Discrete vs Combined Footings 4

[Althalus]

Way back when I took a foundation design course my instructor said that when you have vertical loads close together, it is often more cost effective to provide combined footings when the Aconc >= 0.35 x Agross. This was in the context of typical column spacings for typical building construction.

Since then I've found many people never heard of this. I'm wondering how common this rule of thumb is.

BTW, I've discovered that when a footing is especially deep (frost depth) other variables come into play and a modification to the 35% figure should be employed due to costs of excavation.

Any thoughts?

 

[Althalus]

BA,

I don't think I'm on the same page as you are. Explain why 1) Load reversals necessarily mean LARGE bending loads. And 2) How does this have anything to do with formwork vs concrete pouring.

 

[BAretired]

BA,
I don't think I'm on the same page as you are. Explain why 1) Load reversals necessarily mean LARGE bending loads. And 2) How does this have anything to do with formwork vs concrete pouring.

1) To cause tension in the top of a footing, the bending moment must be large enough to move the vertical load outside the kern of the footing.
2) If bending moments are substantial, a square footing is not efficient because pressure is not uniform. This could favor using a combined footing.

BA

 

[Althalus]

BA,

Yup. We were not on the same page.

I see what you are saying. But I'm afraid if I continue to explain everything, we're going to go down a long rabbit hole. Let's just leave it at that.

I guess I've had my OP question answered. No one else seems to have heard of the 35% rule. But MTU seems to have found a reference that is somewhat close to that rule of thumb. But I think I like the length of formwork rule of thumb better.

Thanks to everyone.

 

[hokie66]

Althalus,
I was addressing in general the common issue of plastic settlement in deep sections. With top bars, and without adequate consolidation by vibration, the concrete will continue to settle under its own weight, thus separating as it moves down past the top bars. This can be prevented by adequate vibration, sometimes requiring revibration, but if the top bars are not needed structurally, as in an individual footing without uplift or overturning, then the top bars are best omitted.

 

[Archie264]

Hokie66,

Thanks. I love what I learn on this site.

 

[Althalus]

Hokie,

What are you talking about? Don't you know that every concrete contractor is perfect and ALWAYS vibrates the EXACT amount that they're supposed to for each pour?

But seriously, folks...

Actually, Hokie, I was not aware of that. That is helpful. Thanks.

 

[TDIengineer]

Brief article on Plastic Settlement I found informative.

http://www.concrete.net.au/publications/pdf/Settlement.pdf

 

[TDIengineer]

And Hokie, I specifically found an australian publication for you since you love california so much.