# Concrete shear wall minimum rebar



## TehMightyEngineer (Aug 24, 2014)

Looking over the special concrete shear wall design in chapter 21 of ACI I'm wondering about the minimum rebar ratios prescribed by 21.9.2.1 when the shearwall has flanges.

The definitions of [SIZE=12pt]ρl and [/SIZE][SIZE=12pt]ρt[/SIZE] in chapter 1 refer to gross concrete area perpendicular to the reinforcement. For the gross area regarding the horizontal reinforcement, that will clearly be the thickness of the web of the wall times the height of the wall.

For the vertical reinforcement I would take the gross area to include the flanges. However, I wonder if they don't mean Acv (the area of the web of the wall times the length of the wall). Essentially the gross area of the wall minus the flanges. I wonder this because the vertical reinforcement is there to resist shear and compression and shouldn't care about the flanges (which are there for overturning moment resistance).

Anyone else come across this? Do you think [SIZE=12pt]ρl should be in regard to the web thickness only?[/SIZE]


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## darius (Aug 24, 2014)

Acv = gross concrete of area section bounded by the web thickness and length of section in the direction of shear force considered.

So, I think I would consider only the thickness of the web * total length of the wall. Also I am thinking, since the flanges are for tension / compression (like a vertical cantilever "I" section) and usually we have more reinforcement concentrated at the end of the wall to resist these forces, don't you always have more vertical reinforcement or at least the same in that area. I know you are talking about a general case and not a specific condition, but the flanges could be part of a shear wall on the perpendicular direction and if it is the same thickness is going to be the same minimum reinforcement per foot of length.

When I am calculating minimum reinforcement of walls I prefer to find the reinforcement area per foot of length (in this case will be 0.0025*thickness*12 inch)

My 2 cents, I hope somebody with more experience will clarify better.


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## TehMightyEngineer (Aug 25, 2014)

That's what I was thinking but isn't the most conservative. Though it does seem to match the intent of 21.9.2.1.

Another question and this one is driving me nuts:

In 21.9.2.1 it states that if Vu &lt;= Acv*Lambda*SQRT(F'c) then we can use 14.3 for [SIZE=12pt]ρl[/SIZE] and[SIZE=12pt] ρt.[/SIZE]
However, R14.3 states that for walls resisting horizontal shear forces in the plane of the wall reinforcement designed per 11.9.9.2 and 11.9.9.4 may exceed the minimum requirements in 14.3. 11.9.9.2 and 11.9.9.4 essentially wipe out any minimum rebar savings from 14.3.

Can we ignore 11.9.9.2 and 11.9.9.4 as 21.9 is in regards to a special shear wall and has it's own design strength requirements beyond those of 11.9.9?


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## darius (Aug 25, 2014)

This is my understanding: 21.9.2.1 check the condition you mentioned, if you qualify you go 14.3, if not minimum 0.0025 for both vertical and horizontal.

14.3 requires a smaller minimum for both horizontal and vertical reinforcement, unless greater amount required per 11.9.8 or 11.9.9.

11.9.8 - if you satisfy the condition you can use either section 14.3 (less reinforcement) or section 11.9.9 (conservative with more reinforcement). If you don't satisfy the condition is mandatory you use section 11.9.9

It is my understanding that if you satisfy condition from section 11.9.8, section 11.9.9 is waived and you can use section 14.3, unless you wanna be conservative (and I would because is a special wall in high seismic area) and still use section 11.9.9


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## Andy Lin (Aug 25, 2014)

darius said:


> Acv = gross concrete of area section bounded by the web thickness and length of section in the direction of shear force considered.




This is my understanding also.



darius said:


> When I am calculating minimum reinforcement of walls I prefer to find the reinforcement area per foot of length (in this case will be 0.0025*thickness*12 inch)




I also do the same.



TehMightyEngineer said:


> That's what I was thinking but isn't the most conservative. Though it does seem to match the intent of 21.9.2.1.




Not sure what you mean by "isn't the most conservative". If you use the "area per foot length", I think it will be just as conservative...



darius said:


> This is my understanding: 21.9.2.1 check the condition you mentioned, if you qualify you go 14.3, if not minimum 0.0025 for both vertical and horizontal.
> 
> 14.3 requires a smaller minimum for both horizontal and vertical reinforcement, unless greater amount required per 11.9.8 or 11.9.9.
> 
> ...




I agree with this also.

So basically, always use 0.0025 unless you really really really need to save the owner a little bit of money or if the exam question is asking for the most economical solution...


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## McEngr (Aug 25, 2014)

I've always done .0025... my .02 cents


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## TehMightyEngineer (Aug 25, 2014)

darius said:


> This is my understanding: 21.9.2.1 check the condition you mentioned, if you qualify you go 14.3, if not minimum 0.0025 for both vertical and horizontal.
> 
> 14.3 requires a smaller minimum for both horizontal and vertical reinforcement, unless greater amount required per 11.9.8 or 11.9.9.
> 
> ...




Thanks, this makes a lot a sense and I had missed 11.9.8.

I will also probably just default to 0.0025 and avoid the headache entirely. Certainly faster for the exam as well.



SEHQ said:


> TehMightyEngineer said:
> 
> 
> > That's what I was thinking but isn't the most conservative. Though it does seem to match the intent of 21.9.2.1.
> ...




I meant that if you considered the entire wall plus the flanges then your required total rebar area would increase. However, if you just consider the wall in 12 foot sections and then verify the flanges themselves have at least 0.0025 ratio of steel to gross area then it would be conservative and definitely easier.


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