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RE: Rigid vs. Flexible Diaphragm

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For reference, the ST-12 document can be found at:

http://www.icbolabc.org/graphics/pdf/st-12v1.pdf

Regards,

Bill Allen, S.E. (CA #2607)
ALLEN DESIGNS
Consulting Structural Engineers
Laguna Niguel, CA
http://www.AllenDesigns.com 
V (949) 365-5696
F (949) 249-2297

||-----Original Message-----
||From: Richard Flower [mailto:RLFlower(--nospam--at)worldnet.att.net]
||Sent: Tuesday, September 05, 2000 4:57 PM
||To: seaint(--nospam--at)seaint.org
||Subject: Re: Rigid vs. Flexible Diaphragm
||
||
||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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