Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

CMU

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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