Slab on Grade 1

[ivanga7]

I am currently working on a project where the tenant is stacking heavy tires and the owner is concerned about the weight of the tires bearing on the slab on grade. The tires vary significantly in weight and size but can be as heavy as 4,150 lbs. each and have a diameter of over 6 ft. The building was constructed in 1972 and the slab on grade is a 5" thick unreinforced slab with f'c=2,000 psi. I ran numbers based on the type of tire and number of tires stacked and the worst case scenario creates a uniform load of 650 psf.

I have never really done calculations for slabs on grade so I am somewhat unfamiliar as to what I should be looking out for. I looked at ACI 360R but I didn't find anything that could help me. It is my understanding that an important factor for slab on grade strength is the quality of the soil beneath it, but what properties of the concrete itself should I be looking at? Any help would be greatly appreciated.

 

[canwesteng]

A UDL is not going to cause stress in the slab, you'll need to identity a footprint for your loading. You'll also need to figure out a subgrade modulus below the slab, which is the hard part. ACI 360R should have the pieces you need for design, but nothing stops you from building a FEA model if you figure out a subgrade modulus (overestimating the stiffness of the slab is conservative for the purpose of slab moments).

650 psf is peanuts for a slab on grade in my experience, but usually the slabs I see are reinforced, so YMMV. Keep in the mind the consequence of failure for a slab on grade is pretty marginal, depending on your client they may find it more economical to just load it up and repair if required rather than soil testing and engineering.

 

[skeletron]

Properties of the Concrete:
-Consider the f'c since this will affect your modulus of rupture (tensile strength)
-Consider the condition of the slab
-Consider where the joints are
-Consider the reinforcement in the slab (there is none)

I've used the PCA Method before to design SOG. The strength is correlated to with the effective loaded area and the required thickness is determined.

5" unreinforced seems a bit thin, but you could run the numbers and check.


...but I can't recall if I have ever solved that problem yet.

 

[jike]

Although I have been retired for 5+ years, I recommend the book "Designing Floor Slabs on Grade" by Boyd C. Ringo and Robert B. Anderson. It was published by CRSI. I believe you can download a pdf version if you search the internet.

The critical bending for a uniform load such as you have is generally at the end of the load or in an aisle between the loads.

You can also analyze it using FEM.

Good Luck!

 

[JoelTXCive]

I had to google this, but "YMMV" means....... "Your Mileage Might Vary"

 

[TLHS]

Honestly, calculated values without real field information won't tell you anything. Damage from a reasonably consistent area load is going to come from soft spots, material loss or other subgrade problems.

 

[catsndogs]

They tires have to get into the building. Won't the wheel loads of forklifts be ***much*** more significant than the uniform load of stacked tires?

Design charts are readily available for slabs on grade subjected to wheel loads. But at 5" thick, your slab might be just outside the scope of many charts.