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RE: 94 UBC and Rigid Plywood Diaphragms??????

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The pertinent section of the 1997 UBC reads:

"Section 1930.6 Horizontal Distribution of Shear.  The design story shear,
Vx, in any story is the sum of the forces Ft and Fx above that story.  Vx
shall be distributed to the various elements of the vertical
lateral-force-resisting system in proportion to their rigidities,
considering the rigidity of the diaphragm.  Se Section 1633.2 for rigid
elements that are not intended to be part of the lateral-force-resisting

Where diaphragms are not flexible, the mass at each level shall be assumed
to be displaced from the calculated center of mass in each direction a
distance equal to 5 percent of the building dimension at that level
perpendicular to the direction of the force under consideration.  The effect
of this displacement on the story shear distribution shall be considered.

Diaphragms shall be considered flexible for the purposes of distribution of
story shear and torsional moment when the maximum lateral deformation is
more than two times the average story drift of the associated story. This
may be determined by comparing the midpoint in-plane deflection of the
diaphragm itself under lateral load with the story drift of adjoining
vertical-resisting elements under equivalent tributary lateral load."
Bill Cain, SE
Oakland, CA

	-----Original Message-----
	From:	vicpeng [SMTP:vicpeng(--nospam--at)]
	Sent:	Monday, May 03, 1999 10:15 AM
	To:	seaint(--nospam--at)
	Subject:	Re: 94 UBC and Rigid Plywood Diaphragms??????

	I don't have a copy of the UBC.  Could someone send a copy of that
definition of rigid vs. flexible diaphragm?

	Thor Tandy  P.Eng  MCSCE
	Victoria BC
	vicpeng(--nospam--at) <mailto:vicpeng(--nospam--at)> 

		-----Original Message-----
		From: Robert J Bossi, PE < rjbossi(--nospam--at)
<mailto:rjbossi(--nospam--at)> >
		To: seaint(--nospam--at) <mailto:seaint(--nospam--at)>  <
seaint(--nospam--at) <mailto:seaint(--nospam--at)> >
		Date: Monday, May 03, 1999 10:08 AM
		Subject: Re: 94 UBC and Rigid Plywood Diaphragms??????

		I was surprised to find that the current definition of
flexible versus rigid diaphragm first appeared in the 1988 UBC (2312(e)6 on
page 148).