Shear in a Basement Wall

[DaveAtkins]

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

 

[KootK]

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.

 

[KootK]

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.

 

[dcarr82775]

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.

 

[manstrom]

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

 

[DaveAtkins]

dcarr82775,

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

DaveAtkins

 

[KootK]

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.

 

[DaveAtkins]

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

 

[chekre]

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.

 

[dcarr82775]

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

 

[KootK]

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.

 

[DaveAtkins]

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 nuts

DaveAtkins

 

[dcarr82775]

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

 

[KootK]

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.

 

[manstrom]

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

 

[KootK]

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.

 

[DaveAtkins]

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

 

[RFreund]

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

http://www.HowToEngineer.com

 

[DaveAtkins]

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

 

[bookowski]

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.