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Shear in a Basement Wall

Shear in a Basement Wall

Shear in a Basement Wall

(OP)
Assume Vu at the bottom of a basement wall exceeds phi*Vn. I contend that the only way to make the wall adequate is to make the wall thicker. But I have seen more than one engineer try to justify a wall that is too thin by checking shear friction in the wall, using the available vertical steel.

I think you must prove shear works based on the thickness of the wall--shear friction is a separate concept.

Thoughts?

DaveAtkins

RE: Shear in a Basement Wall

I agree completely. Many designers assume that if either shear friction or diagonal tension shear work, things are good. I believe that you need both diagonal tension shear (Vc) and shear friction to pass muster in order for a design to be safe. Obviously, in the absence of a construction joint, shear friction is not critical.

So yeah, in my opinion, providing sufficient shear friction capacity does not alleviate one's obligation to also provide diagonal tension shear capacity. For that, one needs shear reforcing of some strife (stirrups, z-bars, stud rails...). Really, a simple free body diagram of the wall with a diagonal crack in it will quickly show the fallacy of the "shear friction fixes everything" concept.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

If you're up for some tedious pedantry, this thread was my version of the same question: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

I agree that the two are separate, but in my experience if you are exceeding Phi Vn on a basement wall (I am assuming out of plane) you have other problems as well.

RE: Shear in a Basement Wall

or bump up f'c

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

RE: Shear in a Basement Wall

(OP)
dcarr82775,

Interestingly, I have seen shear actually control the design of below grade walls on a number of occasions.

DaveAtkins

RE: Shear in a Basement Wall

When I'm dealing with deep basements, I've basically come to the conclusion that walls are not aggressively thin enough unless there's shear reinforcement. If I show a 14" wall without shear reinforcement, I can be reasonably confident that my competition would show a 12" wall with reinforcement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

(OP)
KootK,

So you actually show stirrups in a wall? I have never done that. I am not even sure how it can be done.

DaveAtkins

RE: Shear in a Basement Wall

Hi,

If you make the wall thicker, its nominal shear Vn will increase but since it will be more stiff, it will attract more load. So by increasing the dimension of the wall, it is not sure that u will solve the problem.

Personally, if the other walls in your floor aren't overstressed and critical, u can assign shear stiffness modifiers to your O/S wall and run the calculation. The wall will attract less shear and the adjacent walls will take that residual shear.

RE: Shear in a Basement Wall

Hmmm interesting. Never had it control for me, or seen shear reinforcement in a wall. Learn something new everyday

RE: Shear in a Basement Wall

Yes, I actually do show stirrups in basement walls. They'll be localized near the floor slabs and, so far, have taken one of four forms. I'll rank them twice, first in order of my preference, second in order of perceived contractor preference.

Me:

1) Stud rails.
2) U-stirrups like in beams.
3) Individual tie bars.
4) Inclined Z-bars.

Contractors:

1) Inclined Z-bars.
2) Individual tie bars.
3) U-stirrups like in beams.
4) Stud rails.

Hey, those lists are in reverse order? Go figure. Inclined Z-bar QC is terrible. Sometimes they get installed bass-ackwards which renders them utterly useless. Very often, the spacing gets messed up and, on such a shallow member, that's disastrous. Stud rails are awesome but nobody's willing to pay for it. Ever.

Yes, it is rather impractical to shear reinforce a basement wall and yes, a thicker wall would be much better. The economic realities of shoring, parking, and local market competition just are what they are I'm afraid. Besides, I only sort of believe in earth pressure. That helps me sleep some.

As a related matter of interest, check out the foundation wall detail below. It's by another firm and for a significant project down the road from me. You can infer their assumptions regarding wall fixity from the location of the shear reinforcing. It's never occurred to me to do it this way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

(OP)
Ahh...so they give a vertical AND a horizontal spacing for stirrups.

I think if I did this on a project, they would call me nutsponder

DaveAtkins

RE: Shear in a Basement Wall

Waste of a perfectly good waterstop. I have used that fixity assumption on a few occasions, but always used a thicker wall so no stirrups

RE: Shear in a Basement Wall

I would think that considering the wall laterally pinned at the SOG and footing would almost always lead to shear overstress due to the backstay effect.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

Interesting. I've never had a shear issue. If the force is that big, are you using the slab on grade as a lateral support? Seems like it would want to buckle.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

RE: Shear in a Basement Wall

I do typically use the SOG as lateral support. I've looked at treating the SOG as several cracked, pin connected segments that have to buckle upwards in opposition to gravity. It works okay based on that simplified -- and admittedly rough -- analysis.

