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Re: Residential Flexible/Rigid Diaphragm Analysis

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Dennis -

The following is clipped from my earlier post regarding this issue in October
of last year.  I have still yet to find a good rationale for the proposed
rigid diaphragm approach to wood buildings.  As an added factor, consider that
the 1997 UBC raises the seismic design force from V=0.183W in the 1994 UBC
(for worst case factors and Rw=6) to V=0.26W in the 1997 UBC (also worst case
factors)!  This is a 40% increase in load.  Couple this with the 25% reduction
in plywood shear wall values following Northridge, and we have almost doubled
our factor of safety in 5 years!  So let's not get too crazy with the
analysis, too.

Bruce Resnick, SE
Parker Resnick Str. Eng.

>From my October post:

<< 2. Rigid Diaphram and shearwall stiffness analysis in residential
 Construction - The 1997 UBC (and proposed Los Angeles Regional Codes)
 requires a diaphragm and shearwall deflection analysis.>> 

THIS IS A VERY BAD IDEA in my opinion, of course. Reasons?

A)  Typically, the weak link in a plywood building is the short wall.  Using a
rigid diaphragm analysis, this wall gets even less load and will now be even
weaker!  Our fancy analysis will help to create a soft story in the building!!

B)  The code prohibits (or at least frowns on) rotation in wood diaphragms.
This is justified because rotating wood diaphragms tend to perform poorly (see
Northridge Meadows).  The rigid analysis encourages rotation.

C)  Wood buildings are not like commercial steel and concrete buildings where
the architectural finishes have little effect on the structural system.  In
wood buildings, the drywall and stucco have an enormouse effect on the
stiffness of the walls, at least for the first few cycles of the earthquake.
Where does that go in the model?  The same is true for the ceilings and the
floors.  Is your floor diaphragm stiffer with hardwood or tile or carpet over
your plywood?  You have to think it matters.  Do we model it?  What if the
owner changes it?

D)  Wood buildings (residential) tend to get many design changes over their
life and even the original construction period.  Does the addition of a new
opening in one wall during construction, which weakens the wall and
redistributes forces to the other walls mean that I have to increase the
nailing and HD's for those walls?

E)  Wood buildings are not always built per plan.  Whereas if I specify a
W12x50 column, I will usually get it, the same is not true in wood.  I may
specify nailing at 6" o.c, and get 4".  I may specify 1/2" roof plywood and
get 5/8".  This is impossible to know and to model.

F)  How do all the "nonshear" walls in the building effect the response?

G)  Conclusion:  We must differentiate between Analysis and Design!!  While
rigid diaphragm analysis may be closer to reality the limitations in the
modeling assumptions make it a fruitless task.  And why are we doing this
anyway?  Do well designed and detailed wood buildings under the current code
have severe structural problems in an earthquake?  Let's not be afraid to jump
off the analysis horse and focus on the design issues.