lift and slab question

[Ohmthis]

I'm putting a 2 post 9k pound in my new house. The GC and I have been talking about the floor. The salesman just say a 4" 3k psi is the minimum. I'm thinking of digging a spot around the area of anchoring to 6" or so. What is the thoughts here. If 4" is acceptable, why pay more for a deeper slab (this could be nearly dollars, but everything counts).

 

[metlmunchr]

Rotary probably puts more engineering into lifts than anyone else due to sheer size and the percentage of the market they have. They call for 4 1/4" minimum of 3Ksi mud. But they also say if the slab doesn't have sufficient strength to allow torquing the anchor bolts to 150 ft lbs, then a 4x4 area should be cut out centered on each post and replaced with 6" thick 3Ksi.

If it were mine, on a new build I'd definitely thicken the slab at the posts to 6". It would only take 1/4 yard extra concrete to taper the grade down and get the 6" thick 4x4 areas, and no extra finishing.

If your site requires any fill, its impossible to overstress the importance of proper compaction when you plan to have any heavy point loads like a 2 post lift. For relatively small areas like a garage, its hard to beat a walk behind vibratory roller for getting really good compaction. We had to put in about a foot of fill when we built our shop. The guys said they had it to grade and had packed it with a loaded tandem dump. Although it felt hard on top, you could watch the dump drive across it and see the soil pump. Had them spend a while with a vibratory, and when we shot it again it was about an inch low on average. Brought it on up to grade and have no cracks in 9K sq ft of 6" slab after 25+ years, even with regularly having mobile cranes weighing up to 100K lbs in the shop and raised up on their outrigger jacks for repairs.

 

[astroracer]

Your salesman is correct.
Bend Pak's "minimum" requirement is 4" @ 3K psi.
It's stated in the floor requirements, here: (scroll down about 3/4's of the way)
http://www.bendpak.com/support/frequently-asked-questions/
As an example: The floor in my shop is 4" 3500 PSI. My 2 post has had an Astro van sitting on it for 4 years, straight, no issues at all. I was working under it last night.
Like metlmunchr said, getting the site prepped correctly is 99% of the job.
If you search some of the MANY other threads on this subject you will see some discussion on the goods and evils of having tapered portions in a monolithic pour. Some say the tapered areas induce more stress into the slab then the point weight of a hoist. Something to discuss with your concrete guy.
Mark

 

[Ohmthis]

My googlefu isn't on today. I've searched using tapered slab, tapered mid depth slab, but didn't find anything relating to pros and cons of what I'm looking at. Thanks for the responses and I'll go and search using lift as a key word.

 

[wssix99]

My googlefu isn't on today. I've searched using tapered slab, tapered mid depth slab, but didn't find anything relating to pros and cons of what I'm looking at.

You won't find much because a "tapered mid depth slab" was invented by the maker minds of the Garage Journal and not by any trained engineers.

Just follow the instructions, pour a flat slab, and you'll be the safest and most secure.

The 4" depth is important to make sure you get the proper engagement of the attachment bolts. This thread has some great visualizations on that: https://www.garagejournal.com/forum/showthread.php?t=370803 (You want to get your full bolt depth to spread the forces across a properly sized stress cone.)

Slabs on ground are continuously supported (by the ground) and behave differently than elevated concrete beams. They are more like a giant bottom of a footer all in themselves - this is one of many reasons why your lift doesn't need "footers."