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?
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
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
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
RE: Shear in a Basement Wall
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
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
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
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
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
RE: Shear in a Basement Wall
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
I think if I did this on a project, they would call me nuts
DaveAtkins
RE: Shear in a Basement Wall
RE: Shear in a Basement Wall
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
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
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
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
EIT
www.HowToEngineer.com
RE: Shear in a Basement Wall
DaveAtkins
RE: Shear in a Basement Wall
RE: Shear in a Basement Wall
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
"It is imperative Cunth doesn't get his hands on those codes."
RE: Shear in a Basement Wall
RE: Shear in a Basement Wall
EIT
www.HowToEngineer.com
RE: Shear in a Basement Wall
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
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
RE: Shear in a Basement Wall
RE: Shear in a Basement Wall
RE: Shear in a Basement Wall
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
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."