ACI 318-19 minimum flexural reinforcing / shrinkage and temperature reinforcing 3

[StrEng007]

Per ACI 318-19, what is the difference between a "Combined Footing" per section 13.3.2 for 'One Way Shallow Foundations', versus a "Two-way combined footing" per section 13.3.4

The design provisions for one-way shallow foundations lead the user to Chapter 7 for one-way slab, which will require shrinkage and temperature reinforcing in the 90° direction.

However, Two-way combined footings send the user to Chapter 8, for the design of two way slabs, which does not have a minimum for shrinkage and temperature. I'm assuming here you don't have shrinkage reinforcing because you need to provide Asmin in each primary direction. (Which cannot be split between two faces anymore)

Also, for one-way shallow foundations per section 13.3.2:
13.3.2 One-way shallow foundations
13.3.2.1 The design and 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.

Now, correct me if I'm wrong, but the intention here is to send the user to Chapter 7 (One Way Slabs) for strip footings and combined footings ONLY.
For grade beams, the user will go to Chapter 8 (Beams)? The reason I say this is grade beams don't require min. shrinkage and temperature reinforcing in the 90° direction, but combined footings should have this.

It also looks like we're no longer allowed to split the Asmin reinforcing between two faces for combined footings.

R7.6—Reinforcement limits
R7.6.1 Minimum flexural reinforcement in nonprestressed slabs
R7.6.1.1 The required area of deformed or welded wire
reinforcement  used  as  minimum  flexural  reinforcement 
is the same as provided for shrinkage and temperature in
24.4.3.2. [highlight #FCE94F]However, whereas shrinkage and temperature reinforcement
is permitted to be distributed between the two
faces of the slab as deemed appropriate for specific conditions,
minimum flexural reinforcement should be placed as 
close as practicable to the face of the concrete in tension due[/highlight]
to applied loads.

 

[StrEng007]

It's such a joy when the institutes "update" things for the specifier.

 

[IDS]

It also looks like we're no longer allowed to split the Asmin reinforcing between two faces for combined footings.

I can't comment on the questions related to slab type, but it's important to recognise the reason behind minimum flexural reinforcement, as opposed to minimum shrinkage reinforcement.

Up until the cracking moment, the tension force due to bending is taken primarily by the concrete. After cracking there is a large transfer of force from the concrete to the steel on the tension side. If the steel has inadequate tensile capacity it can result in sudden non-ductile failure of the section. Additional steel on the compression face does nothing to help. That is why codes specify that concrete sections should have sufficient tensile steel that the cracked moment capacity is greater than the section cracking moment.

That is why the minimum flexural tensile steel should all be on the tension face, and this always was the case.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[StrEng007]

Yep, I understand the reason why we use reinforcing. I was referring to this section of ACI 318-11 that I know a lot of us were relying on to keep the reinforcing down.

 

[IDS]

Yep, I understand the reason why we use reinforcing. I was referring to this section of ACI 318-11 that I know a lot of us were relying on to keep the reinforcing down.

I'm saying if that clause was used to halve the area of minimum flexural reinforcement in the tension face, then that use was wrong, and it's a good thing the code has been amended.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[KootK]

Yeah, we've been struggling with pinning down this obnoxious, moving target since at least the turn of millennium. Well before the first, pre-hillbilly Beyoncé album FFS.

Sadly, this is one of those cases where the code writers insist on trying to give you an all encompassing prescription rather than just directing you to do something intelligent.

I feel that all questions related to this could be intelligently answered as follows:

1) Generate a couple of meaningful study sections through your concrete thing. Usually these are orthogonal.

2) Ask your self "do I expect meaningful curvature to develop along this section" for each section considered.

a) IF YES THEN [provide designed flexural tension reinforcement where it's needed, respecting the requirement for minimum flexural reinforcement]

b) IF NO THEN [provide temperature and shrinkage reinforcement wherever in the cross section that you feel is most advantageous to do so.

END

It seems to me that following this simple algorithm will yield the correct answer most, if not all of the time.

A lot of engineers would probably provide at least T&S in any cross section with meaningful curvature but I do not believe that is a code requirement.

 

[StrEng007]

it's a good thing the code has been amended

Have you done many footings that are governed by overall stability and not strength equations? This creates a huge demand by requiring that 0.0018Ag be applied in full at a tensile face. Some of these footing use and incredible amount of concrete.

