A little problem...........

[fred cook]

I'm installing a two post hydraulic lift in my shop and ran into a "little" problem. When I went to drill holes in the slab for the anchor bolts, I discovered that the slab where the left post will go is only about 2 1/2" thick! The installation specs for the lift call for at least 4" of concrete. I called the contractor that built my shop and he is going to come Monday and cut out the slab where the posts sit and pour two new pilings. I think they will be 18" x 18" x 12" with the bottom of the piling slightly larger than the top to make sure that it is locked into place. The aggravating thing is that I was almost done with the installation. Now I will have to wait for the concrete to harden and gain full strength. Oh well, I would rather fix it now than after getting a car dropped on my head!

[Bill Douglas]

Sorry, it's not going to work.

[dad911]

18x18x12 doesn't seem like it's enough to me.

I'd contact the lift manufacturer, you really want a continuous beam between the two posts. That has been compromised.

Frankly, I would drill a few more holes in random locations and see how the rest of the slab is. If it's thin in other spots, make him remove and replace.

[TimT]

You're going to have to have the GC do something better than just place a a little block of concrete for the lift to sit on... cutting some holes in the floor and placing some random sized blocks of concrete to hold up a car is a little scary...It might seem counter intuitive..that the 4 " think slab is OK but a 12" deep block of concrete is not...that was because the 4 in slab is monolithic... bending and rotational forces are spread out on the slab... won't work with a tiny cube of concrete..

The GC will need to construct some sort of pier or column that will be able to resist any force when a car is up on the lift..remember the load of a car is not evenly distributed on the lift (think a 911)

I just scribbled a crude back up for why a random sized block of concrete is no good... Note that I don't do much drafting or sketching anymore...

Also have the GC pony up for high early strength concrete (not quick set) it has a few more pounds of cement/yard than "regular" concrete, and you will be able to get your productivity back sooner...

[masraum]

I'm no structural engineer, and I don't know what sort of ground your shop is sitting on, but unless you built it on rock, that doesn't sound good enough to me.

[Por_sha911]

If your slab is supposed to be more than 2.5" thick then the GC didn't do the job properly. Shouldn't he be responsible to make it right and cut out the entire floor and re-pour it to the correct depth?

[Seahawk]

Originally Posted by Por_sha911

If your slab is supposed to be more than 2.5" thick then the GC didn't do the job properly. Shouldn't he be responsible to make it right and cut out the entire floor and re-pour it to the correct depth?

That is where I am. I just had a 20x40 pour to six inch depth. Even though I know the Amish guy I hired very well, I inspected the area before the pour since I am also adding a lift.

[stomachmonkey]

Was the area under the right post 4" deep?

[Drbraunsr]

MaxJax Lift - Edelweiss

Hope the link works.
In GarageJournal here's a great work up connecting the two posts with rebar and 18" of concrete. I installed mine in similar fashion since I live in earthquake country.

Peace of mind for me.

[sc_rufctr]

The most practical way to fix this is to cut a slot so that a beam of reinforced concrete can be laid to "join" the two posts. (as suggested above)

Lets say 18 inches wide by 32 inches deep with proper steel bars... Complete overkill but what option do you have?

[DanielDudley]

There is no such thing as too much, only not enough.

[Porsche-O-Phile]

Originally Posted by sc_rufctr

The most practical way to fix this is to cut a slot so that a beam of reinforced concrete can be laid to "join" the two posts. (as suggested above)

Lets say 18 inches wide by 32 inches deep with proper steel bars... Complete overkill but what option do you have?

Yes, essentially you'd be creating a grade beam of sorts, you need something to resist overturning moments - the axial loads are pretty minimal.

If you specced 4" and got 2.5" yes, the GC owes you but if you've already paid them, good luck. You'll also need to take several samples spread around the field of the pour to build your case. One sample would probably not hold up (they'd just say it was a pocket or something). If you could get a couple dozen samples from all over with similar results AND find them AND get them to respond you might have a remedy.

The lesson here is to watch everything a contractor is doing - or hire someone else who knows what to look for while work is ongoing, or do it yourself. Contractors are notoriously good for finding ways to skimp / cheat customers and get away with it. I had a subcontractor recently (HVAC tin knocker) who tried to get away with slightly thinner-gauge duct sheet metal. We had a good field / resident engineer who caught it, then went back and put a gauge on a lot of the ductwork that had already been installed. The subcontractor was PISSED. I made him rip out the entire run of ductwork and redo it, and I refused to give them a time extension. As I see it, they tried to pull a fast one and I made sure it ended up costing them. I gave the resident engineer a raise.

[fred cook]

Ok, after giving the matter some more thought and dredging up some old structural engineering from college days, here is plan B. A monolithic pour that will be about 11 1/2 feet wide, 5 1/2 feet front to back and about 2 feet thick. Total weight will be about 5500 lbs. Will use fiber reinforced concrete mix with a re-bar or two thrown in for good measure. The lift posts attach via 5 12mm bolts (each side) in steel anchors. Bolts torque to 95 ft lbs. Each anchor will have at least 6" of concrete around it. Will allow 7-10 days for strength curing time. What say ye to this design?

[MBAtarga]

Tie that structure into/under the existing slab.

[ledhedsymbols]

I don't think there is enough existing slab to tie into. If it was thicker I would suggest using rebar dowels and Hilti epoxy or something like that. With only 2 1/2" to work with, I don't think there is a good way to tie it to existing. Maybe undermine the current slab after you saw-cut out the waste material and then pour so that it locks into the existing slab? Throw a stinger in it while you are pouring to get rid of the voids. Just a cocktail napkin kind of idea.

[Bill Douglas]

Or get a couple of big (30 inches x 30 inches) half inch thick steel plates for each side of the hoist to sit on. This way the stress is spread over a larger footprint making up for the weakness of the concrete under the previous small footprint.

[fred cook]

Originally Posted by MBAtarga

Tie that structure into/under the existing slab.

Can do that without much extra work. I drilled a few more test holes and it looks like the slab is mostly 4" thick. Just my luck to hit the thin spot! It will be easy enough to dig under the edge of the cut slab.

[javadog]

I would just pour a rectangle, not an H shape. I would put #4 rebar in it on 12 inch centers, both ways, top and bottom. I would dowel the top mat into the existing slab with #4 bar on all four sides. I'd make the bottom level. I'd pour 4000HE and make sure the slump wasn't too high, nor would I allow water to be added to the load. I'd make sure it was vibrated into place. I'd make jigs and embed proper anchor bolts into the pour for the lift bases.

JR

[DanielDudley]

Originally Posted by javadog

I would just pour a rectangle, not an H shape. I would put #4 rebar in it on 12 inch centers, both ways, top and bottom. I would dowel the top mat into the existing slab with #4 bar on all four sides. I'd make the bottom level. I'd pour 4000HE and make sure the slump wasn't too high, nor would I allow water to be added to the load. I'd make sure it was vibrated into place. I'd make jigs and embed proper anchor bolts into the pour for the lift bases.

JR

I would only embed anchor bolts if I knew they were proper. J bolts will not hold in that situation. But this would be my plan A.

[javadog]

I use the anchor bolts specified by my structural engineers. I'm betting they are proper...

Time will tell, I suppose...

JR