Vertical Reinforcement in exposed face of Wall

[CWEngineer]

How do you typically design the vertical reinforcement in the exposed face of a wall, when you just have soil in the backside. I have seen the vertical reinforcement in the exposed face of the wall being designed using 10% of the vertical reinforcement in the earth face, but haven't been able to find a reference for this approach. I think another approach would be to just provide vertical steel that meets the minimum reinforcement of flexural members per ACI 318, para. 10.5?

Thanks

 

[hokie66]

It depends. Since you are proposing less reinforcement in the inside face, I assume you are talking about a cantilevered retaining wall. If it is a basement wall, the reinforcement on the inside face would almost always control.

If it is a cantilevered wall, again it depends, mostly on the wall thickness. If thin enough, you may want to use only one layer of reinforcement. If it is thick, and you are concerned about vertical restrained shrinkage cracking, the inside face horizontals would need enough verticals to support them during casting of the wall.

On the compression face, using minimum flexural reinforcement is illogical.

 

[CWEngineer]

Sorry about that, yes I am talking about a cantilevered retaining wall (where the wall is thick enough that vertical reinforcement would be required in both the earth face and exposed face). In ACI 318, Para. 7.6.5 it states that in walls primary flexural reinforcement shall not be spaced farther apart than three times the wall thickness, nor farther than 18 inches. Would this be something you might use to size the vertical reinforcement in the exposed face of the wall?

 

[Lion06]

ACI 318-08 14.3.4 says that one side should have no less than 50% and no more than 67% of the total steel. So if the soil side controls (which it will for a cantilevered retaining wall), then the exposed face should have no less than 50% of what the soil side has.

 

[cvg]

actually, the wording is a bit different and it specifically excludes basement walls. the exterior wall must have .5 - .67 of the reinforcement, the interior wall the balance. This makes sense since you have no control of the loading on the wall in the future, loading and direction of loading could change.

14.3.4 — Walls more than 10 in. thick, except basement walls, shall have reinforcement for each direction placed in two layers parallel with faces of wall in accordance with the following:

(a) One layer consisting of not less than one-half and not more than two-thirds of total reinforcement required for each direction shall be placed not less than 2 in. nor more than one-third the thickness of wall from the exterior surface;

(b) The other layer, consisting of the balance of required reinforcement in that direction, shall be placed not less than 3/4 in. nor more than one-third the thickness of wall from the interior surface.

 

[IDS]

Section 14 of ACI 318-08 also specifically excludes vertical reinforcement to cantilever retaining walls:

14.1.2 — Cantilever retaining walls are designed
according to flexural design provisions of Chapter 10
with minimum horizontal reinforcement according to
14.3.3.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[CWEngineer]

ACI 14.1.2 states that cantilever retaining walls are designed according to the flexural design provisions of Chapter 10. But do not see any information for the vertical reinforcement in the exposed face of the wall. The horizontal reinforcement (temperature/shrinkage) is placed on both the earth face and exposed face. But was just looking for code guidance on the vertical reinforcement on the exposed face of the wall. Was thinking of using the minimum reinforcement requirements per para. 10.5, but as stated previously this is illogical.

 

[hokie66]

The flexural design provisions only require reinforcement on the tension face. For instance, in slabs you often have areas which are always in compression, therefore do not require reinforcement, except for crack control, and that can be all on the tension face.

Horizontal shrinkage control reinforcement is the sum of the amount on the two faces. If part of it is on the inside face, you need support bars to tie the mat together. Otherwise, there is no requirement for vertical bars on that face.

 

[CWEngineer]

Got it, thank you!

I had different question related to the same topic. When you have a retaining wall with soil backfill and water in the exposed side, such as in the attached sketch (retaining wall/u-frame)would you expect that reinforcement is required on the exposed side (i.e, horizontal force from soil - horizontal force from water, would create a net force that would require steel in the exposed side of the wall, etc.)

I ran a few analysis using the CURFBC program and for the case when I have water to the top of the channel, the results showed that the area of steel required was 0.01 to 0.03 square inch (I know the value is very small, but it is still there). Does this seem appropriate or could there be a glitch in the program?

Thanks