CMU Shear Wall Aspect Ratio

[StrEng007]

Outside of the empirical design section 5.3 (Lateral Stability) in ACI 530, are there any restrictions to the aspect ratio of CMU shear walls when doing analytical design?

Seems like there isn't and the best approach is to simply check for shear capacity and drift limits of the wall.

 

[ANE91]

The seismic requirements can get complicated fast. Even with an Ordinary wall, I believe that there are still certain minimum horizontal and vertical steel areas and ratios. This could affect your wall size.

You could also be limited by boundary elements. Of course, the required steel also dictates how long the wall must be, given that you can only shove so much into a cell.

Otherwise go nuts, as long as the P-M diagram works. Recall that nominal axial strength gets a haircut from stability. The hard part is figuring out what the internal stresses are at the corners of openings. I still don’t completely buy into the hand-wavy FEM/FEA stuff.

Masonry shear walls are ridiculously strong. Out-of-plane flexure (panel wall behavior) tends to govern the wall size & reinforcement.

 

[StrEng007]

When calculating the deflection of your walls and comparing against drift limits, do you take 100% of the wall inertia or do you need to consider a reduction due to any cracking?

 

[ANE91]

When calculating the deflection of your walls and comparing against drift limits, do you take 100% of the wall inertia or do you need to consider a reduction due to any cracking?

Well, that depends on if your wall is actually cracked. Compare your Mu to Mcr. I don't see value in designing an RM wall to be uncracked. URM is not permitted to be cracked.

When cracked, TMS 2022 assumes that the entire wall is completely cracked, reducing the MOI to about 15% of gross. This is extremely conservative. You could do gross-section analysis / transformed section analysis to justify a higher MOI, per 9.3.4.4.5. I remembering reading some of the original research that established these deflection limits and being shocked at how much a masonry wall can actually bend (out of plane) at ultimate.

I don't do enough seismic to substantively comment on drift limit issues with shear walls (in-plane action). Never had a problem for wind.

 

[Luceid]

0.15Ig? Man - that's brutal!

 

[driftLimiter]

NEHRP suggests 50% Ig. But its subject to a lot of variation.

https://nehrp-consultants.org/publications/download/nistgcr14-917-31.pdf

Often times CMU walls length will be limited by shear capacity. Don't neglect this when checking length or aspect ratio.

Also out of plane loading as the walls become high aspect ratio, that can commonly mean they also have a large out of plane load to consider be it wind or earthquake.

 

[StrEng007]

Regarding the comments on the wall being governed by out of plane action, how do you all handle the interaction between the two?

ASCE 7 does require that we analyze MWFRS walls that will impart loads as a shear wall, while still experiencing wind loads from side wall loading. However, just about every book I've seen regarding masonry design is silent on the subject. If feel as though slender wall design for out of plane loading that is also considering the P-delta, combined with shear wall design and the consideration of whether or not the wall is cracked and what actual Ig to use, is complicated enough without having to come up with some theory as to how the two combine.

What I'd like to know is WHO here is actually considering the interaction between the two and HOW are you doing it?

From Section 5.6 of the 2012 Design of Reinforced Masonry Structures (Concrete Masonry Association of California and Nevada):


I feel like this is saying in most cases they are considered separate because we don't actually have a codified way to combine them.

 

[Celt83]

The current available method would be a bi-axial P-M interaction surface with either moment magnification or actual second-order moments, similar to how concrete walls are handled.

Generally, the peak demand locations occur at different elevations for in-plane loading the peak moment demand will be at the base of the wall while for uniform out-of-plane loading the peak demand will be at mid-height.