ACI minimum area of steel 4

[nashe]

I have 3 feet deep pile cap. Due to lateral loads, I am placing rebars E.W. T&B. The Ast from analysis is less than minimum Ast = .0018 x b x h as Per ACI 318-05, Section 7.12, 10.5.4.
I am placing rebars each way both at top & bottom. Now I need this minimum Ast each way at top and bottom of pile cap.

 

[271828]

Taro typed: "My head hurts. Anyone have an aspirin?"

Yeah, mine.

Sorry for getting snippy with my earlier post. I was working on a project all weekend and until 2:45am this morning.

Time to go to bed now.

 

[mitchelon]

Taro, I was wrong about the 0.0018bh. Completely agree w/ you that it should be used as min flex reinforcement for slabs and footings. So folks, please listen to Taro.

UcfSE, what do you propose the 3 in 3?(f'c)/fy should be? In my derivation of 3?(f'c)/fy, I assumed a fr of 12*fc^0.5.

 

[271828]

mitchelon, if we won't just take his word for it then why would we take your word for it when you say to take his word for it? LOLOL.

BTW, I now agree with taro, mostly anyway.

 

[mitchelon]

Because I am now one more person who agrees with the definition described by Taro; thus the probabilities that he is correct have increased.

 

[GalileoG]

I think it's time now to let this thread die.

 

[271828]

mitchelon, I guess I'm one more too except that I think it doesn't matter if all the steel is on the tension face. One would usually put it all there anyway, though

Clansman just knocked me off his Christmas card list for restarting that argument!!

So the probability goes up even more? Isn't that like asking 100 people if they've caught Bin Laden? If 90 of them say yes, does that mean it's a higher probability than if 80 say yes?

 

[mitchelon]

Clansman, I guess this thread will not die.

271828, All the steel needs to be on the tension face if your are designing for flex. steel. If desinging for T&S, then it can be distributed.

It is not like asking 100 people about Bin Laden. Most people have no factual data or experience in what happened to Mr. Laden. Conversely, we are practical engineers.

 

[271828]

Clansman hates us (maybe just me) now, LOL.

It's an academic point because that's probably where it'll go anyway, but...

Name a reference that states that .0018 must go on the tension face. I'll back down in a hurry if somebody can produce one! -- a paper, an ACI example, an ACI opinion, anything credible, explicit, and halfway formal.

 

[mrengineer]

Just for the sake of keeping this thread going forever, I would like to point out from personal experience that professors are capable of being just as confused, dazed, and disoriented as the rest of us, regardless of their credentials.

 

[WillisV]

271828 -

See MacGregor, "Reinforced Concrete"- 3rd ed. p713

"ACI Sec. 10.5.4 states that for footings of uniform thickness, the minimum area of flexural tensile reinforcement shall be the same as that required for shrinkage and temperature reinforcement in ACI Sec. 7.12...For Grade 60 steel it is As,min = 0.0018bh. This amount of steel should provide a moment capacity between 1.1 and 1.5 times the flexural cracking moment and hence should be enough to prevent sudden failures at the onset of cracking."

halfway formal enough? When was the last time you put your flexural tension reinforcement on the opposite side of the flexural tension?

 

[mitchelon]

271828,

Sure:
ACI 10.5 - Minimum reinforcement of flexural members
ACI 10.5.4 - For structural slabs and footings of uniform
thickness, As,min in the direction of the span shall be
the same as that required by 7.12.

If you would not split the As,min required by 10.5.1, 10.5.2 and 10.5.3, why would split the requirement in 10.5.4 for slabs between the tension and compression faces?

Now, to quote Taro “Because there is usually more redundancy in a slab than a beam, this is a sufficient amount of reinforcing and the higher reinforcing ratios (200/fy, etc.) are not required.” So, due to this redundancy, ACI prescribes a reduction the As,min for slabs by almost half of that required by 10.5.1 and 10.5.2. And your interpretation of the requirement is that you can reduce As,min in slabs by a quarter of that required for other flexure-controlled members? Would you also split the As,min required in 10.5.1 and 10.5.2?

 

[mendinho]

More or less it is the same in the EC-2. In the Eurocode-2 the minimum amout of steel in plates to control fisuration because of temperature and retraction is

As = 1.8e-3 x Ac

And I apply the same As (not a half) top and botom and in each direction. I was taught that the minimum steel reinforcement tries to equal traction resistance of the concrete and the steel. In equations,

2 x As x fy = Ac x fct

ratio = As / Ac = 0.5 x fct / fy = 2e-3 to 4e-3 aprox.

However in footings there is no recommendend value in most of the norms because the effect of temperatute and retraction is not so important as in plates. At least in Spanish foundations, we tend to use 1.5e-3, and we put in each direction (2/3) to (1/2) of this value in the bottom and a (1/3) to (1/2) on top of the foundation when minimum reinforcement is required. This is done by most of the engineers in practice and it works.