tall stemwall foundation design

[jeng123]

When a typical slab on grade/cmu stem wall foundation (see attached) starts getting higher lets say over 6 ft (exposed face above grade) would you analyze as a cantilever retaining wall? or could you assume you have a pin-pin with the keyed in slab holding back the wall? I often detail dowels in the slab hooked into the top of the chair block an effort to help with any differential settlement but I don't think I these could be adequately(per code) developed into the masonry or 4" thick slab?

 

[ishvaaag]

If your system of walls shows low deformation under lateral loads, a pin-pin situation for forces normal to the walls seems not particularly worrying; anyway a 3D model will be better on showing the expected behaviour for the set of hypotheses.

Respect reinforcement one needs to follow the applicable code specs. You may need more or less depending on your situation, but you will see examples in the generic literature of reinforced masonry where the particular aspects of effective anchorage are (at least) not even addressed or warned about.

 

[msquared48]

Retaining wall would be fine.

As an alternative, you could leave the fill out behind the wall, pour the slab for a distance "H" of the wall back from the inside face, stub out bars from the slab, extend and bend the vertical wall steel up and bend 90 degrees to meet the stubbed out bars and field weld them together, then backfill and tamp by hand, after which you pour the remainder of the slab.

I have used this metho many times for higher walls where the owner did not want the larger footing required by a standard retaining wall.

Mike McCann
MMC Engineering

 

[hokie66]

I wouldn't assume it is cantilevered unless it actually is. If you are going to tie it to the slab, design it as a simple span. You will have to brace the wall until the slab is cast and for a few days thereafter. It doesn't take much force to tie back the top of the wall.

If this block wall is to be exposed, you need to waterproof the earth side. And fill all the cores.

 

[jeng123]

thanks for the responses. Mike-although I like your method I think I would get push-back from my contractor on the welding. hokie-i agree it doesn't take much force to tie back the top of the wall but since I don't want to rely on the concrete's rupture strength I will need to develop some reinforcement into the slab and top of the wall. My problem is that the hooked dowels I typically specifiy can't be properly developed in the wall thickness...If instead of the hooked dowels I just bend in the vert. wall reinf. into the slab this would be better but still per code I have a bar in tension hooked 90° with less the reqd. hook embedment...although I know the bar gets developed in tension vertically in the wall I have read other posts where people have disagreed whether reinf. turning 90° without proper hook embedment should not be considered developed. I know I'm overthinking this one...

running through some quick calcs under retaining wall assumption the foundation does get large (as I expected) but reqd. wall thickness for flexure got larger than I expected. The asd provisions required a much thicker wall then lrfd.

 

[hokie66]

You don't have to fully develop the bars...just enough to take the force.

 

[ron9876]

I think hokie66 has it right.

 

[msquared48]

Just make sure that the slab at the top is large enerate the enough to generate the required restraining force to the top via it's own dead weight and friction.

Mike McCann
MMC Engineering

 

[Teguci]

Several things:
1. Consider how this wall gets built. If you design it as a basement wall (pinned top and bottom) then the top will need to be braced until the top slab gets cured to take the load.
2. If you transfer structural loads into the slab on grade (probably the top and the bottom slabs), the SOG will need to be designed to ACI 318 requirements.
3. You will need to develop the hooks into the wall at the top enough to transfer the required loads. Ref 12.5 for ldh

 

[SteelPE]

I agree with Teguci. I'm not a construction expert but don't you have to back-fill the wall to install the slab? Also, when the back-fill are they not going to compact?

I can't see a contractor installing bracing to resist the back-fill until the slab is cured... although I guess bracing could be handled by loading the left side of the wall temporarily with dirt.

 

[hokie66]

I made the point about bracing the wall in my first post above. This is a common requirement where I am, and contractors get it...at least after the first time they have a wall fall over while compacting.

 

[jeng123]

thanks for all the comments! now say this wall is more like 10 to 11 ft exposed solid grouted 8" cmu w/ #5 @24" o.c. designed by assumming the wall pin-pin. For the foundation I propose designing it conservatively as pin-fixed(at foundation) assuming the foundation accepts some moment. I'm interested in what others thoughts are on this...is it necessary to check flexure of the footing and increased bearing pressure induced by this moment? or can you just assume pin-pin with the bottom anchored by shear friction provided by the vertical reinf.?

 

[hokie66]

I would use #5@16" near the outside face, and #5@8" central for the starter bars from the footing. Shear friction is a concept not addressed in masonry codes, but the dowel action of #5@8" should be enough. I wouldn't worry about the footing rotating, but you should check sliding. You can't necessarily confirm by calculation all these numbers, but a bit of extra reinforcement gives me some comfort in a type of construction that probably won't receive much in the way of inspection.

 

[jeng123]

hokie-why do you say near the outside face...wouldn't it be best to maximize the "d" distance for flexure of the wall?

 

[hokie66]

Yes. The way I read your post and wall section, the outside face is in tension. The soil load is on the inside and the wall is a simple span between footing and slab above.

 

[jeng123]

hokie - thanks for the comments. yes I'm with you...with the pin-pin design assumption the outside face is most efficient placement for flexure reinf. the cantilevered ret. wall assumption is a different matter...

regarding your statement "I wouldn't worry about the footing rotating" so even if the allowable soil bearing capacity at the edge of the footing was exceeded by some rotation due to fixity of the wall to footing...you agree this would be negliable and not cause any setllement etc.?

 

[hokie66]

Yes, but then I wouldn't consider that fixity at all, and my footing would be conservatively sized.