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# Rho in Diaphragms

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: Rho in Diaphragms
• Date: Tue, 2 Nov 1999 09:19:00 -0800

I am starting a new thread for redundancy issues in diaphragms instead of
the 10/lw topic from which it evolved.

Darry H wrote:

The diaphragm load is just what I've been confused about. In my case
I've
got a 1-1/2 story residential where the upper walls aren't in-line
with the
bottom walls.  I'm trying to show that the lateral forces are being
transfered to the lower exterior shear walls via the floor diaphragm
and
roof diaphragm. 1630.2.3.4 says to use 3*Ca/R for Fpx in equation
33-1.
After this, I believe 1633.2.6 allows me to get 1.7 times more
allowable
design strength and 1612.3 equ 12-9 says I get to divide it by 1.4
again to
calculate what exactly is the design strength needed for the floor
and roof
of this 1-1/2 story. I'm still trying to get E for equ 30-1 so it
looks to
me that I have to use rho. (To avoid further confusion I'm simply
using 1.5
as the maximum default value for rho so I don't have to worry about
it.)  DH

I think you have a many issues here.
1) You are using the SIMPLIFIED method, requiring the 3*Ca/R value
for diaphragms.
2) You are describing a transfer diaphragm, where the vertical
resisting elements are parallel but have an offset. That is a Plan
structural irregularity per table 16-M item 4. You need to look at those
sections as well.
3) Rho does not apply to collector design (Section 1633.2.6). You
need to determine the force demand to the shear walls and multiply that by
omega or determine the limiting force parameter to your shearwall (or brace)
that may allow you to use a force significantly lower than that obtained by
using the omega. Examples of limiting factors are overturning, diaphragm
yield, shearwall or brace capacity.
4) I don't think you can divide by 1.4 even in ASD. There were
be true.
5) NO WHERE in the code does it say that E applies to diaphragms, Fp
does. For the diaphragm itself, you do not have to consider RHO. Look at it
this way, How can you improve the redundancy of a diaphragm? I don't know
either.
6) The code needs refinement in its wording, we need the Seismic
Design Handbook Volumes II & III to be published immediately, and we need a
1"997 UBC for Dummies" to be published or "1997 UBC Cliff Notes" that says
the intent of the code in plain english. I know these really helped me
learning DOS and Shakespear respectively.

I guess I better add a disclaimer to my posts, It seems to be the
right thing to do and also a funny anecdote.

Civil Engineer, Associate
CRJ Associates, Inc.
email: gerardm(--nospam--at)crjarch.com
tel: 650.324.0691
fax: 650.324.0927
web: www.crjarch.com

DISCLAIMER:
1st the Funny one (At least to me):

The following post is property of the National Football League. Any
Broadcast or re-transmission without the expressed written consent of the
NFL and the San Francisco 49ers is Strictly prohibited.

Now the Real One:

These views are mine alone. These opinions should not be construed
to be policy at or the opinion of CRJ Associates.

AND FINALLY THE QUOTE:

"It's looks like one huge shit sandwich, and we are all gonna have
to take a bite"
From the movie Full Metal jacket (Scene where the reporters from
Stars and Stripes are meeting to discuss the turn of events in the Vietnam
War at the TET Cease Fire) .

p.s.: Sorry for the foul language, but it kind of sums up some of
the frustrations interpreting these changes.