Mini Storage Slab Design

[jstew1]

I have a client that is building Mini storage all over the country. He has been using a design from another engineer where the mini storage columns are 60" oc, 10 ft apart and supported by a 4" thick slab. This is a new relationship and he wants to use the same design concept. The ACI states that the min thickness of plain concrete footing must be 8". The occupancy cat is Type I. Is there a precedent for ignoring the ACI min requirement in a situation like this? If you did design the slab as the footing you would be required to neglect the bottom 2" of concrete, which gives you a 2" thick footing.

Thanks,

 

[MotorCity]

I am not aware of any loopholes in the code that allow exceptions to this requirement (for non-residential applications).

Keep in mind, plain concrete is concrete that is either unreinforced or contains less than minimum reinforcing. My guess is that you client's other engineer was probably not aware of this requirement, provided some nominal reinforcing (maybe welded wire fabric), and designed it accordingly as a footing.

Do you have access to the other engineer's calcs and drawings? Perhaps there is a more substantial footing for each column below the slab?

I never use 4" unreinforced concrete for anything more than a sidewalk or mudmat.

 

[BUGGAR]

More important are the underlying soils and the base.

 

[DaveAtkins]

I would ignore ACI and use "Designing Floor Slabs on Grade" by Ringo and Anderson. There is a method for checking a point load on a slab.

DaveAtkins

 

[jstew1]

I don't have any calcs, I am in the process of performing my own design/code research and found the issue. The original design calls for 6x6 1.4x1.4 WWF as you would typically see in sog. Although they want to use fiber, which I do not prefer. The studs are 4x2 so if you count on 4" slab and 45 deg load distribution to soil you have a soil bearing area of .83 sqft. (this, of course, also violates most recommendations for min footing area) At 2000 psf bearing capacity your looking at an allowable stud load of 1.66k. I need 1.5k for the roof loads. I'm trying to be creative as it is a very light load in a low occupancy structure, but I'm not comfortable ignoring the ACI without precedent. Is there another publication or standard for low occupancy buildings that I'm missing?

 

[ornerynorsk]

This is not necessarily my area, but I can share anecdotally that I invested in a new mini-storage facility in the early 2000's, and what was specified by the building system manufacturer was 6" slab-on-grade reinforced with re-mesh, thickened to 12" at the perimeter and at the column locations. Site prep was done very well, and more than 10 years on, the slab has held very well. Sawn joints, and not a single crack anywhere.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[901STR]

ACI states: 1.1.7 — This Code does not govern design and construction of slabs-on-ground, unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil.
Since your slab does transmit loads from other portion of the structure (columns) to the soils, I would interpret this to be a structural SOG, and you wouldbe bound by ACI 318.
The IBC also defines a shallow foundation similarly, so section 1809 would apply.

Even in prescriptive requirements for supporting walls of light-frame construction, the min footing thickness is 6". IBC also has requirement for minim depth below undisturbed ground, and for frost protection. I know lots of these mini-storage facilities are not a conditioned environment, so I would interpret the entire slab must be protected from frost. Are you in a cold-climate area?

 

[Once20036]

I'd have a hard time considering this as being a footing and following any footing requirements.
If you can't call it a slab on grade, call it a mat slab, raft slab, etc.

OP mentions that the previous construction used a 4" slab, but doesn't mention reinforcing. Was it reinforced? Is that what you were told? Or have you seen drawings to verify that claim? Typically the reinforcing for a slab on grade isn't for strength, but to control crack widths.

I`m with DaveAtkins on the Ringo book. This would be my first reference. There's also an army publication and an article from Structural Magazine (that I don't trust or utilize very often).

I`m not on board with the frost protection across the entire slab. If you have an interior location, you do not have moisture. If you do not have moisture you do not have frost issues. If you have moisture in your storage areas, you have bigger problems. Frost protection around the perimeter only.

If it were me, I'd be thinking about a 6" slab with some WWF.
The loading would be too high/variable/unpredictable for me to be comfortable with a 4" slab.

 

[jstew1]

No, most will not be in a cold area.

Daveatkins, thanks for the recommendation. The book does ref the ACI requirements I picked up on but it also says the interpretation of "from other areas" is up to the eng to decide. I'm typically conservative with these things so I'm between a rock and a hard place. I think the fact that roof load is being supported is a clear example of "from other areas".

orneynorsk, your situation sounds like a great design. Much more substantial than what I am being asked to produce.

 

[901STR]

Once20036, I agree with you with the interior condition would not be conducive to moisture, frost protection on exterior. However, I interpret that both ACI and IBC are pretty clear that if the slab transmits loads from "other parts of the structure" to the soil it must be designed as a footing (or structural raft slab, structural mat foundation, etc). IBC lumps all of these as "Shallow Foundations". Most of other guidance for SOG refer to slabs that are directly loaded (such as industrial floors, etc) or for single-family residential. A series of columns are, in my view, transferring loads from "other parts of the structure".

jstew1, you could also consider a waffle slab - making it thicker in line with the columns and keeping the 4" elsewhere.

 

[thaidavid40]

I am in agreement with those who are saying that you cannot just arbitrarily ignore the clear statements in the ACI and IBC codes. As for moisture in the interior areas: to assume that there never will be moisture in the interior areas of a storage-unit building, especially in the wintertime, is hubris, and could cause the client untold damages beyond just what the water (ice) alone could do.

Thaidavid