2 post lift concrete question

[Calhouncm]

Good Morning,

I’m looking for some help with planning out a cut in my concrete and to pour a new section for a two post lift. My pole barn and concrete are fairly new and I did pour deep enough floors with the lift planned, but the control cuts I didn’t have planned out so they are right where the lift would go.

I’ve ordered a forward i10 10k lb 2 post lift. I have outside and inside measurements of where the two post lift will sit. The lift instructions call for a 4’ x 4’ with 6” deep concrete. I plan to go 8-10” for some added peace of mind.

For those familiar with setting up lifts, what do you recommend for spacing of rebar in a small pour like this? I want to ensure I don’t have rebar where the post anchors will do. Also for tying the slabs together, how deep do you drill into the existing slab to tie together? And then for tying slabs together do you do all 4 sides or just the same direction the car faces?

Thanks for any and all help.

 

[rooster59]

Things to think about
crushed stone packed?
5000lb concrete
fiber mesh

i would pin the new slab(s) all the way around

 

[Walkers]

If you initially poured thick enough concrete, why are you going to cut it out?

 

[sz0k30]

You are way overthinking this. Have had a 2 post lift for probably close to 20 years on standard 4" pour.

 

[wssix99]

If you initially poured thick enough concrete, why are you going to cut it out?

Because the control cuts in the the original floor are too close to the posts. This cuts down the holding capacity of the bolts holding the post down to the floor.

Since the OP is out of spec, a repair slab is needed.

 

[wssix99]

For those familiar with setting up lifts, what do you recommend for spacing of rebar in a small pour like this? I want to ensure I don’t have rebar where the post anchors will do. Also for tying the slabs together, how deep do you drill into the existing slab to tie together? And then for tying slabs together do you do all 4 sides or just the same direction the car faces?

These are great questions but I would advise on getting professional direction. The instructions in Forward's manual are wholely inadequate and 6" is not enough to "key" into a slab unless you dowel. (As you point out, they don't give any direction for that.)

I would first call Forward and see if they can send you a detailed spec and plan for these repair slabs. If not, you might refer to Mohawk's guide. They have a really good guide for this.

 

[ConCretin]

You might clarify if the existing control joints are really a problem. If I recall correctly, it's usually proximity to the anchors themselves that creates a problem and it's measured in inches. Otherwise it's best probably best to follow wssix99's advice.

 

[wssix99]

If I recall correctly, it's usually proximity to the anchors themselves that creates a problem and it's measured in inches.

This is correct. There is a failure cone around each anchor, which is a function of its depth. If a crack (or control joint) intersects any of these cones, then the holding capacity is decreaced. (Then there's a factor of safety on top of that, so we can't be right up against the cone, either. So the "inches away from the post" measurement takes all that into account.)

 

[NUTTSGT]

I'd listen to the advice above.

If you still need to cut and repour, I'd find out what the minimum load charge is and use the number in addition to how big of section you have to repour.

Before placing rebar, figure out your exact layout and placement of the lift...string lines to screws in the sill plate or lower wall. Using cardboard templates of the lift base and string lines should get you close enough to avoid hitting rebar when you drill the anchor holes.

 

[SmackinHondas]

There's no way to move the position of the lift? Don't you just need the anchors to be 1' away from cuts/cracks?

 

[firebirdparts]

Lots to debate here, but I'll just ignore that and say the lift really is two independent cantilevers which are each holding up half a car. I personally would cut all the way across and do 4' x all-the-way-across and put some long steel in there. There's a big moment at the base plate of each post. Make a concrete beam that has ahold of both those moments.

4' in fact still seems a little bit small to make darn sure you don't drop the car in the front/back direction. They always say to pin the slab and that is to me adequate for this, but I wouldn't say adequate for resisting the fundamental moments that exist even if the car is perfectly balanced.

 

[575cat]

Yes major over thinking , where the hell do you think that thing is going to go ? hoist arm will break first .

 

[SM Racing]

Why not just add a larger steel base plate to push the lift anchors further away from the cracks. Sounds like the floor is plenty strong enough just the cracks are in the wrong place. Bolt the lift to the plates and anchor the plates to the floor. Or anchor through the floor plates into the floor and then anchor the floor plates further outboard from the cracks in the floor?

 

[wssix99]

Why not just add a larger steel base plate to push the lift anchors further away from the cracks.

This would require larger and longer bolts. (Requiring a thicker-than-spec slab.)

 

[Walkers]

There's no way to move the position of the lift? Don't you just need the anchors to be 1' away from cuts/cracks?

That was my point as well.

 

[ItsNemo]

Scroll to the bottom of this page, they have retrofit requirements for their lifts:

Concrete Floor Requirements - 2-Post and 4-Post Lifts - BendPak

Minimum floor requirements for pre-existing concrete floors and new concrete slabs when installing BendPak two-post lift and four-post lift models.

www.bendpak.com

You need to do a lot more concrete than you think.

Also, pretty much all lift manufacturers say not within 6" or so of control cuts.

 

[SM Racing]

This would require larger and longer bolts. (Requiring a thicker-than-spec slab.)

Why? If he was going to install the lift on a 4in slab to begin with, how would having four more anchors further out out increase the required depth of fastener and concrete to meet the requirements of the lift?

 

[CraigStu]

I think that the OP is correct in asking. I don't think it is just the fastners that enter into this. Say one of the posts is near the intersection of 2 control cuts but far enough away to meet fastner specs. As firebirdparts says, "the lift really is two independent cantilevers which are each holding up half a car." I could see the lift cracking off the entire triangle of concrete. Or if the lift post is just inside of a control cut, it could crack out a chunk of the floor. This is one thing I have never understood about 2 posts w/ nothing on the floor. They are putting a thin piece of steel across between the tops of the posts to run hydraulic or cables and release cables. Why not make that piece stronger so then it would keep the tops of the posts from moving inward. Seems to me that would get rid of all the cantilever forces so all the posts would need to deal w/ is vertical weight and forward or backward forces from the car not being at it's center of gravity.

