Horizontal reinforcement in reinforced concrete retaining wall

[Woody1515]

Hi all,

When designing a reinforced concrete retaining wall, I understand how you design the vertical reinforcement (cantilever beam conditions). How should the amount of horizontal reinforcement be determined? Is it simply temp/shrinkage amounts? Or is there something else that needs to be considered?

Thanks!

 

[Woody1515]

To be more specific, I’m talking about laterally unsupported concrete foundation walls in houses. I quite often see much more horizontal reinforcement in these regions.

 

[hokie66]

The horizontal reinforcement is for temperature and shrinkage. How much depends on how far control joints are apart, and what degree of crack control is required. The main concern is restraint shrinkage cracking. The footing restrains the wall.

 

[JoshPlumSE]

If it's not supported on the sides then I agree with hokie and it's all about temperature / shrinkage. 0.0018 * gross area of concrete, something like that. My tendency is to go with smaller, more closely spaced bars to better control cracking, but that's more important when people are going to be seeing the inside of the wall as part of a basement or such.

 

[Woody1515]

In the case of a basement foundation wall, it would be supported at each end by the walls that run perpendicular to it. Assuming the wall is laterally unsupported, how would you analyze the wall?

 

[KootK]

In the case of a basement foundation wall, it would be supported at each end by the walls that run perpendicular to it. Assuming the wall is laterally unsupported, how would you analyze the wall?

You've got some choices then:

1) Ignore the horizontal spanning action and design the wall as a conventional, vertical cantilever. In this case, I'd throw in some nominal detailing rebar to acknowledge crack control issues that might arise from disregarding the horizontal spanning action.

2) Truly design the wall in a way that pays homage to the horizontal spanning action. With regard to working out force demands for this case, you could use FEM, the Moody monographs, etc.

 

[elreedyman]

As you mention it is a cantilever type wall in this case choose the horizontal bar as the minimum based on your design code and standard

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[hokie66]

Minimum (.0018Ag) reinforcement is not enough to control cracks due to footing restraint. But if you don't care about crack control, go for it.

 

[Woody1515]

Thanks for the replies. I guess I was most concerned about adding additional reinforcement closer to the perpendicular walls at each end. It looks like there is no standard for this?

 

[YuleMsee]

BS code has a limit of 0.25% of wall area.
Can you view the wall the way we look at slabs, one way spanning action vs 2 way. If the ratios for two way action are met, then you can consider horizontal bars taking some tension with supports at the perpendicular walls. in my experience, if the wall is just retaining one storey of earth, the minimum required area of vertical bars (to control crack widths) is enough to resist moments if the wall was analysed as a cantilever / propped cantilever

 

[JAE]

There is no standard, so to speak, for horizontal reinforcement that is used to resist bending in the wall. That is simply engineering design.

Initially you mentioned "reinforced concrete retaining wall" and I was thinking you were asking about a typical cantilever retaining wall with very little horizontal bending.
That would usually demand at least 0.0018Ag for the horizontal bars.

With a basement wall, usually the lateral restraint is via the basement floor slab and the floor framing above....so a vertical spanning wall.
But with offsets, corners, etc. in basements you do get horizontal bending.

In our area we have a local code that doesn't require horizontal bars at all and usually you are lucky to get vertical #5 @ 40" o.c. With minor foundation settlements, and the horizontal bending in the wall where a stair opening eliminates lateral restraint, you get very large cracks...we've seen 2" to 3" wide.

You can analyze the top of the wall as a horizontal spanning beam and bunch up additional reinforcement in the top of the wall to "span" horizontally to areas where there is restraint.
Then the wall below spans vertically from footing to internal wall-beam.

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[Deadblow]

I typically use 0.0020Ag for #5 or smaller bars and 0.0025Ag for larger bars based on ACI 318-11 14.3.3. In a true cantilevered retaining wall, for horizontal temperature & shrinkage steel, I call for 1/3 of the horizontal reinforcement in the earth side of the wall and 2/3 of the horizontal reinforcement in the exposed face of the wall based on the Concrete Reinforcing Steel Institute design guides.

EIT

 

[WARose]

It's typically temperature and shrinkage steel. However if the wall is long enough.....I tend to check it for the case where there is active pressure on one part of it and none on another part (adjacent). This can produce some significant moments.