CMU Slender Wall - Design and Cost Implications

[Martino8]

We're working on an industrial project where the owner wants the exterior shell to be CMU.

It's quite tall in places + 29 feet and in a high seismic area (Sds = 0.816) so we're definitely needing to use the slender wall design procedure.
Several designs are possible involving multiple variables. Off the top of my head we could adjust the following:

1. Nominal Block Size​

2. Assembly design strength F'm​

3. Size and Spacing of Reinforcing​

4. Ceiling Height Reduction​

5. Other ???​

From this list, any experience on what is the most cost effective variable to adjust when you're running up against out-of-plane bending strength and deflection limits?

 

[strucbells]

You're likely going to be running into issues with an 8" wall at that wall height + Sds, especially near openings. I assume this is single story (no intermediate floor diaphragms that can brace the wall?)

My inclination would be to go to a 10" or 12" block wall. Check for local availability. Then you can get 2 mats of reinforcing in your wall and get a lot more flexural capacity out of your wall.

Another option could be to stick with 8" blocks and add pilasters at whatever spacing you need to help out with out of plane issues. Economics probably come down to what your roof framing looks like and what plan area we are talking.

 

[Martino8]

Yes, Not going to be able to avoid 10-inch block and possibly even 12-inch.

I'm guessing the masonry subcontractor will prefer no bigger than #5 vertical bars which may result in a pretty tight spacing (trying to avoid getting clear down to 16" oc). If that's a little tight, how much is the premium for going to higher strength block? (say 3,250 psi block for F'm = 2,500). This feels like it might have merit because it will assit more directly in the deflection limit calc.

I don't know anything about availability/cost differences. So I left a message with a local block supplier.

 

[JAE]

For larger blocks, like 12", it is not uncommon to use double bars (increased d) for bending in either direction. There's usually plenty of room for the double bars and it increases your spacing and stiffness to a degree.

 

[strucbells]

Block strength can vary quite a bit regionally so best to reach out to local suppliers. Once you start specifying f'm > min values you may want to consider doing prism testing rather than the unit strength method. The unit strength method is pretty conservative and you may get higher f'm values for the same strength block.

FWIW I wouldn't even blink at specifying #6 bars for this wall. #7s might be appropriate at some jamb steel if you have big openings. Just make sure you're not exceeding the TMS max reinforcing limits for whatever size block you use.

 

[Martino8]

We have some decent size openings that will need heavy reinforcing and likely pilasters for the jambs.

One more thing, we've got an in-house spreadsheet that does the iterative slender wall analysis and a generic software package that has a slender CMU wall option. In the software, there are two options for stiffness.
1. Ieff based on Mu as it varies by height
2. Icracked x Full Height

In addition, service load moments can often be below the cracking moment of the wall. However, seismic forces include reductions (R-values) for design loads that are significantly less than the instantaneous forces. The reinforcement ensures ductility for the intial overstress with the concept that the system can disipate the excess energy. However, the wall still cracks first, so using Ig for the service checks doesn't seem correct.

So what should we assume for I at the strength level for the P-delta checks.
And how is that different than the I at service levels for deflection limits?