Replies

1. 
   oops | Sep 30, 2012 03:59pm | #1

   conc. pad

   I'm not sure what they mean either.  I don't know what kind of loads you are talking about when you say, "install a two post lift".  Often in structures that have point loads, such a support columns, they will isolate these from the slab.

   I don't see this much different than when one thickens the slab under , say a fireplace.  Check with your friendly engineer. 

2. 
   DanH | Sep 30, 2012 04:14pm | #2

   It won't "weaken" the slab,

   It won't "weaken" the slab, but the two areas of the slab (because of their different weight per square foot) are apt to settle at different rates, causing cracks near the thickened area.  For this reason it's typical to isolate the two areas with an expansion gasket or such between them.

3. 
   wedge40 | Sep 30, 2012 04:31pm | #3

   Second try.

   Here is what the guy posted.

   "You don't want to do this... You should follow the instructions that come with the lift and go with a slab of uniform thickness.

   Its a natural expectation that making the concrete thicker around the post or reinforcing the concrete will make it "better," but in reality it will overstress the transition area between the thicknesses, could cause cracking, and the whole thing could fall over.

   If you look at the lift from the side and a cross section of the slab underneath, you'll see that the two form an upside down "T," which counteracts the tipping forces when the car is not evenly loaded on the lift. Small "footers" under the posts don't counteract the tipping forces unless they are very very deep."

   Wedge

4. 
   wedge40 | Sep 30, 2012 05:14pm | #5

   Second try. Here is what

   S

1. 
   DanH | Sep 30, 2012 04:54pm | #4

   You should probably look at the install instructions for a typical lift and see what they specify for a slab if you were pouring a slab just for the lift, independent of any other slab.  One I see calls for a 4'x4'x6" slab under each leg.  Any seriously out-of-balance vehicle will (if it does not simply fall off) bend the lift or rip the mounting bolts from the slab before it will tilt the slab.

   (In any event, one thing you want to be sure to do is keep your in-floor heating coils well away from the lift area.)

1. 
   wedge40 | Sep 30, 2012 05:20pm | #6

   Lift will be a BendPak XPR-10A.  Their manual say's 4" of concrete minimum.  I will have 5", that's 25% extra.  My concrete guy wants to put 4'x4'x8" pad where the posts will be mounted because he had another cutomer who drilled through the 4" cement and it was difficult to fix.  If it would have been me I would have cut a section out, undercut the slab and poured new.

   Yes I have laid out the pex with the two pads being avoided completely.  Last thing I want is to worry about hitting the pex when drill the anchor holes for the lift.

   Wedge

1. 
   DanH | Sep 30, 2012 06:18pm | #7

   Note that if you go with the thickened sections you want to avoid those with the tubing (even if you don't install an expansion gasket) since there WILL be cracking around the thick sections.

1. 
   wedge40 | Sep 30, 2012 06:59pm | #8

   See my reply above!!!!

   Wedge

2. 
   DanH | Sep 30, 2012 08:11pm | #9

   I was just pointing out that it was not merely the danger of drilling through the PEX, but also you don't want the PEX in an area that's likely to crack/move.