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Rigid VS Flexible Wood Diaphragms

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Rigid VS Flexible Wood Diaphragms

The following may not be statements of facts. The 
This document is an inquiry and is open to discussion.

The nature of the tributary area method leaves the 
shear walls mostly balanced with the center of 
gravity. The exception may occur with a 
cantilevered diaphragm. 

Assumption: 
The shear wall flexibility is directly related to its 
capacity. An eight-foot wall will deflect a little less 
than half an inch when loaded to its capacity.




Torsion is a global affect, not local. A test for a rigid 
diaphragm may not be realistic if the deflection is 
considered in a short span between two interior 
walls. 

Torsion may increase loads at the most distant shear 
walls. However, the rigid diaphragm in most cases 
of residential homes will reduce shears in 
overloaded walls and share the excess with over 
designed walls.

Proprietary shear wall systems are recommended 
not to be mixed with standard shear walls in the 
same wall line. Then, for a rigid diaphragm, 
all or no walls shall be of a stiffer 
proprietary shear wall. If one models with  
the rigid diaphragm, the proprietary shear walls 
should not be needed. Deflections of longer 
cantilevered diaphragms can be justified. Over 
loaded walls will be unloaded by adjacent 
stronger and stiffer walls.

If the rigid diaphragm is only proven with 
an interior short span, the rigid model should not be 
used to conclude loads less than that of the tributary 
area method. It is probably physically correct to 
reduce loads if the test for a rigid diaphragm is for 
the most flexible span possible by ignoring the 
interior shear walls.

I propose creating a simplified design method that 
In theory would never violate a code procedure. 
Make a proof using the code to its most detail demands
and reducing the base shear with the high period 
response that the flexible shear walls allow. The 
zone four base shear, on firm ground, is about a 30% 
reduction. The proof is then attached to your quick
seismic analysis for lateral distribution.

Attached to the last response to this subject was of 
a story of a site visit. Missing, is the proper 
response. What is the proper response when a shear 
wall is dangerously compromised by the contractor? 
Who do you communicate with and how. What is next 
if the dangerous condition is not abated? What to 
do if you never get the opportunity to determine the 
consequence of your report.