 

[SM Racing]

I have also wondered the same thing. Why not tie the top together? I guess the biggest issue is the clearance above the lift and hurting the lifting height with a tall profile vehicle.

 

[like2wheel]

I had the same problem & I have the same lift. 5" 4k psi concrete. I think the installation instructions said 3 3/8" from the slab edge, so I offset my closest anchor that much from the control cuts. Been 1 1/2 years with no problems.
I'm in shops regularly, often see posts straddling cuts & much worse.

 

[Garrym]

I have also wondered the same thing. Why not tie the top together? I guess the biggest issue is the clearance above the lift and hurting the lifting height with a tall profile vehicle.

I’ve installed a 2 post in each of my two sheds. No more than 4” of concrete in either. But I tied each post to the roof trusses. Works a treat. Cars are solid as a rock when in the air.

 

[like2wheel]

I have also wondered the same thing. Why not tie the top together? I guess the biggest issue is the clearance above the lift and hurting the lifting height with a tall profile vehicle.

Forward I10 already has a bar across the posts. It is a clear floor lift.

 

[firebirdparts]

Forward I10 already has a bar across the posts. It is a clear floor lift.

some connections in fact could be structural. but it should be very obvious this is not. And I mean very obvious.

to me, a wise person tying it to the building makes a ton of sense in the "couldn't hurt" category, but I sure wouldn't ask for advice on it. If you can't take responsibility for it yourself, then I'd say just follow the instructions as written. Nobody anywhere is going to do an engineering evaluation on installing the lift not as designed. It's too much trouble.

 

[like2wheel]

some connections in fact could be structural. but it should be very obvious this is not. And I mean very obvious.

to me, a wise person tying it to the building makes a ton of sense in the "couldn't hurt" category, but I sure wouldn't ask for advice on it. If you can't take responsibility for it yourself, then I'd say just follow the instructions as written. Nobody anywhere is going to do an engineering evaluation on installing the lift not as designed. It's too much trouble.

Yeah, I didn't mean to imply it was structural.
I do know of a lift installed in the center bay of a 3 car garage with 8' walls & a hip roof.
Most of the rafters have been removed & the lift is supporting the roof at the ridge board.

 

[CraigStu]

I do know of a lift installed in the center bay of a 3 car garage with 8' walls & a hip roof.
Most of the rafters have been removed & the lift is supporting the roof at the ridge board.

I love it. That sounds like something my brother and I would have done 30 years ago. And I bet the one you know of will be just fine.
Edit to add. I realized the way I wrote that might make it seem like we wouldn't do it now cause we are smarter. No, not at all. I'd do it tomorrow.

 

[Hobby_Man22]

So is a 4" slab typically more than 4" some lifts require 4.25" was wondering why not an even 4?

 

[wssix99]

So is a 4" slab typically more than 4" some lifts require 4.25" was wondering why not an even 4?

It depends on the bolts required to hold down the posts. There are some threads here noting that Bend-Pak changed their spec from 4" to 4.5" without changing any of the lifts. This is probably due to people having problems with the embedment of the bolts. The extra space gives some insurance that the bottom of the slab won't blow out if one drills all the way through it, etc.

In the end, the required depth is a function of the anchor bolts being used.

 

[Hobby_Man22]

It depends on the bolts required to hold down the posts. There are some threads here noting that Bend-Pak changed their spec from 4" to 4.5" without changing any of the lifts. This is probably due to people having problems with the embedment of the bolts. The extra space gives some insurance that the bottom of the slab won't blow out if one drills all the way through it, etc.

In the end, the required depth is a function of the anchor bolts being used.

I noticed that. I think mohawk called for the same thing. How long are the bolts? Do they not go all the way through?

 

[wssix99]

I noticed that. I think mohawk called for the same thing. How long are the bolts? Do they not go all the way through?

No, they don't go all the way through. Take a look at the picture in post #8. The wedge at the end of the bolt bites into the concrete. Starting at that bite point, a stress cone forms where the maximum tensile stresses in the concrete build. There's no stress below that bite point.

However... if the bottom of the slab blows out when one is drilling the hole for the bolt then there isn't anything for the wedge to bite into. This causes the bolt to ride up closer to the surface of the slab before it grabs hold, which weakens it. (The shallower the bite, the less holding power it has copared to what it was designed for.)

 

[Hobby_Man22]

No, they don't go all the way through. Take a look at the picture in post #8. The wedge at the end of the bolt bites into the concrete. Starting at that bite point, a stress cone forms where the maximum tensile stresses in the concrete build. There's no stress below that bite point.

However... if the bottom of the slab blows out when one is drilling the hole for the bolt then there isn't anything for the wedge to bite into. This causes the bolt to ride up closer to the surface of the slab before it grabs hold, which weakens it. (The shallower the bite, the less holding power it has copared to what it was designed for.)

So then just utilize some tape on the drill bit so you don't go too deep?

 

[wssix99]

So then just utilize some tape on the drill bit so you don't go too deep?

Yes, that's the text book way of doing it. If I recall correctly, my instructions called for drilling an extra 1/2" over the depth of the anchor.

Others here will advise drilling all the way through in the event that you want to uninstall the lift. That way you can tap the anchor down into the ground and grout the hole. This risks a blow-out of the bottom of the slab, so you would need to weigh this, with the required depth for your bolt, with the thickness you have to work with in your slab.