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Design of Residential Retaining Wall (Which Criteria to use???) 1

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mfstructural

Structural
Feb 1, 2009
230
I've been engaged to design a retaining wall in the backyard of the residence. I completed the design, which includes minimum rebar per ACI. The footing dimensions (heel and toe) were requested by contractor due to a sewer pipe. The contractor is the one that retained me as the city is required a stamped drawing from an engineer. I've included the drawing below. I'm getting pushback on the amount of reinforcing in the wall and footing, as they are saying they never put that much at that spacing. After reviewing the IRC which states:

R404.4
Retaining walls that are not laterally supported at the top and that retain in excess of 48 inches of unbalanced fill, or retaining walls exceeding 24 inches in height that resist lateral loads in addition to soil shall be designed in accordance with accepted engineering practice to ensure stability of the wall against overturning, sliding, excessive foundation pressure and water uplift. Retaining walls shall be designed for a safety factor of 1.5 against lateral sliding and overturning. The section shall not apply to foundation walls supporting buildings.

So, that contradicts all the minimum steel requirements set forth in ACI. If you take a look at my drawing you can see #5@12 vert and #5@14 horz in the wall plus bottom and top steel in footing. Also when looking at IRC, foundation walls in general that are 10 inches in width require reinforcing of #4@56" for a wall height of 7'-4" which is the total wall height (see attached image).

Which is the correct steel to use in this situation? There is no mention of conforming to ACI minimum T&S and flexural steel.

Screenshot_2022-04-22_171007_m5tr8u.png
 
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Also, Table R404.1.1(5) shows that no reinforcing is required for a 4' tall cantilever retaining wall with 4' unbalanced backfill.
 
mfstructural said:
Also when looking at IRC, foundation walls in general that are 10 inches in width require reinforcing of #4@56" for a wall height of 7'-4"
Where are you seeing this? I can't find this specific spacing anywhere in the IRC tables.

mfstructural said:
Table R404.1.1(5)
This must be a table specific to your state? I can't find this table in the IRC. Your wall is taller than 4' so I don't think the table would apply here.

My opinion is that if the city is asking an engineer to stamp a retaining wall design then you're no longer in the IRC, you're in the IBC. IBC 1807.2 talks about retaining wall requirements. The wall will need to meet at least a minimum amount of steel per ACI, but the vertical steel will also need to be designed to handle the bending moments in the wall from the lateral soil load, which may or may not be more than the ACI minimums.
Curious how you're getting a 12" thick footing with #5 dowels. For a #5 bar I think the minimum embedment into the footing would be 9.6" if you use all of the applicable reduction factors. Not a huge deal, and I could be wrong, but that would mean the footing should be at least 12.6" thick. I'm probably missing something there.
 
Your design looks appropriate, but one detail struck me as wrong. The starter bars from the footing should be in the outer layer, and should have more lap than shown.

If your wall is designed based on the assumption of active soil pressure, that means it will rotate, so the visible face should be sloped.

But if the slab provides bracing for the wall in the long term, the design could be overly conservative.
 
But if the slab provides bracing for the wall in the long term, the design could be overly conservative.

If the slab is assumed to be permanent (it will not be removed during the service life of the wall, then overturning and sliding shouldn't be a problem, but regardless of the permanency of the slab, the structural design should account for at-rest pressure from the soil behind the wall. It should be designed for the counterclockwise moment at the slab, and there's a slight chance of a clockwise moment at the top of the footing. IF the slab is not considered permanent, then you'd have to also consider the 6' retained soil height, but I would say active soil pressure conditions would be a reasonable assumption for that case.

For our retaining walls, we generally extend the hook of the dowel to 2" clear of the front of the toe, and don't put any other steel in the bottom of the slab. AASHTO may be different than ACI or IBC in that respect, so if it's required by your governing spec, then it's required.

If the aforementioned clockwise moment from the stem wall is substantial enough you may need the bottom steel for that, but again, I doubt you'll have much if any clockwise moment to worry about.

Rod Smith, P.E., The artist formerly known as HotRod10
 

I don't know if the 'starter bars' are the dowels... if so, there is a dimension of 0" which indicates they are in the outer layer. I like Hokie's idea of sloping the outside face. I do that with nearly all the ones I've done. As they rotate, the face becomes more vertical. You could taper the wall from 8" to 10"... it's easy and relatively inexpensive tapering a wall form. As far as reinforcing, it doesn't look bad (I don't know what's required), and you have your dowel leg in the right direction.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
Thanks for the responses. I'll take a look at the dowel hook development length. The design of the wall/footing did not account for the slab being there (conservative). My feeling is that it's a retaining wall but not a classic retaining wall in the sense that it's retaining soil along the full stem height . 3.5' has fill on both sides of stem wall, above which 3.5' of soil is being retained. With the slab in place, which to my knowledge will be there permanently, as there will be a below grade patio at the walk-out basement, there should be little rotation of the wall. I've read a lot of threads on this forum about designing cantilever retaining walls and the mass consensus is to design for active pressure if it's not supporting a structure. This wall is only retaining soil...no structures will be supported on it nor will there be any structures near it...it's just the backyard. Some other threads on this forum also indicated that an at rest pressure designed retaining wall will turn into an active pressure wall once it displaces very small percentage of the stem wall height.

