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RE: IBC and Rho factor

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Dennis, please see my answers below:

-----Original Message-----
From: Structuralist [mailto:dennis.wish(--nospam--at)]
Sent: Wednesday, October 02, 2002 2:29 PM
To: seaint(--nospam--at)
Subject: RE: IBC and Rho factor

I'm no longer an SEA member so I won't be receiving the notes from this
presentation. What I want to ask is this:

1. The 20% increase that you are refering to is the difference between
2.5Wd/(1.4R) and 3.0Wd/(1.4R) I take it???


2. Don't you require V=3.0Wd/(1.4R) to be multiplied by a Rho of 1.5?


3. Do you consider plywood covered in lightweight concrete to be
flexible or rigid?

(BY) Yes. The concrete topping is not a continuous diaphragm, it is
discontinued by wall sill plates and there is no documentations that proves
it causes the plywood to act rigid.

4. Doesn't the code still require the engineer to calculate the
deflection of the diaphragm before assuming it is flexible and using the
Simplfied Static Method? This was my interpretation of the code as I
could not find any specific wording that stated the engineer may use
flexible diaprhagm analysis for a wood structure three stories or less
based on Simplified Static proceedures. My interpretation has always
been that the engineer must first calculate the diaprhagm deflection. Is
the assumption of flexible diaphram only the Tri-County's comprimise on
the issues?

(BY) No, if you comply with the simplified method provisions. For all other
cases you are correct. The new policy (amendment) that I was referring to
which will be adopted November 1 by most LA and Bay area jurisdictions, is
modeled after the new language in 2003 IBC and it reads as follows:

"1630.2.3.4	Horizontal Distribution.  Diaphragms constructed of untopped
steel decking or wood structural panels or similar light-frame construction
are permitted to be considered as flexible"

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