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RE: Rigid wood diaphragms, more to know

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Who is this "plywood official?"  I like to know who is offering advice!


Bill Cain, SE
Oakland, CA


	-----Original Message-----
	From:	merrick group [SMTP:merrickgroup(--nospam--at)compuserve.com]
	Sent:	Tuesday, March 23, 1999 18:32 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Rigid wood diaphragms, more to know

	UBC earthquake rigid wood diaphragm

	A plywood official reported that to analytically test for a
rigid 
	diaphragm (94UBC, 1628.5 third paragraph) there needs to be an 
	equation for unblocked diaphragms. Therer is no equation for
such. He 
	believes it is not then  intended for unblocked diaphragms to be

	considered rigid. He then hints of existing fudge factors for
blocked 
	diaphragm equations to represent the unblocked.

	Possible factors are.
	******Increase the deflection six times for loads parallel to
unblocked 
	edges.  
	***Increase the deflection three times for loads parallel to the
joists. 

	Most wood floors are glued and are most likely rigid. No fudge
factors 
	For this one. 

	The method of tributary area reduces the likely result of highly

	eccentric shearwall systems. Tributary area method (most of the
time) 
	will control the seismic design. But does allow larger
cantilevered 
	diaphragms to be justified.

	The code only suggests a rigid diaphragm system for seismic and
not 
	wind forces. But it sure creates a great back up to justify
reduced 
	deflections to extend  cantivered diaphragms! (do not assume
rigid 
	please) 

	This might free the need for the cantilevered columns.

	Wood diaphragm force distributions by tributary area, are
modeled as 
	being flexible in shear and rigid in flexure. Following is a
summary of 
	models that I have heard of.

	1. Flexible shear, rigid flexure: This is the tributary area 
	method
	2. Rigid shear, flexible flexure: This is where interior shear 
	walls are increased per a continuous beam model
	3. Rigid shear, rigid flexure: By demand of the code section 
	of 1628.5.  This is the method used for simple concrete 
	structures, and seems to be the recommended approach 
	per code.
	4. All of the above enveloped has yet to be recommended as 
	enveloped.
	5. The enveloping of case 1 and case 2: is found in ATC-4

	I have yet to find a program intended for 1994 UBC wood shear
walls, 
	with the capability of rigid diaphragm checks. I know of one 
	programmer who is now trying to implement the issue as a
secondary 
	check. Evidently he got enough calls.

	The above is mearly a group of expressed ideas and not my
opinions or 
	recommendations as to how to design. I hope that the concepts
can be 
	intertained and responed to. Of course with the same limit of
liability.