In this case, flexure does not control any of the reinforcing requirements in the stem. The reinforcing is strictly based on minimums, which is where my original main question comes in....
I feel like the reinforcing here is very high (even though I agree it meets ACI minimum). However when reading through IRC, an 8' tall concrete retaining wall 10" thick does not require #5@12 Horz and Vert...much less is required and much less is used in practice. One can argue that those 8' tall walls even though pinned at top and bottom see more lateral loading that my retaining wall here...

Then I start reading into IRC and the language (included in first post) is vague. Basically saying that doesn't need to be engineered. I take that as it's the engineers calls and in most cases engineers aren't even required by municipalities, right or wrong. I don't believe that just because an engineer is being required, they mean we need to conform to IBC. Using that logic we'd never or rarely use IRC for residential design. My experience is that the city is looking for a licensed party to design for liability reasons...i.e. if something goes wrong they can come back and point to me.

 
I'd normally use the slab as restraint and the granular backfill would go full height except for filter cloth and topsoil for grass.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
The IRC tables for concrete leave anyone coming from an ACI background scratching their head. But if you start looking into basement wall problems you'll see why most engineers don't trust the IRC for this stuff. (Haven't dealt with it much myself - I'm in a swamp - but there have been some good threads about it.)

IRC is for non engineers to design cheap structures that'll stand up okay. If the contractor wants an IRC retaining wall, they should hire a freelance drafter to draw it, sir the table, and submit it. If they (or the AHJ) wants it engineered, then engineer it. And that's minimum flexure steel.

Though 10" seems a little thick for a 3'6" retained height (assuming they know how to compact soil).
 
The reason for the 10" wall is to match an existing 10" concrete retaining wall adjacent to stairs leading from walkout basement to grade, requested by client.
 
dik,

Either the 0” dimension is incorrect, or the horizontal bar is not between the starter and the vertical. Can’t have it both ways.

‘Starter’ is Aussie terminology to distinguish flexural bars from dowels, which are for shear only. I once called starters dowels, but accepted what I believe to be more descriptive language.
 
I've read a lot of threads on this forum about designing cantilever retaining walls and the mass consensus is to design for active pressure if it's not supporting a structure.

Active pressure is appropriate where the wall can move away from the retained soil enough to activate the interlock of the soil particles to make it partially self-supporting, which reduces the pressure exerted on the wall. If the wall shouldn't (or, as in this case, can't) move, the higher at-rest pressure is more appropriate.

Rod Smith, P.E., The artist formerly known as HotRod10
 
The drawing shows it wrong, but I would have assumed the dowels were in the same layer as the vertical wall reinforcing because of the 0" dimension; I usually place dowels in the middle of the wall, but not for cantilevers where they are used for flexure... Didn't know that about starter... thanks.

It's dangerous to show it wrong, because that's the way it could be built...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
Isn't As (min) = 200/fy, say d = 10", it requires #6@12". For #5@12", it turns out "d" in cal was 8". Correct? Also, the retaining height is 7' to the finished grade, but you only specified 42" (3'-6") drainage backfill from the grade, what type of fill is to be provided below the drainage backfill then? I suggest providing a cal sheet per ACI to the city for clarity.
 
No. That's a requirement for beams. Minimum reinforcement for walls is less.

Disagree, I consider the earth retaining wall is a cantilever beam, which is dominated by flexural behavior rather than the typical walls that are subjected to compression with small eccentricity, and are supported on both ends.
 
I would call it a slab, just oriented vertically. Most codes distinguish between slabs and beams for minimum reinforcement.
 
I've always designed it as a slab...FYIW.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik
 
le99 said:
Disagree, I consider the earth retaining wall is a cantilever beam, which is dominated by flexural behavior rather than the typical walls that are subjected to compression with small eccentricity, and are supported on both ends.
You are welcome to your opinion, but the ACI 318 code does not support your opinion. The OP's AHJ would probably follow the code.

Screenshot_2022-04-23_190909_qvrty5.jpg
 
It's not a beam or a slab; it's a semi-gravity retaining wall. Doesn't ACI have provisions for that?

Thanks OldDawg; apparently it doesn't. I'm a little surprised. I'm beginning to like the AASHTO bridge spec more and more.

Rod Smith, P.E., The artist formerly known as HotRod10
 
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