Minimum Reinforcement in Shallow Footings (Beam or Slab?)

[ahypek]

We've recently opened up discussion in our office about minimum reinforcement requirements in shallow footings.

ACI318-11 13.3.2.1 states that the design, detailing of one-way shallow foundations, including strip footings, combined footings, and grade beams, shall be in accordance with this section and the applicable provisions of Chapter 7 and Chapter 9.

Chapter 7 "One Way Slabs" states that minimum reinforcement shall be: 0.0018x60000xAg/fy or 0.0014Ag

Chapter 9 "Beams" states that minimum reinforcement shall be: 3xsqrt(f'c)xBwxD/fy or 200xBwxD/fy

I've normally followed 0.0018Ag as my minimum guideline but as per Chapter 9, my minimum is greater.

 

[wannabeSE]

The chapter 9 minimum is for when reinforcement is required by analysis (flexural reinforcement). It also allows for less when the reinforcement provided is 1/3 greater than what is required by analysis. When reinforcement isn't needed for bending, it is common to provide temperature and shrinkage reinforcement in footings.

 

[dik]

Does it work as 'Plain Concrete' without any reinforcing... if so, then any added reinforcing would be considered as a bonus...

Dik

 

[ahypek]

I don't acknowledge any tensile strength in concrete so in my opinion flexural reinforcement is always required. With that I assume that Chapter 9 would be my required minimum.

 

[EngineerEIT]

Generally the requirement of minimum reinforcement is to prevent a sudden failure by ensuring enough capacity to develop the cracking moment of the section and start yielding the reinforcement. The attached excerpt from the Wang & Salmon reinforced concrete design textbook gives some good background on how the ACI specified minimums were determined. As the pdf states, the minimum reinforcement for structural slabs and footings required by ACI is 0.0018bh (ACI318-11 10.5.4)

 

[dik]

It is likely that a kazillion (real big number) lineal feet of plain concrete strip footings have been constructed in the last century... ACI used to have a provision for plain concrete... I'm not sure if it's in current code.

Dik

 

[JAE]

First off I think you are actually referring to ACI 318-14 correct? Not 318-11.

In 318-11 the provisions for footings are in 10.5.4 as EngineerEIT indicates above....and are very very confusing to some - see this thread: thread507-251751

However, it appears that 318-14 has cleared up this mess from 10.5.4.

One thought I have on this is that it may be that this section, 13.3.2.1, refers to footings and then to "grade beams" and that the chapter 7 provisions are for general one-way footings while the chapter 9 provisions are for grade beams. That to me is rational.

Having said that I would possibly concur with you that the higher of 7 vs. 9 would be prudent.

But it does say "as applicable" when mentioning 7 and 9 in 13.3.2.1. which implies that 7 applies to some and 9 applies to some.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[ahypek]

[pre][/pre]JAE

You're absolutely right. It is 2014 not 2011. I think I'm also onboard with Chapter 9 as my minimum.

The conversation came up as our min reinforcement has always been 5 @ 12 for a 12" thick footing under light loading. Now it is 6 @ 12.

 

[JAE]

If you go with "beam" then don't forget that you can use As(min) = As(calculated) x 1.33. That sometimes drops you down lower.



Check out Eng-Tips Forum's Policies here:
faq731-376