Min. reinforcement for footing

[johndeng]

I notice 0.0018bh min tensile reinforcement discussed in this board.
I have a sign footing, only need min. reinforcement.
for my 20" thick footing, that requires #6@12" spacing each way.
for my 42" thick footing, that would mean #6@ less than 6" spacing each way. would this be too much?

For my sign footing (say 5'x10'), do I need to make it below frost line? If so, I need to make it 42" deep.

 

[structSU10]

Did you look at unreinforced footing? If it is 42" deep you could likely get it to work, but maybe not. How about a pilaster that makes up some of the below grade depth, using your 20" thick footing or whatever would be needed?

 

[johndeng]

client don't want pilaster.

Just wondering whether we need to go down below frostline. I read the code, it did mension small area/uncritical project may allow.

 

[msquared48]

Is the project not critical when the sign fails due to frost heave and damages the neighbor's or your client's property? You AND your client will be liable then.

42" is not that far down. Just do it.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

structSU10,
Don't be silly. A footing with those dimensions must be reinforced.

johndeng,
Mike is correct. As sign footings go, this one is small. Just do it.

 

[johndeng]

hokie66,

I did some pier foundations with plain concrete. I never designed plain concrete footing with larger size. from what you are saying, is it needed for construction purpose or t&s? any limit for plain concrete footing?

Just feel too much steel bars. Say footing b=5', h =42", 0.0018bh= 4.536 sq inch. so we need (10)#6 for 5' footing.
if h=24", I only need (6)#6 for 5' footing.

 

[hokie66]

If the only reason for the depth is to get below the frost line, you could place the lower part of the footing first, with low strength concrete if you want, then build your structural footing on top of that. But I still think trying to minimise on reinforcement, for a single footing, is false economy.

As this is a sign footing, you will need reinforcement both top and bottom, so using the actual structural depth does save a bit.

In this footing, you do not need "T & S" reinforcement. Perhaps the minimum requirement for flexural reinforcement concides with the amount required for T & S, but it is for flexural purposes. Thus, you actually need 0.0018 bd x 2.

 

[asixth]

I don't think No.6 at 6" is over the top for a footing 42" thick. I've regularly done footings with that thickness and reinforcement (sometimes even heavier reinforcement).