It was my previous understanding that, as long as you satisfied the As >4/3 As(required by analysis) you could forgo the need to use that 0.0018Ag at one face only, and thus be able to distribute it at each face. It would meet the shrinkage and temperature provisions of ACI 318-11 7.12.2.1 and allow you to cut down costs.

I think what this is saying now is, it doesn't matter if your As is 4/3 greater than what is required by analysis, You have to use 0.0018 at one face only. And if you have any uplift forces that govern, you might be talking about 0.0018Ag at each face of the footing (not entirely sure about the last statement).

[highlight #FCE94F]I agree that all tensile faces must satisfy their minimum requirements[/highlight] and it would make sense that if you're providing 33% more than required, you should be good.

 

[IDS]

StrEng007 - I should mention that I don't use ACI 318 in my design work, but if As > 4/3As(required by analysis), then surely that is the minimum As that is required to be placed at the tension face.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[StrEng007]

then surely that is the minimum As that is required to be placed at the tension face

Yeah and that's the discrepancy that I'm talking about. The way the new Code is worded, you cannot use the 'required by analysis' steel even if it meets that 4/3. You get stuck with have to use .0018Ag at the face experiencing tension. So much steel!

Disclaimer: this also took place in the ACI 318-14 but somehow I missed that.

So this now goes back to my OP. Is the intent of one-way shallow foundations to apply BOTH the rules of Chapter 7 and Chapter 9 to strip footings and combined footings. Based on the new Code, only the 4/3 rule applies under the section for beams. I cannot see how the beam section should govern the strip or combined footing section. My understanding is the reference to the beam chapter would ONLY be for grade beams.

IF we're allowed to apply the Chapter 9 beam rules to that of a combined footing or strip footing, then I could see the allowance to split that S&T steel at two faces.

It looks like this 4/3As rule DOES NOT APPLY to two-way combined footings/mat foundations, or two-way isolated footings since these section have absolutely no reference to section 9.6.1.3.

 

[JoshPlumSE]

I think what this is saying now is, it doesn't matter if your As is 4/3 greater than what is required by analysis, You have to use 0.0018 at one face only. And if you have any uplift forces that govern, you might be talking about 0.0018Ag at each face of the footing (not entirely sure about the last statement).

Yes, this is the part that absolutely frustrated / outraged me when it first came out. So, a beam is allowed to have less than As_min so long as you provide 4/3*As_required.... But, you're not allowed to do this for slabs on grade and combined footings?!

That just seems flat out stupid to me. And, always has. I've never gotten a satisfactory response from anyone to justify this change. Well, I understand the As_min requirement. That makes sense. But, no one has ever explained why the 4/3*As_required is no longer allowed to apply to slabs.

To me, this was a dramatic change to the ACI code and will make foundations more expensive without really making them any safer. I can't remember exactly when it happened. I think it was when the chapters were re-organized. I want to say sometime around 2014. And, that this may have been an unintended consequence of the re-organization that we're now stuck with.... Until someone can prove that it's not necessary.

 

[rapt]

I think the ACI code is the only major code that still allows the 4/3 concession. Most now use the cracking moment rule that IDS mentioned without any concession for over strength. Even ACI uses that for PT.

.0018 is still not a good rule for higher strength concretes. It should be dependent on the tensile strength of the concrete (cracking moment).

The other thing that codes do not require a limit on is reinforcement strain. With low reinforcement ratios, reinforcement strain is far too high at the point of cracking, expecially for lower ductility reinforcing steel..

 

[BridgeSmith]

I think the ACI code is the only major code that still allows the 4/3 concession.

AASHTO still does. Subsection 5.6.3.3.

 

[StrEng007]

Any ideas why ACI specifies Chapter 8 (Two-way slabs) for the design of 'Two-way combined footings and mat foundations', while both chapters 7 & 8 are specified for design of 'Two-way isolated footings'?

The way I see it, Chapter 8 doesn't make reference to 90° shrinkage & temperature reinforcing. Therefore, I'd assume that each primary direction need to satisfy 0.0018Ag at the tension face. In the event that tension exists at both faces in both directions (for different load combinations) you'd need 0.0018Ag top and bottom, each way, each face. That's a lot of steel!

Chapter 7 is the only one that references 90° shrinkage & temperature reinforcing split between two faces, but I don't see how that would apply for 'Two-way combined footings'. Wouldn't Chapter 8 supersede this?