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wood diaphragm revolt.

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Rigid Diaphragms 
The code is the law. The standard of care is as a minimum, the code. There
are only a few legal avenues to changing the law. So lets stick to the code
and make it work for us.

There are differences in results from flexible vs. rigid diaphragm
analysis. 20 cases of designs that are not affected by a rigid diaphragm do
not change the physics of the one that is affected. 

As quickly as one can create a system sensitive to a rigid diaphragm, one
could develop a generic set of rules of enveloped cases to prove when a
rigid diaphragm analysis is not needed.

If all walls are of similar size, strength and spacing, a full rigid
diaphragm analysis may have no affect,, or a limited affect. Why not have
possible theorems of rigid diaphragm results not being able to exceed
maximum factors of results for a flexible diaphragm.

If one doesn't have stiffness information for an unblock diaphragm then
envelope the results from a range of possible values. Or do testing.

 I read one interpretation that if there are no stiffness values for a wood
diaphragm then the rigid diaphragm law does not apply. If my car does not
have a speedometer then I am not speeding?

The best suggestion has been to consider the non-linear behavior of plywood
shear walls. They are much more ductile than other systems. Their ductile
resisting force is far more constant than in other systems (steel and
concrete).  A plywood shear wall maintains a constant resistance through
deflections long after reaching its load capacity. 

Consider an over loaded shear wall. Let it further deflect and prove that
it will begin to share that load with neighboring walls. Demonstrate that
the deflections do not exceed limits.

One might adjust an analysis by removing an overstressed wall and replace
it with a constant force equal to that wall's capacity with a vector
opposing the seismic direction of loading. Then check the deflections and
the  loads added to the adjacent walls and diaphragms

There are cases where rigid diaphragms change the flexible diaphragm
results. A front narrow wall at a window is too flexible because of the HDs
or the wall is too stiff because of theuse of the new proprietary systems.
This changes the assumed flexible diaphragm load path. It will change the
demands on the floor diaphragm. 

The code does allow reduction of base shear due to high periods. Wood
systems have very-very high periods.

There is good reason why there is little damage occurring to wood systems
designed by the flexible diaphragm method. Unfortunately those structures
most likely don't behave as flexible diaphragms. (what  if the contractor
glues the floors to the joists) It's the other physical properties
mentioned above that could allow the flexible diaphragm model, with certain
limitations, to represent the rigidly behaving diaphragms.


David Merrick, SE
merrickgroup(--nospam--at)compuserve.com