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Interaction Diagrams - Buckling and Shearwalls

Interaction Diagrams - Buckling and Shearwalls

Interaction Diagrams - Buckling and Shearwalls

(OP)
I might have a brain cramp here but I'm trying to sort this out:

Typically when we create Axial-Bending moment (P-M) interaction diagram we use "material limits" and don't really consider any sort of buckling whether it be local buckling or lateral torsional buckling. For example:

Lets say you were creating a P-M diagram for a concrete or masonry column/pier whatever. You would iterate your N.A. location and sum the moments based on the compressive strength of the concrete and yield strength of the steel (what I'm calling material limits). However you would limit your compressive capacity to the maximum allowable axial load for the entire section based on Ag (ACI eqn 10-1 or around there). However this does not consider any buckling effects, which I suppose would need to be addressed by finding the exact capacity of the section based on the applied loads and then check via moment magnification or something along these lines?
However where it gets confusing for me is when looking at an interaction diagram for shear walls. Again you iterate the N.A. limiting your capacity to the material section but there is no local buckling check. I suppose you could compare just the axial load on the wall to the allowable (as a separate check), but what if you have a very low axial load and a very high bending moment with a very tall thin wall, is there a check to make sure you 'end piers' will not buckle? Is there some way to incorporate this into a P-M diagram? Or are they typical separate checks - i.e. for a shear wall you check the P-M diagram then you check axial only your wall to ensure it has required strength for axial only?

Hopefully this is clear? If not I can post a sketch. Thanks!

EIT
www.HowToEngineer.com

RE: Interaction Diagrams - Buckling and Shearwalls

The local buckling checks are usually handled seperately and pretty crudely.

1) usually there's a code h/t limit. In Canada, it's 25.

2) MacGregor's book is the only place I've seen coverage of this. He recommends h/15.

3) Similar to your proposal, some folks will just treat the boundary element as a column, especially where plastic hinging is expected/desired.

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: Interaction Diagrams - Buckling and Shearwalls

(OP)
I see. I like the boundary element / column idea but that makes it tough to make an interaction diagram.
hmmm...

EIT
www.HowToEngineer.com

RE: Interaction Diagrams - Buckling and Shearwalls

The slenderness/buclkling check is assured with minimal moment M2 requirements.

Also by calculating the amplification factor 1/(1-pf/(p_euler))­, you can see how sensitive to buckling your columns are

In reinforcing concrete, you generally increase resistance by adding rebars without increasing stiffness. So, it's possible to get an buckling critical column even if you are have a short column.
(Because your short column resistance is higher than the euler critical load factored)

RE: Interaction Diagrams - Buckling and Shearwalls

(OP)
Thanks Pico, how would you apply that to a shear wall situation? Imagine a tall slender wall with a large overturning moment and very small axial load.

EIT
www.HowToEngineer.com

RE: Interaction Diagrams - Buckling and Shearwalls

Local buckling of thin wall segment of shear wall are covered in code by requiring minimal thickness calculated as a fraction of the net wall story height.
Note that seismic clause requires thickier wall than usual shear wall.

RE: Interaction Diagrams - Buckling and Shearwalls

Typically I'll get a buckling moment of sorts and you can apply it in conjunction with strong axis moments (edit: to clarify, as a weak axis moment in conjunction with the strong axis moments and then run biaxial). ACI includes a provision for moment magnification that can be used. I've found in several cases now that moment magnification is really pretty conservative for walls because of the minimum factored moment they make you take. If you've got a ten foot floor to floor, your minimum eccentricity is going to be over 4 inches with moment magnification, really quite large. Have taken to actually carrying out an elastic second order analysis instead and getting the actual buckling moment of the wall, which is usually significantly less. Just have to make sure you still meet the overall stability checks of ASCE 7 and/or ACI 318.

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