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I have the following inquiry
Comments please, SEND TO 
UBC 1997 section 1630.6 and
UBC 1994 section 1628.5 requires a diaphragm to 
be analyzed as flexible when the deformation of the 
diaphragm is more than 2 times the average story 
drift of the associated story.  

If deformation was less than 2 times..., could it be
analyzed as flexible?

There is no wood house exclusion directly made in 
the code.  Most residential wood frames have rigid 
diaphragms per code. 

For a wood diaphragm that is not blocked, the 
deflection equation is missing in the Standards.  
Does this mean that the rigid diaphragm rule 
excludes unblocked diaphragms?  

Many code sections require the use of equations 
that are not given.  An adjustment factor could be 
made to the equations for the blocked diaphragm 
equations found in the Standards.  

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. 

Code sections are given with deflection equations 
for some plywood diaphragm cases and not others. 
Does an excluded unblocked diaphragm equation 
of a family of included equations exclude the 
unblocked diaphragm case from the code section 

Does a building official have the power to add the 
exclusion of unblocked diaphragms for residential 
construction to the code section 1628.5?

Does a popular vote of engineers create a Standard 
of Care over the legally accepted Standard of Care, 
the code?  Legal cases exist where the answer is 

Are there wood analysis programs using the 1994 
code with a rigid diaphragm option? 

Wood diaphragm force distributions have been 
treated as being flexible in shear and rigid in 
flexure. Another method uses the envelope of two 
combinations of shear or flexure and rigid or 

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 the continuous beam 

3. The enveloping of case 1 and case 2: 
Enveloping case 1 and 2 is found in 

4. 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.

5. Is an enveloping of all cases, or cases 
1 and 4, reasonable?