Thick Foundation Design 1

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I've started doing some foundation design for exterior equipment pads in areas with deep frost depths. This particular footing is 8'-6" long, 4'-0" wide, and 4'-0" deep. The length and width is defined by the equipment footing and the depth is defined by the frost depth. The forces from the equipment are completely negligible.

I've always designed for T&S steel using the full depth on footings (when flexure does not define), but it seems like significant overkill in this situation. I use to know an engineer who would design the reinforcement on a face-by-face basis, using only the first 1'-6" of depth as a required design depth. Is anyone else familiar with this idea? Does it actually have any basis in code anywhere?

 

[jec67]

I have used the 1/3 increase of required steel for flexural design in situations like you describe. It is in the ACI code (don't have a copy with me to give you the exact section - I believe it is in Chapter 10). T&S steel is intended for flat slabs, according to the commentary in the ACI code. I agree that the reinforcing steel for a mat such as you describe is excessive.

 

[jayrod12]

That seems like a hell of a lot of wasted concrete for an inch or two of insulation. I'd be inclined to make it 2 ft thick (or less) and provide an inch or two of insulation (get a geotech to recommend the amount required).

2" of insulation under entire slab and extending 4 feet beyond face of pad is roughly 500 bucks in material.

2 feet of concrete is about the same price for the concrete supply only, doesn't account for the additional rebar, formwork etc.

Just my 0.02$

 

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jec67,

Thanks for your thoughts. I've always checked the Asmin given by 4/3 the required flexural steel (ACI 318-08, 10.5.3), and I definitely plan on keeping T&S steel in my typical footings, but it looks like the the requirements for T&S as listed in ACI 318-08 sec. 7.12 apply to "structural slabs" in particular. I might be using that as a way to limit my reinforcement size. I'll have to look into that a little deeper.

jayrod12,

Comparing the two, going the insulation route increases the excavation area by almost a factor of three (16.5 ft by 12 ft by 2 ft compared to 8.5 ft by 4 ft by 4ft, 14.7 cubic yards compared to 5), adds the insulation cost, adds formwork of 50 square feet (below grade because now we are overexcavating instead of casting in place), and adds soil compaction of approximately 12 cubic yards of soil. That compares to just doubling the concrete and steel. I find it hard to believe that the insulation route would be the cheaper route in this situation, especially if I can find a logical code argument to limit my reinforcement.

 

[jayrod12]

Fair enough, Around here the earth is rarely used as formwork, So I actually considered requiring more formwork for the 4 foot deep as opposed to 2 foot.

Here they would dig out a hole that is 12 ft by 6 ft by 4 ft with 45 degree sloped sides (14.2 cubic yards, with an additonal width at the sides to fit workers and formwork included) compared to a 16.5ft by 12 ft by 2 foot excavation with straight sides (14.7 cubic yards), So I considered it a wash. In all honesty they would likely straight cut the 4 ft excavation as well, but they'd quote and charge for the 45 degree.

However every area is different on how they do (bid) their construction.

 

[wannabeSE]

Have you considered a 4' deep continuous footing (turned down slab edge) and a 6" structural slab on top with void form between the footings. This might save a little reinforcing, concrete and excavation.

 

[KootK]

This is going to be a bit of a journey. Stick with me, there's beer at the end.

Consider slabs on grade which can be unreinforced if the control joints are spaced tightly enough, say 24 x thickness. As we move from one control joint towards the next, tensile stresses build in the slab due to shrinkage restraint provided by the subbase. Implicit in our 24x number is the expectation that over that distance, not enough tensile stress will build up to exceed the tensile rupture of of a slab of a thickness equal to joint spacing / 24.

I contend that the situation with temperature and shrinkage steel in OP's footing is analagous. The point of the reinforcing, were there to be one, would be to reduce the size of t&s induced crack widths caused by soil restraint. However, unless some dimension of the footing is greater than 24 x 4' = 96', there's no reason to expect any tensile cracks to form. This leads me to the conclusion that there is no t&s demand whatsoever in OP's footings.

I know, a footing is not a slab on grade. There will be large temperature swings, mass concrete thermal effects, and maybe even some flexural cracking. But still, 96' is a LOT longer than 8'-6". And some of the most long lived concrete out there has survived so long precisely because it's unreinforced.

In practice, I go with #5@12" min top and bottom for equipment pads like this. No particular reason other than I probably saw someone else do it at some point and it feels reasonable-ish.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

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wannabeSE,
The client wants to pour it as a full mass of concrete rather than the method you described. I've detailed equipment pads in the manner you suggested, but I prefer that way once the perimeter to square foot ratio becomes more favorable.

KootK,
Thanks for your thoughts. That's definitely another good way to look at it. The concrete on itss own is more than adequate to handle the loads being imposed upon the pad, so I feel that the function of steel reinforcement in this case only needs to prevent a sudden tensile failure (from flexure) from occuring. Your discussion helps confirm my thoughts that full T&S steel for a four foot deep pad.