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Re: Rigid vs. Flexible Diaphragm

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Bill:

No, it is not a "stupid question." I am in constant battle with the city of
Los Angeles, which is among several local jurisdictions that is presently
trying to resolve this all important question about horizontal diaphragms.
It is true that we cannot mearly "assume" that a given diaphragm is either
close to truly flexible or close to truly rigid. The actual behavior of
diaphragms is dependant upon so many variables that it will prove to be a
formidable task to come to a more knowledgeable understanding of actual
diaphragm action.

Refer to the Information Bulletin Number ST-12 (Structural) Interim
Alternate Method for Distribution of Lateral Forces in Wood Frame Buildings,
Assuming Flexible Horizontal Diaphragms, Issued by the City of Los Angeles
on January 27, 2000, revised May 15, 2000. This document has also been
adopted by LARUCP on May 25, 2000.

This document only addresses the seismic question; wind analysis is not
addressed here in this document. Sorry 'bout that, chief!

-Richard L. Flower, P. E.
RLFlower(--nospam--at)worldnet.att.net

-----Original Message-----
From: Bill Polhemus <bpolhem(--nospam--at)swbell.net>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>; aec-residential(--nospam--at)polhemus.cc
<aec-residential(--nospam--at)polhemus.cc>
Date: Thursday, August 31, 2000 7:47 PM
Subject: Rigid vs. Flexible Diaphragm


>Okay, I'm going to have to finally come clean, and admit my abstruse
ignorance.
>It pains me to have to do so, but unless I ask stupid questions--and
thereby
>expose myself as stupid--I know I'm not going to get wise to the answer. So
here
>goes.
>
>My understanding of "rigid" vs. "flexible" diaphragm design is that the
former
>is appropriate for seismic design, where the dynamic nature of the
force--it's
>sudden introduction and just-as-sudden change in magnitude and direction
(or, in
>short, it's impulsive characteristics)--means that the actual flexibility
of the
>diaphragm itself and its connection to other elements is meaningless. The
forces
>are introduced solely on the basis of the relative stiffness of the
shearwalls.
>
>The latter, however, is used, I might say "exclusively", for lateral loads
such
>as wind, which are characteristically more a "steady state" force whose
>influence due to impulse is much less. In that case, the "flexible"
diaphragm
>imparts its force equally to the active shearwalls.
>
>Now, did I get that right, or did I just display irrevocable ignorance?
OR...is
>it ignorance, but their is hope?
>
>Please, enlighten me.
>
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