Concentrated Load on Slab on Grade Next to Control Joint

[KHoff]

I designed an auto repair shop that is intended to have vehicular lifts installed on the slab on grade. The lifts consist of two vertical columns with a horizontal beam rigidly attached at the top. The columns have arms that swing under the vehicle and lift it. The concentrated load from one of the columns is approximately 7 kips when the lift is at max capacity. The slab is 5" thick with #3 bars at 12" on center each way. Seems to me the 7 kip load is a relatively light load for this slab to support.

The lift manufacturer has indicated they do not recommend the columns be installed within 36" of a crack in the slab. In this case, there is a control joint within 36" (about 12" from the column). The control joints are detailed to be saw cut with a depth of 1/4 the slab thickness. Reinforcing is continuous through the joint. I am looking for input on if the manufacturer's concerns are justified. I might be more concerned with a heavier load, but I'm not sure a 7 kip load warrants making modifications to the newly constructed slab. Obviously the owner would prefer not have to do anything to the slab.

Are there any references I could use to quantify a slab capacity for a concentrated load adjacent to a control joint? Also, I feel that it might be conservative to assume the load is at a slab edge since reinforcing is continuous through the joint. I appreciate any input.

 

[WARose]

Typically loads near edges/corners control the design for slab on grade. As to your question on resources......if you have FEA software, that is a pretty good way to model it. Another is ACI 360. It has charts for slabs with vehicular loads and concentrated loads.

Any attempt at estimating the slab's capacity will require a sub grade modulus. Hopefully you have some geotechnical info.

And by the way, I'd be careful about going too much on what the lift manufacturer has to say about the support of their product. Unless they don't know what they are doing, likely their recommendations on support will be quite limited. Because (to put it simply) for all they know, it could be sitting on quick sand. Most of them will have a (max) settlement recommendation......and the rest is up to you.

 

[dik]

If you are close to a joint, you may want to cut out the concrete in the shape of a footing to support the equipment. The top of the footing can be flush with the top of the SOG.

Dik

 

[XR250]

Don't forget about the overturning moment on the slab from a vehicle which is not balanced on the lift. If you have an 8000 lb truck on the lift and it's CG is 2 ft. off center of the columns, then the base moment would be 8 ft-k at each column which is pretty significant. I would be careful about installing on on a 5" slab from a moment and fastener perspective.
It used to be that the lift manufacturers would say 4" min. slab. Dunno how they got the fasteners to work although a friend of mine has one on his 4" slab with no issues (yet)
I put a lift in my garage and was fortunate that my slab was 8" thick in the area of the posts.

 

[OldBldgGuy]

You're kidding. An 8" thick slab in a residential garage??? I've seen contractors who couldn't level granular to save their lives & then a separate contractor poured the concrete which could vary in thickness from 3" to 13", but for at least the past 20 years most concrete contractors that I know do their own excavating & backfilling to avoid exactly this. I can't imagine it was intentional??

 

[Ron]

Use Westergaard's equations for a rigid pavement section near a slab edge (3 conditions...interior, edge and corner). A joint or crack, assuming little or no load transfer across the joint/crack, acts as a slab edge and will limit the load capacity accordingly.

 

[XR250]

You're kidding. An 8" thick slab in a residential garage??

It was a garage that I built 17 years ago. The whole slab was not 8". Somehow the gravel in the middle was really low. It was not intentional by any means. I can't remember what happened.
Ironically, i used the same concrete guy for another house where I had placed some floor receptacles. The were set at exactly 4". After he placed the concrete, he told me my receptacles were 1/2" low. Turns out he has a 1/2" convex belly in the slab. Fortunately, it is not noticeable.

 

[jike]

Do you have enough embedment for your anchors in a 5" slab?

If the WWF goes thru a sawcut joint somewhere in the mid-depth and is not laying on the bottom then you can probably count on it holding the joint together and transferring some shear load thru the joint by shear friction. If not, then you could restore the load transfer by adding dowels (sawcutting slots from the top and epoxying them into the joint).