Technical Tired of 2 post lift questions?

I'm days away from pulling the trigger on a two post lift, or tapping out and buying a 4 post.

My question relates to concrete and that's why I'm hesitant today. I had what should be a 4" span poure about 2 years ago. It is 48x40 poke building. There is no metal at all, they used fiberglass fibers and I think they speed 4,000 psi.

Everywhere I look they say wire mesh is required, but is it? I'm looking at an Atlas PV-10 from a local Greg Smith store, picked up only $2,745, about 8 miles from my house. They are closing the location and this is all they have left. I have some square body suburbans but probably not lifting anything more than 6,000#.

Is the purpose of the metal to help with cracking and lifting? I have pretty straight and flat concrete and the section where the lift will go was virgin soil with good compaction.

I'm not looking for someone to tell me it will be fine or not but to educate me so I can evaluate the install risk.






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Well, you could rent an electric jack hammer, they are a work horse....cut out an area, get as deep as you need to go...then go more.


You'll have a piece of mind that way.

 

check on garage journal. my best advice is drill all the way thru the concrete when you put your anchors in.

 

If.... I had to use a two post lift, I would extend the landing plates about 12" on both sides of the post using 1" thick steel, welded on. Then large bolts into the concrete.
But, for $2700 or a few dollars more you can get a four post lift that does not have to be bolted to your floor. And you can move it. A 4" thick floor is not really that strong to hold a two post lift.

 

The reason for steel in the concrete, is that concrete has low tensile strength. Adding steel to it makes it strong enough to resist the bending load that the lift will put on it.

at least that's how I see it...

 

You've been educated, you're just ignoring it.

 

Think of it this way:

If your two-post lift is putting any stress on the slab other than downward force, you're loading it wrong. Measure the contact patch of the four tires on any given vehicle you're going to lift and compare that to the base of said two-post lift. Mine is about double, if not more. When you drive your car into the garage and park it, does the concrete crack and sink? Lifting that car evenly 20-feet into the air doesn't put any more stress on the contact patch as it does sitting on the ground.

If you're still weary about it, do some rough calculations regarding the PSI of your concrete and the foot print of the two-post lift's bases. Mine are something like 24x24-inches.

A 4-post lift isn't any safer than a 2-post lift, they're just harder to load incorrectly. If in doubt, grab a couple jacks for the lift. In fact, I wouldn't have a 2-post lift without a couple. Good, cheap insurance.

But as far as actually working on a car, I wouldn't use anything but a 2-post lift. We had four in the last shop and two 4-post lifts. The 2-posts were used daily and the 4-posts were used to store cars.

 

4” uninforced concrete ain’t gonna cut it.


Your going to want to make a hole 24 “ x 24”. X 24” deep

google how to make a proper footing, will need rebar.

thrn pour your slab with mesh over top.


Or simply look up the building code or talk to a company in your area and see how it’s done.
This is the way it’s done here.


I wanted to move a structural post in my last house roughly 18” to one side so it would get hidden in a wall. My uncle who’s a contractor told me “ sure ! No problem! Why even move it, just get rid of it and save yourself some time !”
Me: huh ?

my uncle : just remember kid if you do it wrong, your house won’t fall down right away !

me: ok let’s get and engineer here to look at it.

I was able to move the post but had to make sure it was sitting on a footing as I described to take the load.

 

uh....I think that's exactly why the 4 post lift is indeed safer than a 2 post lift. Because it's harder to load incorrectly. A big part of making something safe, is making it so the operator can't use it improperly.

 

I’m not the smartest guy but if I wanted to know if the lift was ok on my floor I would talk to the lift manufacturer...not the internet


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...or maybe read the instructions that came with the lift, and follow them?

 

i built my shop in 1991 the guy's that poured the inside concrete were telling me about this greatest new thing ,fiberglass fiber's in the concrete no rebar needed. well over the year's i learned exactly what rebar do's. when it crack's and it's going crack it keep's it from lifting unevenly. i can't believe there still pouring concrete with that worthless fiberglass and no rebar

 

I've got fiberglas in the concrete of my basement floor, steel in the shop floor. I haven't gotten a lift,yet, know that 2 posts are good for working, but backyard mechanic of some age, I'd be getting a 4 post lift.

 

You say "what should be a 4" span". If store bought 2X4's were used to form the circumference be advised that they are 3 1/2" not 4" so that cuts your thickness. At a minimum, any floor that a vehicle will be driven on should be 5 1/2" thick with steel wire mesh used if poured on the ground. This mesh needs to be placed in the middle of the concrete, not laid on the ground and poured on top of. The two post lift needs a good solid base and a base thick enough that it will not flex the concrete. Your unreinforced concrete floor is adequate for a sidewalk but not a working shop with a two post lift. That was the original design intention for the fiberglass reinforced concrete, aesthetics without the cost of steel. Structural concrete is just that, structural and uses steel for the strength. Structural concrete is what you need to attach a two post lift to. PLEASE DO NOT let us have to read about how your two post lift broke the concrete and the car you were working on crushed you in the news paper.

 

Two post lifts do not just have compressive stress (the weight of the car pressing downward). The also have torsional stress due to inevitable unbalance in car location. This places tensional loading on the base as the lift tries to tip over. That is why the lift suppliers specify concrete at least 6" thick, with steel rebar and not near expansion cracks. My last shop had a 4" floor with steel and the cement under one of the posts began to crush. I cut out a 2' square, dug the hole to +6" and put in several rebars that extended into the surrounding cement. Problem solved. The other post never failed. I would not put a 2 post on the slab you describe.