Footing uplift-discontinue reinforcing?

[haynewp]

I have a combined footing under uplift (2 columns) reinforced for the positive and negative regions for flexural strength or by the ACI minimum reinforcing requirement.

But can the reinforcing be discontinued where the plain concrete section (designed by ACI 318-05 Chapter 22) is adequate to resist the bending in the top or bottom face of the footing? Or is the intent of 22.2.2 to only be applied to footings that do NOT have uplift? I don't think it matters if it is a combined footing or a single column footing under uplift, my question is can the plain concrete section be used by Chapter 22 to resist the footing's own self weight under uplift if the plain section works by analysis (per 10.5.1)?


22.2.2 — Use of structural plain concrete shall be limited
to (a), (b), or (c):
(a) Members that are continuously supported by soil
or supported by other structural members capable of
providing continuous vertical support;


10.5 — Minimum reinforcement of flexural
members
10.5.1 — At every section of a flexural member where
tensile reinforcement is required by analysis

 

[JAE]

haynewp,
I think that you can certainly use a plain concrete footing if it has net uplift IF the section works under Ch. 22.

However, I've always perceived that under ACI 318, you EITHER have a reinforced element or a plain concrete element and to suggest that part of a footing is reinforced and part is plain concrete doesn't fly.

 

[rapt]

Agree with JAE but would add that I would not like the idea of a plain reinforced concrete footing with uplift loads.

My understanding of a plain concrete footing is that the material around the footing will provice some support for a compression member so that the footing cannot really fail. The same does not apply in tension as cracking of the concrete under uplift would put the footing into tension and resulting cracking would reduce the weight of the footing acting against the uplift as the footing concrete connected to the column separates from the remainder of the footing at the crack.

 

[haynewp]

Then you would agree that the intent of ACI 10.5.1 is that whenever there is tension on concrete under flexure, that reinforcing is required and that any tensile flexural strength of concrete shall be ignored?

 

[JAE]

I just read the commentary section R22.2.1 - [red]"...use of structural plain concrete should be limited to members that are primarily in a state of compression, members that can tolerate random cracks without detriment to their structural integrity, and members where ductility is not an essential feature of design."[/red]

I think then that if you have a footing in net tension, then even if it is a very low tension and less than chapter 22 limits, I'd be wary of using plain concrete.

 

[haynewp]

That's what I was thinking too.