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- Subject: CMU
- From: Mark Baker <shake4bake(--nospam--at)earthlink.net>
- Date: Mon, 28 Jul 1997 09:30:11 -0700
This seems to be the year for design of CMU buildings in my office: I am presently designing an 8" thick 27' high cmu wall utilizing slender wall design with the following results: Option 1: f'm = 2500 #5 @ 16" o.c. Ratio of Mu/phi*Mn = .92 Allowable reinf. ratio = .0089 Actual reinf. ratio = .0051 Axial stress ratio = .12 height/deflection ratio = 415 Option 2: f'm = 2500 #6 @ 16" o.c. Ratio of Mu/phi*Mn = .68 Allowable reinf. ratio = .0089 Actual reinf. ratio = .0072 Axial stress ratio = .12 height/deflection ratio = 473 This leaves me wondering: Option 2 brings one much closer to tension steel being 50% of the balanced design steel which gives excellent wall ductility. The ratio of Mu/phi*Mn is well below 1.0. Option 2 seems the obvious choice. The ratio of Mu/phi*Mn is acceptable in Option 1 but, the reinf. ratio is a long way from 50% of the balanced design steel ratio. It seems there should be a lower limit to the reinf. ratio. This wall would be very flexible in comparison to the Option 2 wall. I don't really know how to word a question with this post, I guess I am looking for any discussion from those who would like to comment on our ability to design a wall such as Option 1 which by code is acceptable but by instinct looks weak. Regards, Mark D. Baker Baker Engineering
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