and would only be needed for industrial settings.
Please share a pic.
The requirements regarding buffer from the edge of the slab and distance from cracks are non-negotiable. (Your saw cuts in the floor count as "cracks.")
You can get a diamond disk for a hand grinder and make flush any edges you have to chip out.
The physics of this are non-intuitive but the reason you would have to go to a repair slab is because you don't have enough depth in the concrete for pull-out resistance in the concrete for anchor bolts. It has nothing to do with "strength" of the floor.
All you need to do is follow the guide in the instructions for building a repair slab.
For a slab-on-grade, all the static stresses get transferred to the dirt below the slab. (by a number of ways) Having well-compacted fill underneath the slab is the most important thing structurally. (Assuming no defects in the concrete.)
If the slab was elevated and reinforced concrete, things would be different. (The lift instructions should also say/imply that it can't be installed on an elevated slab.)
This doesn't matter for the lift and is why you won't see it mentioned in the instructions.
You don't need to worry about the concrete you have right now. Your 9000 lb truck parks on the slab as it is now, contacting the slab only on 4 small tire contact patches. The lift has plates on the bottom of the posts equal-ish to the are of the contact patches on the truck. The pressure that the lift puts vertically on the slab is roughly the same as the truck parked on the slab with no lift at all!
BigMike62
Well-known member
I just installed an Atlas 9k baseplate lift and they have specs in the manual. If you don't meet the specs, they say to cut out a section and pour and new footer for the lift. I was lucky, I used an 8" bit and my 4" garage was 6" minimum.
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No. Weight + Moment + Bolt Preload will put an order of magnitude more compressive stress on the face of the slab than tire pressure would. But it is the concrete shear stress at the car-side edge of the base plate that determines the required minimum slab strength and thickness. The bolt pattern and length, base plate size, and slab thickness and strength all have to work together to resist the applied bending moment.
AC-WC
Well-known member
Here-solved it for you
When I did my install I followed the Bendpak guidelines. My lift folks just told me I needed 4" with no mention of anything similar to Bendpak for a retrofit. I figured as big as Bendpak is and all their lawyers and CYA instructions I would be more than safe to follow their guidelines.
I ended up doing a 4'X12' and 12" thick, 2" compacted stone, 2 layers of fiberglass rebar, bottom layer on chairs, pinned to the surrounding concrete.
You can do pinned OR keyed OR both.
DO NOT use 3.5" thick. It's not enough.
When I did my install I followed the Bendpak guidelines. My lift folks just told me I needed 4" with no mention of anything similar to Bendpak for a retrofit. I figured as big as Bendpak is and all their lawyers and CYA instructions I would be more than safe to follow their guidelines.
I ended up doing a 4'X12' and 12" thick, 2" compacted stone, 2 layers of fiberglass rebar, bottom layer on chairs, pinned to the surrounding concrete.
You can do pinned OR keyed OR both.
DO NOT use 3.5" thick. It's not enough.
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wssix99
Well-known member
In a simplified sense, most people freak out about the shear and compressive strength of the concrete assuming the "massive" weight of a vehicle would punch through the slab. The discussion above is an illustration that this isn't even remotely the case or even possible. A lift puts a vertical stress of tens of psi on the concrete and even "bad" concrete with 2500 psi isn't even phased in this mode.
Bolting is another thing entirely and typically why we need the depth in the slab. Concrete is extremely weak in tension and we need the anchors sunk to a depth where the concrete can develop enough tensile/pull-out resistance to the lift's potential tipping forces. ("Potential" is a key thing here. If the load is properly balanced and/or braced with tripods, tipping forces and bolt pull-outs become moot. If people are worried about safety, IMO - they should worry more about lifting procedures than the concrete; assuming the installation instructions are followed.)
Moment is another odd thing because concrete doesn't deal with tension well. When there is tipping of the lift, the compression strength of the soil below, the bottom of the slab on one side of the lift (under compression) and the top of the slab on the other side of the lift (under compression) take all the load. This leads to a fascinating exploration with Mohr's Circle, but the math works out the same -> the concrete isn't stressed very much relative to its max.
wssix99
Well-known member
Show us your math and I'll show you where you you have made errors. The design methods used for reinforced concrete or columns/beams or other structures don't apply to slabs on grade.
The OP has a BendPak lift as I do. They have a great support organization and I expect, as they did with my installs, will be fine with advising regarding factors of safety and alternate install methods when things don't go according to plan.
It may be unlikely that BendPak has a 3.5" install option under the desk but a bunch of Garage Journal Einsteins won't/don't know. A call to BendPak will be authoritative.
Is there any sort of thing people do to improve support of existing slabs, such as maybe injecting some type of structural foam or epoxy into the base beneath it? Sort of like doing a foundation crack repair only injecting to fill the base rather than a crack.
As someone who has a 2-post installed on a less than ideal slab, this is something that I have been wondering and meaning to look into.
As someone who has a 2-post installed on a less than ideal slab, this is something that I have been wondering and meaning to look into.
mike93lx
ALLIANCE MEMBER
No. Foam under the slab won't strengthen it in the way a 2 post needs.
Injecting foam or mud jacking is for leveling slabs
wssix99
Well-known member
At the point you have a void under the slab, you would need to cut out a section and pour a repair slab per the lift manufacturer's instructions.
dura eagle
Well-known member
- Joined
- Dec 25, 2023
- Messages
- 65
i have seen 1/2” plates welded around the legs and bolted down to give more surface area.
I would cut the concrete and repour it. There are several YouTube videos on do it.
I would cut the concrete and repour it. There are several YouTube videos on do it.
Based on the content of your posts, you would never understand what you were looking at.
Yes they absolutely do. Slabs have loads from above, loads from below, and loads in the plane whether on-grade or hanging in the air.
Great advice. The slab requirements stated by the 2-post lift manufacturers almost preclude using existing slabs in any case.
I dont suspect there is a void, just a less than ideal base. If I was confident that my slab had very good support beneath it, I probably wouldn't consider eventually cutting and repouring the area since I am only just barely shy of BendPaks slab spec and do not use the lift at even half its capacity.
WNYflyer
Well-known member
When I peruse two post lift manufacturers concrete support requirements I'm shocked that they don't explain how far away control joints need to be from the foot print envelope of the two post lift. Talking about when they are talking about requirements for slab-on-grades which are typically designed as un-cracked "Plain Concrete" rather than cracked "Reinforced Concrete". A control joint is a crack so need be very aware of the control joint locations relative to the lift and plan their locations accordingly.
Thank you all for your advice! I will definitely be looking into bendpaks concrete requirements and cut this stuff out and replace with new stronger and thicker concrete. Unfortunately I have found several examples of things being half a**ed in the construction of this building so I have to assume they kept with their theme with the concrete so I would never feel even remotely safe putting a diesel truck on there. I appreciate all your input!
1984shovelhead
Active member
The people I spoke to at bendpak were assholes regarding the second 4 post lift I got from them