Need a book? Engineering books recommendations...

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

Re: CMU

[Subject Prev][Subject Next][Thread Prev][Thread Next]
In a message dated 97-07-28 13:02:21 EDT, you write:

<< Subj:	CMU
 Date:	97-07-28 13:02:21 EDT
 From:	shake4bake(--nospam--at)earthlink.net (Mark Baker)
 Reply-to:	seaoc(--nospam--at)seaoc.org
 To:	seaoc(--nospam--at)seaoc.org (seaoc)
 
 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
  >>

Mark,

I have done a similar design, 25 foot  8" CMU walls, but we used fm=1500 in
the design which only allowed a maximum reinforcing ratio of 0.0053.  Because
of the higher block strength you get a higher maximum allowable reinforcing
ratio. 

In our case we used the maximum allowable d distance (edge distance) which
resulted in placing two curtains of reinforcing ( #7 vertical at 24 each
face).  We had  wood roof  trusses spanning about 50 feet to the bearing
walls.  The design also considered additional vertical load eccentricity due
to diaphragm deflection under seismic loading at the top of the wall besides
the ledger eccentricity. 

This did cause some reinforcing placement problems.  For detailing we showed
all the horizontal reinforcing staggered to one side for a given lift, and
then switched the horizontal reinforcing to the other side for the next lift
above.  This was to facilitate getting the vibrator in the wall.  The
building is located towards downtown L.A. (north of the 10 freeway) and had
no damage due to the Northridge Earthquake.

The mason on the job would have perferred to have used 10 inch block for
rebar placement.  I have heard it is the same cost as 12 inch block but you
save on the amount of required grout.  For either reinforcing size that you
have shown, I would definitely go to two curtains (try to use as big a "d"
distance as you can get to provide more reserve capacity).  I would also
check for additional vertical load eccentricity due to seismic diaphragm
deflection pushing/pulling the wall inward if it is a long span between cross
walls bracing the wall and diaphragm. 

I might suggest for your own comfort level, how large can the eccentricity be
on the wall before the wall bending moment exceeds allowable and do you feel
comfortable with the possibility of  this displacement will not actually
occur.  You might also want to look at some of the slender wall tests which
the Masonry Institute conducted and see what their opinion is.

Hope this helps.

Michael Cochran
BCASE1356(--nospam--at)aol.com