ACI Minimum Wall Reinforcement - Out of Plane Bending 2

[cpb213]

I am designing a deep shaft (100' deep) that has multiple landings for stairs at different elevations and the contractor is questioning our design stating that it is too conservative.

The shaft resists soil on one face only so the reinforcement was controlled by out-of-plane bending. We designed the reinforcement in accordance with ACI 318-14 Chapter 11.1.4 which determines the required reinforcement due to flexure. We provided a minimum tensile flexural reinforcement ratio equal to 0.0018 which was based on the minimum flexural reinforcement ratio for a structural slab (ACI 318-14 Chapter 7.6).

The contractor thinks that we don't need to follow the Chapter 7 minimum reinforcement requirements for slab and just need to provided the required reinforcement from the flexural forces and follow the minimum requirements for a wall (ACI 318-14 Table 11.6.1). They also believe that we can use the provisions of ACI 9.6.1.3 which allows you to ignore minimum reinf. requirements if you provide 33% more steel than required.

To summarize my question:
For a wall in which the reinf. is controlled by out-of-plane flexure, do I need to meet the min. tensile reinforcement requirements for structural slabs? Does ACI state that somewhere?

 

[JAE]

The code logic here, as I see it:

1. Assuming you have a bearing wall, you must start with Chapter 11.
2. Section 11.1.4 deals ONLY with cantilever walls, so that doesn't directly apply.
3. From 11.5.2.1, your required strength Mn, is based on 22.4 (actually it should refer to 22.3)
4. Section 22.3 deals with the general flexural assumptions and Chapter 11 doesn't specify minimum reinforcement.
5. Section 11.5.3.4 points you to wall minimum reinforcement provisions of 11.6, which are dealing with minimum SHEAR (in-plane) conditions and my not apply to your wall.,..but does need to be met.
6. Section 9.6.1.3 only applies to beams, not slabs or walls...so the 1.33As(req'd) provision wouldn't apply here.
7. I don't see how your "wall" (a vertical element - see Chapter 2 definition) can be designed under the Chapter 7 "Slab" section (a horizontal element).

So from your post above - Your use of 7.6 for the 0.0018 minimum is doubtful correct as you have a wall, not a slab.
The contractor's assertion that only Table 11.6.1 should be used is correct in my view.
The contractor's assertion that you can take 1/3 greater than As(req'd) is wrong in my view (it's not a beam).



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[ALEXZ0622]

To my knowledge, there's only one way to get rid of shrinkage reinf. for wall out-of-plane design, and it doesn't worth it:

Based on ACI318-14 Chapter 11.8, if you can prove out-of-plane action on "Wall" is not a problem, then you can have whatever longitudinal (vertical for wall) bars you want. However, the horizontal bars need to meet Table 11.6.1(see Chapter 11.1.4), which is more than 0.18%.

IMHO, a thorough analysis just to avoid min. reinf.? You need to think about this seriously.

If above is not your type, then that retaining shaft wall is not a "wall" anymore, it is a structural slab. Then just design it as you are dealing with a structural slab. No matter one-way or two-way, the intent is the same: you will have at least 0.18% bars through out your cross section.

BTW, 4/3 excessive reinf. only applys to beams, not slab, don't even mention to walls.

 

[JAE]

I'm not sure you can argue that your vertical "wall" can be designed under the ACI "slab" provisions. I may be wrong but can't find any definition in ACI for "slab"...only that "walls" are vertical and that they can have any combination of axial, moment and shear.

So just saying "Hey, I have a lot of moment - therefore it's a slab" isn't necessarily true.



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[cpb213]

Usually, I would just go with the conservative option but this is a design build project so I need to justify the conservative approach.

I completely agree that the 4/3 Asreq provision is not applicable at all and I won't have trouble pointing that out to the contractor.

Does a cantilevered retaining wall have a minimum tensile reinforcement requirement? ACI 11.1.4 says to use Chapters 22.2-22.4 for reinf. design but there are no min. requirements for the vertical steel.

 

[ALEXZ0622]

@ JAE
You are right, I just searched online, at least "slab" is defined in below linked file as "horizontal".

https://www.concrete.org/portals/0/files/pdf/ACI_Concrete_Terminology.pdf

So...I would say Table 11.6.1 is the way to go, Unless you want to avoid min. reinf. by demonstrating "adequate strength and stability" by structural analysis.

Second thought after submitted first post, since retaining walls are vertical, thus compressed by upper structure & its self weight. Shrinkage reinf. doesn't really apply here. You shouldn't have shrinkage problem as long as you don't have anything restraining it from deform due to shrinkage or temperature.

 

[JAE]

Does a cantilevered retaining wall have a minimum tensile reinforcement requirement? ACI 11.1.4 says to use Chapters 22.2-22.4 for reinf. design but there are no min. requirements for the vertical steel.

For cantilever retaining wall, section 11.1.4 refers you to 11.6 for minimum horizontal reinforcement - saying nothing about vertical (or transverse/longitudinal....wish ACI would remain consistent).
So it appears that ACI 318-14 has a "hole" in it with regard to cantilever walls min. flexural reinforcement.

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[cpb213]

So I read through the ACI SP-17(14) "The Reinforced Concrete Design Handbook" last night and with regard to minimum vertical tensile reinforcement for cantilevered retaining walls they state that because of "lack of redundancy" in the cantilevered stem wall to use minimum requirements for beams rather than the requirements of one way slabs.

This would increase my minimum tensile steel ratio to 0.0033 instead of the 0.0018 allowed for slabs. We have redundancy in the system though so I should be able to justify 0.0018.

 

[rapt]

Defining something as a slab in this instance because it is horizontal and wall as it is vertical is wrong in my opinion.

A vertical wall with minimal axial force will act as a slab in out of plane bending and should be designed and detailed as one.

For columns the Australian code says if axial stress < .1F'c, then it should be designed as a beam! And for walls/slabs it suggests .03F'c .

 

[hokie66]

I have always considered walls to be the same as slabs unless they have a lot of axial loading. And 0.0018Ag is not much reinforcement. For a deep shaft as described, I don't understand why the builder is quibbling.