Mat foundation reference

[RareBugTX]

[ignore]Hello:

I would like to know if any of you knows of a resource for the design of a small mat foundation for a heavy safe (70k). The proposed footing would be a mat foundation supporting (4) 8" concrete walls. I have a book from my engineering courses, but it lists only mat foundation design for isolated columns, not perimetral walls. Does any know of a reference for such type? Thanks!

 

[TDIengineer]

Provide a sketch / layout and cross section.

I would work toward 1500 psf allowable soil bearing pressure, 1000 psf if your soil is poor. See chapter 18 of CBC for examples of allowable soil bearing by soil type. An 8' square footing will support 8^2 * (1.5-0.15) = 86.4 kips.

Provide a 12" thick footing with at least 0.0018 (#6 @ 12" at bott). 8" concrete walls with #5 @ 18" ea way. should work for most condition I would think. If your walls are going to receive earth pressure, you may want to detail the footing with 2 layers of reinforcing (#5 @ 12" top and bott should work for most cases)as you will have some cranked moments coming in to the footing. Punching shear may be an issue depending on your layout, so thicker than 12" may be needed.

What is the size of the safe footprint?

Most foundation design guides should have situation for concentrically loaded bearing walls. This is more of your situation.

 

[RareBugTX]

TDI:

Thanks for replying. The footprint is 9'x9'. I estimated 18" thick mat. All my textbooks and examples I have found online are mats with isolated spread footings supporting columns and I have four walls. I took a mat strip 1' long and checked shear and moment due to walls vertical load and is just fine, just wanna make sure I am not missing anything. Yes I assumed 1500 psf per IBC for soils without report. The walls are not supporting any lateral loading except for a 9'x9'x8" top slab and the gravity axial as the safe is isolated in a basement.

Thanks

RareBug

 

[topsisu]

You might review TM 5-809-1/AFM 88-3, Chap. 15 CONCRETE FLOOR SLABS ON GRADE SUBJECTED TO HEAVY LOADS. It includes a discussion for wall loads at the edges of slabs on grade.

 

[RareBugTX]

Thanks to you Topsisu! I will check it out.

 

[hokie66]

References for mat foundations won't help you, as a 9'x9' footing is not considered a mat. It is just a spread footing.

 

[msquared48]

So, what Hokie says is applicable here.

However, I also assume from your comment that the footing is loaded by the four walls at the periphery of the 9X9 square as opposed to a central column. Is this correct?

Mike McCann
MMC Engineering

 

[RareBugTX]

Yes, the footing is loaded by four walls. It is pretty much a box. One wall has a 40"x84" opening for a steel door. I dont think this is a spread footing. It is a mat to my view.

 

[msquared48]

Looking at the picture, it is a slab supported by four walls and loaded vertically by the soil. (Looking at it upside down).

It is a two-way thickened slab that will have the steel at the top or center, instead of the bottom or center, as is normal. Does this footing have to take an OTM as well as vertical?



Mike McCann
MMC Engineering

 

[msquared48]

Take a look at this paper...

http://www.gobookee.org/get_book.ph...vdyBGb3VuZGF0aW9uIEFuYWx5c2lzIGFuZCBEZXNpZ24=

Mike McCann
MMC Engineering

 

[hokie66]

Call it what you like, but it is just a footing. As Mike says, with walls at the perimeter, the top of the footing will be in tension.

The term "mat foundation" is normally used in describing a large footing, typically over the entire area of a building. I don't know why you would call a small footing a mat.

 

[RareBugTX]

Yes, we agree on that it is a footing. I think that it acts, under load, as a mat, or two way slab on ground as someone suggested above.It is stressed the opposite of a spread footing. In this mat, I do have as you said a negative moment at the top and center as the walls push down, in a spread footing, the tension is on the bottom and towards the edges. Anyway, so much for defining it. Thanks for your valuable posts to all.