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RE: Wood framed walls

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-----Original Message-----
From:	r marmaduke [SMTP:avi(--nospam--at)az.com]
Sent:	Monday, August 18, 1997 12:18 AM
To:	seaoc(--nospam--at)seaoc.org
Subject:	Re: Wood framed walls

Joe:

In my attempts to penetrate the residential market with the 
assistance
of zealous building officials, I've actually developed a 
spreadsheet
which automatically calculates panel stiffness based on UBC 
Standard
25.923, and then input all the shear wall lengths and offsets 
from
the origin to calculate center of force and center of 
resistance.

The usual assumption of 10% incidental torsion (what you call 
rotation)
has RARELY been valid for residential construction, where more 
often
the view wall is entirely or substantially glass, and the rear 
lower
wall is often (buried) concrete retaining wall. A more 
appropriate
value of incidental torsion for engineers not wanting to go to 
all
the trouble is 30%. It can make a huge difference on holddown 
design.

By the by, I've found nailing is not all that critical as UBC 
25.923
might imply. That is, as you increase nailing and the stiffness 
of
the individual panel, you increase it's contribution to load 
distribution,
and the solution diverges instead of converging (e.g. a wall 
with 2"
nail spacing will drag down more than it's share of lateral 
loading),
so it's rarely been necessary to re-iterate a solution more than 
once.

If you'd like a copy of the spreadsheet for wind and seismic 
lateral
analysis of plywood shear walls, let me know. I'm trying to get 
time
to put together a user's manual, if for no other reason so I can 
remember
how to use the program. Me, I'm trying to get entirely out of 
residential.


[Dennis S. Wish PE]  I agree with you. I, too, have written 
section 25.923 into my lateral analysis spreadsheet which was 
uploaded to the SEAOC web. I find it to be the best shearwall 
deflection design available today. I have not really noticed the 
effect of nailing, however, I found that the design of the end 
post (i.e., thickness) has a great deal to do with deflection by 
increasing the chord strength. You must remember, though that 
nailing is an important factor when the wall is sheathed on both 
sides - essentially making if much stiffer. The effect of 
nailing on the end post at both sides does make a noticeable 
difference in deflection.
In my spreadsheet, I use the values that Simpson publishes for 
holddown deflection which seem to be appropriate.
I have not considered the stiffness proportions of walls in one 
line. I hear this is the way to go, but am weary of the flexible 
diaphragm verses rigid diaphragm approach. I fear, too, that if 
the community allows the diaphragm to be designed as a rigid 
member, it will justify the design of open-front (cantilevered) 
type structures such as store-fronts and garages. Historically, 
these have proven to be poor designs in high wind and seismic 
zones.
Nice to know that someone else out there uses UBC Standard for 
deflection criteria. It may not be perfect, but it's the best we 
have.

Dennis Wish PE