Keep an eye on the concrete under your lift.

About a month ago, my middle son had his XJ up on the lift. He had run it up and down several times and didn't keep an eye on the pad locations. As the suspension worked, the panhard bar inched the front over just a little every cycle of the suspension. He had the 3" blocks under the pads, and the front slipped off the pads, dropped 3" and landed on the arms. It was quite the bang, enough to bring me out of the shop wondering what had failed. We set the vehicle down and checked the lift and concrete out. No issues were found so we had a discussion about proper use of the lift and the laws of gravity and he went back to work. 

Since then, my eldest's son has had his Suburban up on it for brakes, and just last weekend the Crown Vic for a steering rack and tie rods. The concrete was checked before each vehicle and no issues were found.

I habitually check the concrete around the lift every time I use it or move it. It has been the only part of the lift that had made me a little nervous since I installed it. There were no projects coming up, so Friday evening I pulled it and stuck it back in the shop. Once again I checked the concrete for signs of failure. 

This is what catastrophic concrete failure looks like. It wasn't there a week before. I don't know if dropping the XJ started it or just the contraction of the concrete with the cold weather. Looks like I have a project to do before the lift gets used again. I'll be cutting this entire section out and replacing it with 6" of concrete. 

If you have a MaxJack sitting on 4" of concrete, keep a close eye on it and inspect the concrete regularly.

It doesn't take much of a drop for an impact with a rigid surface to see a 5 g  or more deceleration.  

That's a bending failure from excessive moment applied.    Punching shear would be a funnel shaped crater roughly the shape of the item which applied the excessive weight. 

 We  had a punching shear failure at the fab shop I used to work at when the shop guys were moving an 130 ton piece of equipment on Hillman rollers (similar to the flat rollers for moving vehicles around but bigger).  Sounded like a shotgun going off and shook the whole building.

That, and rolling forward/backward as the suspension cycles with the handbrake on or trans in Park, is why you never set a car all the way down once it is on the lift.  Every time the lift pads lose contact, the vehicle WILL move.

Had to zoom in the picture to see the crack. Yeah,that's scary. Also a good reason not to paint or exopy the floor near the lift towers, could mask a crack.

I think if it were me and I was unsure of the cement thickness I would try to spread the load. Maybe have some steel or aluminum plates (1/2" thick) made increasing the foot print by maybe twice the size. Then drill and tap plates to fit lift legs and then attaching plates to floor. Maybe I'm over simplifying the problem and not seeing the whole picture.

In reply to pirate :

I like the idea, however That might mask cracking. 

When you do replace the area, how big are you cutting out?

I'd do 6 sack with fiber and rebar dowled into all sides on a tight grid. If it's small enough......even deeper than 6 inches too

This is one of the reasons I am not thrilled with MaxJax installations.  People are overly confident in their existing concrete.

MaxJax's website specifies: 

"The floor on which the lift is to be installed must be 4" (101mm) minimum thickness concrete, with a minimum

compressive strength of 3000 psi (20 MPa) and reinforced with steel bar."

The STANDARD residential concrete slab for nearly 30 years was poured with 2500 psi concrete without steel reinforcing.  It was CALLED 4", but it was almost always formed with 2x4's which are 3 1/2", and there was always a possibility of thin spots (depending on the accuracy of the grading work).  1/2" variation in thickness may not seem like much, but it is 12.5% less than MaxJax's minimum requirement.

The ONLY way to check an existing slab for thickness and strength is to core drill and do a break test.  No one does this in residential applications.

Toyman: I'm really thankful no one was hurt!

Now I am glad I made the areas under mine 8" thick with iron when i built the shop.

Good job seeing that crack and not ignoring.

In reply to Antihero (Forum Supporter) :

I haven't laid it out yet, but I'll probably end up replacing a section 5' wide the width of the driveway rather than just cutting a hole and patching it. I'll pour that entire section 6" thick with plenty of steel. 

At a minimum, the blue section will probably get replaced. That will give me a good foundation for the lift and allow me to back it away from the shop some. It's a little too close to the shop now due to the expansion joint you can see inside the blue area. I may also remove the tree at the top corner of the shop and pour the red section as well. 

I'll call a concrete guy after the first of the year and discuss it with them. 

In reply to SVreX (Forum Supporter) :

Very true. I've had to remove slabs that went from 6 inches thick to 1.5 inches thick. I wouldn't put a lift on anything I didn't pour or at least cut out and made sure to strengthen.

I recently did a job where the neighbor swore he knew concrete. He went on and on about his super strong concrete driveway he poured in Arizona with " 2000 psi concrete". He told me I was wasting the homeowners money by doing 6 sack/ 4000 psi mix.

