Temp. & Shrinkage Steel in Retaining Wall Stem at Footing 2

[Deadblow]

Hello,

I am designing a retaining wall and the concrete needs to be designed according to ACI 318-14. Please refer to the attachment for a general sense of what I am talking about. However, my specific design has different dimensions and loading than shown in the attachment. The vertical flexural steel at the back face of the wall stem that is anchored into the footing is adequate for the moment demand in the stem but the area of steel where the top of the footing meets the base of the stem does not meet the requirement of 0.0018x(gross concrete area). As shown in the attachment, I plan to have vertical steel in the front face of the wall resting on the top of the footing to meet the min. temperature and steel requirement for the stem. Do I need more area of steel crossing the plane where the stem meets the footing to meet the temperature and steel requirement at that specific location?

EIT

 

[BAretired]

The wall is not tied to the 8" slab?

BA

 

[r13]

Many designers have routinely ignored the top face reinforcement for the base slab of retaining wall, and the spread footing, if not required for the effect of uplift, as temperature effect is none-issue, and significant/noticeable shrinkage has occurred prior to back fill. But, it is prudent to provide minimum reinforcement to counter the stress due to rotation of the wall, and due to the uneven settlement of supporting soil. However, provide or not provide, and the amount, are very much of a judgement/practice call, I don't think the code have specific requirement for that.

 

[Deadblow]

BA,
Thanks for your response, indeed the 8" slab for that project was tied into the top of the wall.

Retired13,
Thanks for your response, I was specifically talking about the vertical rebar in the stem of the wall that crosses the plane at the top of the footing. I only have one set of bars crossing that plane but as soon as you go up the stem, I have vertical bars on the front face and the back face of the wall.

EIT

 

[r13]

I see. Still, many retaining walls have been constructed without front face reinforcement. I don't encourage it, usually I provide T/S reinforcement and start the verticals from the base slab, or provide dowels with development length for bars in compression. Again, this is purely a personal preference, no code or theory behind it. But I highly recommend to extend the verticals into the base slab to develop tension required to minimize the cracks, and protect the stem from damage due to water and cyclic weathering effect.

 

[r13]

I don't know the function of the top slab. As it is tied to the retaining wall, differential movement can occur, which is un-predicable, and might result in cracks in the slab.

 

[JoelTXCive]

I'm confused.

You have enough steel to meet your flexural demands for your main vertical bars, but this does not meet the minimum temp & shrinkage requirement for the stem.

But.. you are adding a second vertical curtain of steel on the front face of the stem. Since temperature and shrinkage steel can be located anywhere in the cross section; shouldn't the combined front and rear curtains of vertical bars get you over the threshold; so you are good to go? (i don't think it matters if the front bars cross into the footing)

 

[hokie66]

Temperature and shrinkage steel requirements for a cantilevered retaining wall would be for the horizontal bars. Vertically, the concrete is free to shrink. You need to provide flexural reinforcement at the tension face (rear face) of the wall, and that may or may not be the amount you proposed.

 

[JoelTXCive]

Hokie-
You're right..that front face vertical bar is really there just to support the horizontal bars prior to casting.

I'm thinking this must be short wall if the main tension face flexural steel does not exceed the temp & shrink value.

 

[KootK]

For walls and slabs, is not the 0.0018 also the means by which we satisfy the requirement for minimum flexural reinforcing? If so, I think that OP would want to either abide by the requirement or seek recourse from whatever the latest code "out" is (4/3 over design etc).

 

[hokie66]

I haven't looked at the ACI code in some time, so will defer to others. But I thought the code interconnection between shrinkage reinforcement and flexural reinforcement had been removed, as it confused many. But at any rate, minimum shrinkage reinforcement is based on gross area, which flexural reinforcement, minimum or otherwise, is based on effective depth.

 

[Settingsun]

You need more vertical flexural reinforcing on the tension side to meet the code minimum. Then you also meet the letter of the code for T&S although there's usually not much problem as Hokie said.

Walls often have cracking problems due to too little horizontal reinforcement. 0.18% won't help much with that. You need close contraction joints if you have only minimum reinforcement. You can still have a problem at the bottom because the footing restrains the stem shrinkage but it may be buried so not visible. Your call.

 

[KootK]

But I thought the code interconnection between shrinkage reinforcement and flexural reinforcement had been removed, as it confused many.

I just checked that this the truth. That said, doe not the requirement below then govern and make 0.0018 a moot point?

 

[r13]

Let's clear up all these arguments on flexural reinforcement and T/S reinforcing steel.

On stem of retaining walls,
1. the amount of soil side steel is determined by strength requirement, thus it follows the rules on minimum flexural steel requirement. The basic form been ρ[sub]min[/sub] = 200/fy, and the As is placed in the tension side of an area b[sub]w[/sub]*d. Strictly speaking, the flexural reinforcement shouldn't be considered serving the purpose of T/S control, but practically, I don't think anybody has ever added T/S steel on top of the flexural steel.
2. unless designed as compression member, per code, no front face reinforcement is required. But many designers prefer to place reinforcement per code minimum requirement. However, the amount and the location of placement are confusing, as for slab, ρ[sub]T/S[/sub] = 0.0018, and for wall, ρ[sub]min[/sub] = 0.0015 (V) and 0.0025 (H). As I recall, the minimum steel is to be placed in the area b*h, but the code does not address the location/method of this placement, it can be one layer in the middle, or two layers, one each face, again, it is a judgement call by the designer.

The question of the OP is concerning the necessity of the front face vertical bars develop into the base slab. IMO, yes, to better protect the toe of the stem, but it is not required by code.

 

[KootK]

The question of the OP is concerning the necessity of the front face vertical bars develop into the base slab.

OP's question is not limited, or even primarily about, the front face bars as far as I can tell. Rather, it's about whether or not the total quantity of rebar crossing the joint is sufficient. And it's not unless it's in the neighborhood of 0.0025 or possesses more than 1/3 reserve flexural capacity in my opinion.

 

[r13]

Let's make it simple,

1. There is no argument that the flexural reinforcement must develop into the base slab.
2. If classify this is a structural wall, the front face bars also must develop into the base slab, but I doubt the justification on this classification, and obviously the OP did not.
3. Code does not mandate the development of T/S steel, but the designer shall make call based on the effect desired. It can go either way, I'll develop the T/S bars by practice, not per code.

 

[Settingsun]

Kootk, 200/Fy and one-third spare capacity are for beams? 7.6.1.1 is for one-way slabs = 0.18% of gross area.

 

[KootK]

Kootk, 200/Fy and one-third spare capacity are for beams? 7.6.1.1 is for one-way slabs = 0.18% of gross area.

Yes and yes. If neither of these things applies to walls loaded predominantly in flexure, what does?

 

[Settingsun]

0.18% for slabs applies as it is functionally a slab.

Off topic: I wonder why one-third spare applies to beams but not slabs. I'd have it the other way around.

 

[KootK]

0.18% for slabs applies as it is functionally a slab.

Oh for Pete's sake... that's where I started before I was led astray.

For walls and slabs, is not the 0.0018 also the means by which we satisfy the requirement for minimum flexural reinforcing?

Off topic: I wonder why one-third spare applies to beams but not slabs.

Perhaps because 0.0018 is a bit liberal to begin with. Just based on my numbers here: 0.0025/0.0018 = 4/3 +/- 4%.