Mini Storage Slab Design
Mini Storage Slab Design
(OP)
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,
Thanks,






RE: Mini Storage Slab Design
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.
RE: Mini Storage Slab Design
RE: Mini Storage Slab Design
DaveAtkins
RE: Mini Storage Slab Design
RE: Mini Storage Slab Design
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.
RE: Mini Storage Slab Design
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?
RE: Mini Storage Slab Design
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.
RE: Mini Storage Slab Design
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.
RE: Mini Storage Slab Design
jstew1, you could also consider a waffle slab - making it thicker in line with the columns and keeping the 4" elsewhere.
RE: Mini Storage Slab Design
Thaidavid