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Rigid wood diaphragms, more to know

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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 
***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 

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 
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 
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.