Retaining wall as One Way Horizontal Slab

[AlpineEngineer]

I have a tall basement retaining wall (18ft) that I wish to design as a horizontal one way slab system (not a typical vertical one way slab/beam)because my horizontal dimension is much less than my vertical dimension. I have perpindicular walls at the ends of the subject wall that can restrain the horizontal one way slab. I assume I can design the wall as 12" wide horizontal strips and in accordance with ACI 318 one way slab provisions. I'll verify I have enough mass/friction on the perpindicular walls to resist the end reaction and develop the horizontal steel into the perpindicular walls.

My questions are: Do you see any problems with this? Do I really even need to worry about much of a footing on this wall (other than self weight of course)? Do I have shear concerns at the ends of my wall? Reading ACI I don't get the impression I have to worry about shear at the ends for this case. I plan to install #4 vertical at 12" o.c. for temp, shrink, and force distribution.

 

[Lion06]

It makes sense to let the wall span the direction that it wants to span (the short direction).

For shear, you shouldn't have any problems. You can use full phiVc.

Regarding the footing, you'll have to evaluate if there are any other loads acting on the wall. Is it supporting any framing?

If you're retaining on both sides you shouldn't have to worry about having enough mass on the perpendicular walls to resist any sliding.

#4 @ 12" only works for T&S steel if you're less than 10" thick.

 

[JAE]

Agree with everything Lion06 says.
Check [φ]V[sub]c[/sub]

 

[AlpineEngineer]

If phiVc is less than Vu/2 what are my options? Two way slab? I have never heard of shear failure in a one way simply supported slab. Have you? Even in a two way slab I see no consideration for shear except for punching shear at concentrated bearing points. For a simply supported one way and two way slab how do you design for shear? I have never seen such a detail, only details for punching shear, guess you could spec some goofy stirrups.

 

[rapt]

Design codes require a one way shear check on all slabs as well as beams. The only difference in the rules is generally that shear reinforcement is only requiresd for slabs if the applied shear is greater than Vuc rather than .5 Vuc for beams.

If the panel between supports is 18' (vertical) * 12' (horizontal) then it is a two way slab supported on 4 sides!

Also, make sure you do not use active pressure for the soil pressure calculation! You need to design this for full passive pressure.

 

[hokie66]

I wouldn't use passive pressure for loading a basement wall. At rest soil pressure is appropriate, with due allowance for water behind the wall.

 

[JAE]

ACI 318 only requires slabs to have a nominal strength, [φ]V[sub]c[/sub] greater than the factored shear V[sub]u[/sub].

The [φ]V[sub]c[/sub]/2 limit is only for beams as rapt suggests.

With this basement wall & horizontal spanning concept, you would use at-rest pressure not active.

 

[AlpineEngineer]

Thanks Guys. I thought for slabs you had to divide Vc/2. That helps alot being able to use full Vc and that way my slab is fine. My slab is not restrained by a basement slab or floor at the top of the wall; hence my reason for designing it in the horizontal direction. Thus, I don't think I have a two way slab, I think it is one way-horizontal. I see no logic for using passive soil pressure.
I used active pressure, not at rest, so I should be conservative.

 

[a2mfk]

Assuming you also have bars into the footing, you will have moment along the footing whether you want it there or not. I see this as supported on three sides. Though if you size the rebar for the horizontal direction and then use the same in the vertical direction, I think this leads to a conservative design. Maybe only a few more rebar than the min required anyway.

 

[Lion06]

a2-

I don't think you get a moment at the footing just because you have bars there (well, ok, some small nominal moment, but certainly not a fixed condition). We design footings for vertical load only all the time and there are always bars from the wall into the footing. It's about the amount of bars. If you provide enough bars for shear friction to work and these bars can't develop a significant moment, I would ignore it.

 

[msquared48]

If you are spanning horizintally, you will just have a normal basement wall, but spanning horizontally. The MINIMUM design soil pressure should be 50 psf. It will not be a yielding wall, so higher pressures are warranted here.

As for two way action, if you rely on such to reduce the horizontal steel, then you are saying that there is yielding wall action in the vertical direction, and you will correspondingly have to perform another analysis for the wall acting vertically. This will affect the size and positioning of the wall footing.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[AlpineEngineer]

msquared48, why do you say its not a yielding wall? Because of the end conditions? Speaking of which, I analyzed it as a simply supported. I realize there is some moment at the ends due to monolithic pour of perpindicular walls and steel, but i don't think it is a fixed/fixed. Seems to me its restrained where as a cantilevered wall is not. Also, do you guys have any thoughts on overturning moment? Do I need to consider this?

 

[Hemifun]

Using active soil pressure is not being conservative. At rest pressures are usually greater than the active pressures plus, as already mentioned, saturated soil may need to be considered.

 

[Lion06]

When Mike talks about an unyielding wall, he just means that it can't move enough to allow active soil pressure to develop. That's why you need to design for the at-rest pressure (plus any hydrostatic pressure that may be present).

 

[msquared48]

Alpine:

In a basement wall design, which is simply supported, the wall is considered "non-yielding". However, a cantilever wall design is a yielding condition in that the top of the wall will move or deflect laterally. In that regard the pressure is reduced by the movement. Such is not the case for non-yielding walls such as the scenario you have. Use 50 psf minimum.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

That should be 50 psf/ft of depth.

Right?

 

[Lion06]

Nice catch, JAE. I was looking at the magnitude and didn't even pay attention to the units.

 

[a2mfk]

Good point Lion, I guess with this specific case the deflection will be minimal so the bending stress along the top of the footing will be low. Still, any amount of restraint will develop flexural stresses in the slab at the base of the wall, though the minimum amount of vertical reinforcement will take care of that I would assume. In theory, greased dowels into the footing would take care of this issue but I would not bother.

 

[Hemifun]

ACI 350 recommends using 60 lb/ft3 above the water table and 95 lb/ft3 below the water table for "at rest" values of equivalent fluid pressures.

 

[BAretired]

The wall is hinged at the bottom, restrained at each corner and free at the top. Moments and shears may be obtained from PCA tables for rectangular tanks (or similar sources).

If the foundation is well drained, there should be no water table above the basement floor. Otherwise, the design requires special attention.

BA