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RE: Rigid plywood diaphragms

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Thank you Charles Greenlaw, Michael Cochran, Ernie Natividad and Dave Puskas
for your detailed and insightful responses.

To tell you the truth, I am not that much worried about the possible
litigation.  But I do understand the merit behind the rigid or rather
semirigid behavior of wood structures with closely spaced shear walls.  As
somebody said the building does what it wants under the loading. Our job is
to use our engineering judgement, available research and codes to safely
design them.

I will in fact use tributary area method as the base for my horizontal shear
distribution (it does not take that much time and it seems to be still
working).  It should not be that difficult to put together the spreadsheet
using UBC Standards formulas for shear wall deflection either.  I am
wondering though if it would give me the realistic shear distribution ( I
don't think so)  and would it be any closer to the actual performance of the
system than the customary method above.

Let's not forget that we are talking about the RELATIVE RIGIDITY of shear
walls along different lines of resistance.  For the sake of the argument
let's look at the direction when framing is parallel to shear walls.  So the
overturning moment and boundary member forces are the function of the unit
shear times the height of the wall.  I can drop out several components of
the total deflection from my relative rigidity calcs that would cancel out.
These elements may include deflection due to oversized holes in holdowns
(which in fact are 2 to 3 times more than the holdown deformations itself),
plywood shear deformations, slip in sill bolts.

I liked Charles's suggestion to use the surplus capacity rather than the
tabulated capacity of the shear wall.  If we only could design all of our
walls stressed to the same percentage of the allowable... (:<).  But that's
not going to happen.

Well, it's late, and I don't have any answers yet, only thoughts.  Anybody


Sasha Itsekson

-----Original Message-----
From: Mlcse(--nospam--at) [mailto:Mlcse(--nospam--at)]
Sent: Friday, November 20, 1998 11:08 PM
To: seaint(--nospam--at)
Subject: Re: Rigid plywood diaphragms

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



 Sasha Itsekson

I am not sure it is completely appropriate, but it might get you rather
depending on how you handle the holdown assembly.  The code provides
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
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
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
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
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
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
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
the possibly of a rigid diaphragm, especially if it governs the design of
of the walls when compared to a tributary analysis.

Michael Cochran