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# RE: rigid diaphragm chord forces?

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: rigid diaphragm chord forces?
• From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
• Date: Tue, 26 Aug 2003 22:55:49 -0700

```Sounds like you have it correct with one point to add. You assume a
flexible diaphragm, but you need to prove this by calculating the
diaphragm deflection. If the wall mass is high - such as in a concrete
tilt-up or Masonry building, then the roof may calculate as a flexible
diaphragm, but to make a light-frame diaphragm into a flexible one, you
will need to encroach on the 4:1 aspect ratio of the diaphragm to force
excessive deflection - at the risk of trying to tie the perimeter (at
least on two sides) with shearwalls.
Not only are you required (in seismic zone 3 and 4 that I know of) to
check for flexible diaphragm, you are required to do so by comparing
wind and seismic in each orthogonal direction and then using the worst
case condition. Beyond this, you are required to compare the results in
each line of shear with that of the rigid analysis.
What the code does not tell you is that is up to your professional
judgment as to how you distribute the shear from flexible (wind and
seismic) along with the results of the rigid analysis.
You should consider the ICBC Seismic design Manuals - Volume 1 through 3
with Volume 2 as the most important in my opinion.

Dennis S. Wish, PE

-----Original Message-----
Sent: Tuesday, August 26, 2003 9:28 PM
To: seaint(--nospam--at)seaint.org
Subject: rigid diaphragm chord forces?

in flexible diaphragm design, i can see the deep beam analogy and the
resulting chord forces

however, in rigid diaphragm design, i thought the whole point was that
the diaphragm (concrete slab over metal deck) was assumed rigid, has a
center of mass, has a center of rigidity and is controlled by rotation.
this rotation transmits forces to the lateral resisting elements below
based on their relative rigidities in shear only.

have i missed the point here?  if i did can you provide a design
reference / document that shows how rigid diaphragms develop chord
forces?

tia
da

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