Thread: Car lift delima
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WinRice WinRice is offline
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Join Date: Jun 2005
Location: Colorado
Posts: 700
I'm a Structural Engineer, so let me shed some light on this.

Small pads or even a good 4" slab is fine for a 4 post lift, when you go with a 2 post it's a whole new ballgame. If the center-of-gravity (CG) of the load is not directly over the lift posts you create an imbalance condition we Engineers call Overturning Moment (OM) due to eccentricity. In other words, the lift falls over. We keep this from falling over by the strength of the anchor bolts, the mass of the slab and the soil below the slab. The slab has to be deep enough to fully develop the strength of the anchor bolts, also the slab has to be heavy enough to counter act the OM, and the slab has to reinforced sufficiently so it doesn't break in half in the process. Think of a kids basket ball goal with water filled base to keep it from falling over.

That said, in most retail oil changing centers that use 2 post lifts you'll find something like a 6' x 10' pad, 9"-12" thick with #4 bars (1/2" dia) at 12" on center each way set in the center of the slab depth. Center the pad on the posts with the 10' dimension parallel to a line drawn through the posts. Use isolation material between the pad and the remaining slab to allow the new pad to move a little without effecting the old slab as you load and unload the lift.

And this is really depends upon the soil conditions in your area. Here in Colorado my house is on piers and grade beams, and the expansive clay soil can barely support the plain slab.

My 2 cents.
1986 911 Turbo
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Old 11-15-2009, 10:16 AM
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