I don't even think you can use 4 sack in my area( it might be 3.5 sack I can't remember) and definitely not for a driveway that has a huge heavy motorhome driving on it

In reply to Toyman01 (Moderately Supportive Dude) :

That looks big enough that you won't get hit with a short yard premium.

I'd probably super over do it since you already had a failure. #5 steel 1 foot on center with fiber ( Ive been trying buckeye fiber with good results) and a 6 sack/4000psi mix.

SanFord was parked where the lift is installed for 4 years with no signs of cracking. He weighs in at 13k pounds. I also drilled 1/4 test holes to determine thickness. The slab is just over 4" where the pads mount. When Mike (Curmudgeon) had it poured he must have had them do a pretty good job. 

Another thing I need to consider is I'll be moving from this house in under 2 years. I may just do without the lift for a while or take it down to my dad's shop and install it there. He poured his shop floor at 8"  with lots of steel because he parks a 22k pound backhoe on it. 

Decisions, decisions. Man I hate to be without a lift. I've gotten spoiled. 

I am so, so, so thankful you were cautious and caught this before there was an issue!

I've installed a couple hundred lifts in the past few years.  Most of them are commercial installations- often Rotary brand.  Up to 16,000 lbs for 2 post (30,000 for 4 post).

The installation specs always say they can be installed on 4" concrete, but then there is fine print.

Here's an example of Rotary's installation chart for their basic 10,000 lb lift:

So, they claim they can be installed in 4" concrete, but their fastener schedule says a minimum of 4 1/4".  That rules out most residential slabs.  Different fasteners require thicker.

But then there is more...

The chart shows a minimum torque rating of 100 Ft-lbs.  That will likely stress the concrete and pull the fastener.  When that happens, they recommend cutting out the concrete and pouring 6" thick.

It also shows a minimum PSI strength of 3000 lbs.  Most residential work has historically been 2500 psi.

THEN they give you a non-descript column that says "Concrete pad size if concrete does not meet requirements".  THIS is where they tell you 6" thickness.  (Note: the other disclaimers pretty much determine that residential concrete will not "meet requirements").

Manufacturers figured out they sell more lifts if they claim they can be installed on 4" concrete.  But then they give enough disclaimers that they pretty much force you to install on 6"concrete, or your installation will not be within manufacturer's specs.

Toyman, also note the manufacturers DO NOT give specs for a seismic zone.  You are in a seismic zone.  (They also say these lifts should not be installed outside, though I am not sure why).  They are pretty good at CYA.

I agree with AntiHero.  A 6" reinforced pad should do the trick.  The pads should be 4'x4' square minimum under each post.

Note that a 22K backhoe has a very large footprint.  The psi is distributed over a large area.

Technically, a rather petite woman wearing 1/4" diameter spiked heels can exceed the psi rating of that backhoe.  

Oh heck.  I guess a petite man wearing spiked heels would exceed it too! LOL!!

If you want to take the seismic into consideration, I built a dealership shop in your area last year.

The engineer designed the floor for seismic.  It was 5" thick, but was thickened to 8" under each lift post.  The 4'x4' thickened pads had a 12" grid of 5/8" rebar in each one.  The concrete was 3500 psi.

Feel free to steal that.  It's an engineered solution for your area.

I'm betting the inside only is because of freezing water around the anchors spalling the concrete. I don't have to worry about that down here.

 If I get caught under the lift in a earthquake in South Carolina and mashed flat, I'll just assume the Good Lord desided it was my time. I've lived here for 53 years and the only time I've noticed the ground move is when a train goes by.

I also was concerned about water freezing under my columns. Ran a bead of caulk around there to be sure. Also helps keep salty melt water away.

Is that installation outside?

I'm pretty wary of the maxjax causing issues and I actually do have reinforced concrete installed in my garage and it's at least 5" deep at the area where the anchors are installed, but I didn't test it for hardness, cause well, who does? I did also use quickrete anchoring epoxy when I installed the anchors just for some extra peace of mind.

In reply to Harvey :

Yes. It's outside. 
 

You have a 5" slab?  Sounds newer. That's unusual in residential. 
 

If it's newer, it's more likely to be 3000 lb. The testing is not as important on newer slabs. 
 

Older slabs rarely meet spec. Sounds like yours exceeds spec (at least In a couple ways).

If it's less than 6 or 8 years old, is 5" thick, and has no visible deficiencies, or if it is commercial,  I'd feel comfortable using it. 
 

I am also a big believer in epoxy (but leery of QuickCrete products).

You should be fine. Check regularly like Toyman did.