The detail above is not my own and I've never used it. It would produce a substantially greater axial load in the SOG than I typically deal with which is probably your point.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

(OP)
For the project I am working on, the wall is connected to the top of a mat footing. So no issues with slab-on-grade buckling.

We have a very high water table on this project, so our geotech has recommended 95 psf per foot depth for lateral earth pressure. I have never seen anything this high!!! I always thought lateral pressure on a wall could not be greater than the lateral pressure due to water (62.4 psf per foot depth).

Anyway, that is why we are struggling with shear at the base of the wall.

We are going to detail stirrups in the wall (there is always a first time for everything!).

DaveAtkins

RE: Shear in a Basement Wall

You know if you use the friction reinforcement concept, wouldn't you just need to make the shear reinforcement long enough so that it is developed on both sides of a section that no longer exceeds Vc? Meaning if the section exceeds 6" up from the base you would basically have a crack there which you would provide shear reinforcement for. Seems wrong but I'm just curious as to why it is.

EIT
www.HowToEngineer.com

RE: Shear in a Basement Wall

(OP)
That's the point--shear friction reinforcement does not make the shear problem go away. Only stirrups, or thickening the wall, can do that.

DaveAtkins

RE: Shear in a Basement Wall

I would think you'd need a plan detail to make that clear to the contractor - I'm having a bit of trouble with it myself. I assume the '@500 Hor.' spacing is into the page, if so how does that reconcile with the 150 spacing on the verticals - seems like you'd want them to coincide. Same for vert. spacing with horiz. bars. Unless I'm missing something.

RE: Shear in a Basement Wall

Yeah, I've seen it accompanied by both plan and elevation details.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

I tend to avoid shear reinforcement in walls. Thicker wall is less work and less chance the contractor will muff it up.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Shear in a Basement Wall

Count me in the thicker wall camp. The only retaining wall cases I have encountered where shear controlled were deep basement walls, and these are normally some type of deep foundation system walls such as slurry diaphragm walls.

RE: Shear in a Basement Wall

OK, I think I have one logical argument that suffices my wondering. If you have a section "x" inches away from the support that fails in shear i.e. Vc<Vu then that length of wall would be theoretically cracking at all sections between x and the support thus the shear friction reinforcement turns into some dowel bending type thing. Although I suppose you could still argue that the shear friction bar keeps these cracked pieces in compression as so as it starts to deform. I mean the shear friction reinforcement is crossing the same crack as the stirrups just in the orthogonal direction and not parallel to the direction of force. hmmm... I'm just trying to to prove to myself that mechanics of it don't work... not sure this argument is sufficient...

EIT
www.HowToEngineer.com

RE: Shear in a Basement Wall

Oh, there's definitely a post Vc path for the shear in the form of dowel action. It's just a lousy path. Given normal placement of wall vertical reinforcing, there would be a tendency to spall behind the bars on both faces.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Shear in a Basement Wall

That's some heavy water.

Of course, geotech engineers round pi up to 5.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

RE: Shear in a Basement Wall

95 pcf isn't unreasonable for a poor soil. I typically end up with loads in the 80-85pcf range for submerged conditions. You need to design for more than just the unit weight of water.

RE: Shear in a Basement Wall

And if it were wet concrete, you would be designing for 150 pcf. Water can get heavier if you add heavy stuff to it.

RE: Shear in a Basement Wall

Yes, the pressure in submerged conditions has to include the water plus the buoyant soil pressure. The buoyant soil pressure is based on the density of the soil minus the density of the water. I think it is poor practice for a geotechnical engineer to just specify a single number like 95 pcf. While the water pressure always varies linearly with depth, the soil pressure is often trapezoidal rather than triangular.

RE: Shear in a Basement Wall

Quote (hokie66)

Yes, the pressure in submerged conditions has to include the water plus the buoyant soil pressure. The buoyant soil pressure is based on the density of the soil minus the density of the water.

That is sad we have to remind people of such a rudimentary concept.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Shear in a Basement Wall

Feel free to correct any of this:

I think it is important to realize that the concrete shear we design stirrups for is not a purely horizontal shear - the stirrups are really resisting a principal stress. The direct shear (being resisted by shear-friction in this discussion) is just that, and does not have a principal tensile stress. Pure beam shear is complementary, but it is clear that it is only part of the total stress. If you could rely solely on the complementary nature of shear, than your direct shear demand *would* equal your horizontal shear demand. But, it doesn't because of principal stresses.

As I have recently learned from KootK and the ACI-318 commentary, you can utilize the main tension bars for shear-friction without adding more area, as long as the area provided satisfies both separately.

"It is imperative Cunth doesn't get his hands on those codes."

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