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Re: Rigid plywood diaphragms

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In a message dated 11/20/98 3:03:10 AM EST, itsekson(--nospam--at) writes:

<< Thanks to everybody who answered to my post especially to Charles Greenlaw.
 I appreciate opinions of all that responded to my post on the applicability
 of the rigid plywood diaphragm design.
 But what I am trying to figure out is how can I do the rigid analysis if I
 decide to, without any increase in my fees or with the minimum effect on my
 If this is all I am required to do in order to avoid any future litigation,
 I am willing to spend a couple of hours doing the comparative analysis of
 flexible and rigid diaphragm distribution.  If this requires an estimate of
 rigidity for all the sheetrocked partitions and other structural and non
 structural elements, then forget it.  As Lynn Howard said, the plywood
 diaphragms and properly detailed plywood shear wall performed well in the
 past earthquakes.  So why fix what is not broken.
 Sasha Itsekson

I am not sure it is completely appropriate, but it might get you rather close
depending on how you handle the holdown assembly.  The code provides allowable
shear design values for wood sheathed walls with nails at a certain spacing
(6", 4", 3", etc.)   For a given wall length, using a certain nail spacing,
and assuming a certain load in the wall in terms of pounds per square foot (so
you can calculate nail slip), you can then calculate the shear wall
deflection.  If you know the deflection, you can calculate the rigidity.  If
you omit the holdown portion of the shear wall deflection you might be able to
directly use the walls rated capacity (pounds/ft) times the wall length to
represent the wall stiffness.

The problem as I see it is that for the short shear walls, less than 1:1
aspect ratio, the holdown assembly (deflection of holdown body, oversized
holes in post, crushing of sill, shrinkage of lumber) is a major contributing
factor to the total shear wall deflection.  For multiple shear walls in the
same line, you will not obtain the same  equal distribution of shear to all
walls (say all walls have 300 pounds per foot) until all the slack in the
holdown assembly, as described above, has been removed by the wall uplifting
and reaching the capacity of the holdown.   The even distribution also assumes
all walls along the same line have the same nail spacing.  By the time you
reach the holdown capacity, you  probably have exceeded the allowable code
drift limits and now possible damage control issues.

I think you would have to do the tributary analysis first, including holdowns
and calculating shear wall deflections.  Since you now have the shear wall
deflections, you can assign stiffness to each wall and then do a rigid
analysis.  This is a lot of work to obtain an envelope of design forces.  I
don't think you can do the rigid analysis first because of the holdown
assembly deflection problem.    If you can eliminate the holdown assembly
deflection issues,  then you could probably do the rigid analysis first.
Would you rerun your analysis if the forces in the shear wall exceed it's
rated capactiy that you assumed.

Using the shear wall unit capacity method for stiffness will probably be
closer to a complete rigid analysis when all the shear wall aspect ratios
exceed  1:2.  As the   aspect ratios get larger (1:1, 1.5:1, 2:1, etc) the
holdown becomes a very siginificant factor and the rigid analysis will become
most likely more inaccurate.  The other problem I see is that 

I would not include the nonshear wall elements in the analysis such as the
gypboard walls at this time.  I don't know if this would keep you out of
litigation, but I think it would address the fact that you at least considered
the possibly of a rigid diaphragm, especially if it governs the design of some
of the walls when compared to a tributary analysis.

Michael Cochran