ACI 318-05, 7.12.2.1

[dcStrucEng]

I was asked by a superior in my office a fairly simple question (or so I thought):

"What is the minimum amount of steel required for a footing?"

I immediately went to section 10.5.4, which then prompts me to go to section 7.12. Assuming grade 60 steel, the answer is .0018 (7.12.2.1b), piece of cake, right?

He says, "no, it's 0.0014" per 7.12.2.1. I don't agree, especially if we have grade 60 bars. After reading and re-reading this section, I don't understand how the minimum area of steel will ever be 0.0014, regardless of the bar grade. Doesn't (a), (b), and (c) cover every possibility?

Advice/opinions?

Thanks.

 

[JAE]

Check these threads out - especially the last free-for-all one:

thread167-225434

thread507-185223

thread507-207910

thread507-251751

 

[dcStrucEng]

I appreciate the links to the other forums. But I didn't see the answer to my question in those threads, which is 0.0014 vs. 0.0018. Basically, I want to know when, if ever, the minimum reinforcement ratio of 0.0014 (as stated in 7.12.2.1) comes into play? Regardless of steel bar grade, when will this ratio ever control?

 

[JAE]

It is a bottom limit based on the (c) portion of that provision - so if you are using higher strength rebar, this creates a minimum when the (c) forumla puts you below 0.0014.

 

[hokie66]

Agree with JAE about the (c) provision, but I have always resisted using less steel for crack control, just because the steel yield stress is higher. Cracks occur while the steel is elastic, and the yield stress doesn't affect the elongation in service conditions.

JAE, you ruined my day by posting that last link. Had almost erased it from memory. Our friend StructGen never came back.

 

[dcStrucEng]

Thanks for the clarification. That makes sense.

 

[JAE]

hokie66, yes I found myself re-reading that entire thread and getting my blood pressure up! But I think I learned a bit from that thread, which is what ET does for us.

 

[Lion06]

I just had to head to that last post to see what all the clamor was about. I got three posts in and remembered it. About halfway through I had to quit reading!!