Small Footings - Rebar Development Lengths 4

[RM87]

Hello all,

I was wondering if someone can give me some advice I've been wondering about for some time. I work in low-rise and mid-rise residential in a high seismic area and I'm sitting here staring at my design for a 18"X18"X12" (12" deep) shallow footing to support a very light deck. My question is in regard to the rebar and the development lengths associated with these shallow footings.

To allow the bars to develop, I prescribe that the contractors hook the bars. Am I reading into this correctly, or can the contractors just place the bars flat? Any suggestions to make these sort of designs more constructable?

Thanks,
R

 

[msquared48]

Footings this small in plan with the thickness you describe do not really need horizontal steel at the bottom, not only for the problem you describe, but primarily due to the thickness of the footing which will cause shear to control failure, not bending, particularly if the pedestal (column) above is 6 to 8" or greater.

I know that the code calls for rebar in foundations of all high seismic areas, but omitting the steel here is really a no brainer. Usually, the vertical stub steel to a concrete column or plinth above can function as horizontal steel in the footing if the bars have a 6" or so bend at the end into the footing.

Mike McCann, PE, SE (WA)

 

[RM87]

Hi Mike, thanks again for your reply. The pedestal is 4" +/-, so as you can imagine, it's just there to keep the deck floor joists from resting on the soil. I'm trying to find a particular code section within the ACI318 which I can point to which will let me get away with just leaving the bars flat. The truth is, I do have bending concerns because I've spaced the footings far enough and up'd the anticipated loads to compensate for a relatively heavy finish.

If bending governs, I figure that the bars need to be hooked to have them develop adequately for tension?
Even if shear governs, I feel that I would still need to hook the bars to argue that the bending has been compensated for?

 

[msquared48]

4" pedestal seems awful small to me. I would use a minimum of 6" to properly utilize and space the vertical stub steel.

Mike McCann, PE, SE (WA)

 

[racookpe1978]

OK, so assume he goes 6x6 inch concrete.
1-1/2 cover each side leaves a 3x3 "square" to put rebar within.
So, you bend a 1/2 dia rebar into a square 3x3, right?

 

[msquared48]

If you are talking stirrups in the 6" pedestal, I would use no more than #3 bar, not #4's. The verticals are probably #3's too.

Mike McCann, PE, SE (WA)

 

[BAretired]

Is the pedestal size an important cost consideration? Why not use 8" square to permit easier reinforcement placing?

A footing 18"x18"x12" deep doesn't need to be reinforced at all according to the Canadian code because the depth is greater than the overhang, even if the pedestal is 4" x 4". There used to be a section in ACI 318 dealing with plain concrete in footings, but I don't have a recent standard. Is it still included?


BA

 

[hokie66]

I don't think the OP has a 4" x 4" pedestal, but rather he has a plinth 4" high. But I could be wrong.

I agree that in such a small footing, the reinforcement does nothing, but as standard practice, I cog (hook) all bottom bars.

 

[KootK]

It is still included. In general, I've not trusted unreinforced concrete. This a pretty darn small footing however. I'm surprised that it even meets geotechnical minimums.

Anyhow, to answer the OP's question, use small bars and provide more reinforcing than required for flexure so that you can justify a partial development length. In a footing so small, the reduced rebar length (no hooks) will likely offset the increase in steel area provided. And, of course, you'll accrue that constructibility improvement.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

It's silly but, in my jurisdiction, the proportions of your footing, as reinforced concrete, would trigger strut and tie design. And the outcome of that would be... more reinforcement, again.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[racookpe1978]

What proportions, KootK? 4x4, 6x6, 12 x 12? (inches, obviously)

 

[KootK]

Shear span : depth < 2 I think. Here that would be about one. It's resulted in some very complicated spreadsheets for pad footing design.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

Some people make complications out of the darndest things.

BA

 

[KootK]

Yep. Plus it implies that a few zillion deep footings that I designed in the US using the CRSI manual must be on the verge of collapsing as I type. That feels good.

I've run across deep, moment resisting footings that technically also need to be designed STM per CSA. Almost intractable.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[msquared48]

This is a perfect example of code over regulating and requiring so much steel that there is no room for the concrete. OK, I'm a little sarcastic. But just a little.

Mike McCann, PE, SE (WA)