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

[mfstructural]

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.

 

[le99]

I wouldn't against the code, but to call for caution, as the cantilever structure has no redundancy.

 

[gte447f]

To the OP, first and foremost, don't listen to the contractor... he has no idea what he is talking about. Your steel quantities are fine.

Second, why do you start off your post stating that there is a contradiction in the IRC regarding design of retaining walls? There is not a contradiction in the IRC. The IRC does not contain any prescriptive reinforcing designs for retaining walls. Section R404.4 is the extent of it, and it simply says (paraphrasing) that walls over 4 feet high must be designed in accordance with accepted engineering practice (presumably by an actual engineer and presumably in accordance with ACI 318). This also implies that walls under 4 feet do not have to be designed at all, nor are there any prescriptive requirements for the construction of walls under 4 feet high (i.e., the builder can literally do whatever he wants and the wall will most likely perform poorly and fail prematurely in due time).

All of those wall reinforcing tables in the IRC, to which you keep referring and which are all conspicuously light on reinforcing steel, are for either foundation stem walls or basement walls, neither of which are at all comparable to a cantilever retaining wall, so ignore those tables.

 

[mfstructural]

I agree. I used minimum steel for T&S. Thanks for all the comments!

 

[hokie66]

Minimum steel for T&S would apply only for the horizontal reinforcement. Vertically, the wall is unrestrained for T&S. The vertical steel should be whatever is required flexurally.

 

[OldDawgNewTricks]

hokie66, you are technically correct. But it is more of a semantic distinction than a design issue. The quantity of reinforcement (0.0018Ag) is identical for minimum flexural reinforcement and for temperature and shrinkage reinforcement. The only difference is that the entire amount of minimum flexural reinforcement must be placed near the tension face but T&S can be distributed to both faces.

 

[BAretired]

It would be more economical to use a strip footing centered under the wall. The footing needs to be designed to support only gravity load. There is no need for an eccentric footing if you are using the slab to resist soil pressure. A 10" deep x 24" wide plain concrete footing is likely sufficient.

If the slab cannot resist the at-rest soil pressure, the retaining wall is effectively 6'-0" high, not 3'-6", but could be designed for active pressure only. In that case, the footing would need to resist the resulting bending moment.

BA

 

[hokie66]

OldDawgNewTricks,

I thought that unfortunate relationship between flexural reinforcement and T&S reinforcement had been revised in later versions of ACI 318, but perhaps not.

 

[OldDawgNewTricks]

hokie66, ACI stopped referencing the T&S provisions to also apply to minimum slab reinforcing in 2014. But it takes a while for various jurisdictions to adopt new codes and old habits die hard. Also, the values are still identical so I can see how people could get them confused.

 

[BridgeSmith]

Under the AASHTO provisions, we are allowed to (and do) ignore T&S reinforcement for earth retaining structures, as they are not subject to the temperature fluctuations that freestanding walls, etc. are.

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

 

[Settingsun]

BridgeSmith, isn't it the contraction from the temperature at concrete setting that matters? Still going to shrink, still going to get cold. Is there a limit on joint spacing? That's what matters in slabs on ground despite the supposed science behind reinforcement quantities. Maybe the same here.

One Australian main roads department published a technical note requiring that we not skimp on reo for their walls. I understand this originated from observed problems. But our concrete is shrinkier.

 

[JLNJ]

A lap of 18 bar diameters? That's woefully insufficient in THE most critical part of the whole wall.

The second most important part of a cantilever wall is the embedment of the dowel bars and this detail doesn't address the placement depth.

Is that a key I see in there? What is it and what's it doing?

Sometimes I think people just "draw" what they think looks like structure without actually understanding what is happening.

 

[BridgeSmith]

Apparently, I was somewhat confused as to our policy regarding T&S steel for retaining walls. We exempt culvert wingwalls (typically 12" thick) from the T&S steel requirements, but we do provide T&S steel for the stems of retaining walls 12" thick, or more.

5.10.6—Shrinkage and Temperature Reinforcement

Reinforcement for shrinkage and temperature stresses shall be provided near surfaces of concrete
exposed to daily temperature changes and in structural mass concrete.

It does say in other sections that T&S steel is required for "conventional walls", which would include concrete cantilever retaining walls, so we're apparently deviating from the spec in this regard (as a DOT, we're allowed to do that, as long as it's documented, which we've done in our design manual).

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