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RE: Redundancy/Reliablity Factor (ro) in Wood Frame BLDG

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My special thanks to the following contributors on my question:

Gerard Madden, SE
Suresh Acharya, S.E.
Mike Cochran S.E.
Bill Cain, S.E.
Ben Yousefi, SE
Steve Gordin SE

Your valuable comments are highly appreciated.

Casey (Khashayar) Hemmatyar, PE
Private email: <khemmatyar(--nospam--at)>
Southern California


From: Mlcse(--nospam--at) [mailto: Mlcse(--nospam--at)]
Sent: Friday, September 22, 2006 10:29 PM
To: seaint(--nospam--at)
Subject: Re: Redundancy/Reliablity Factor (ro) in Wood Frame BLDG




It looks like I steered you in the wrong direction, and it was a good thing to have posted on the web server.  I haven't worried to much about the rho value in the larger wood buildings since they typically have a lot of wood walls and when checked, rho is always very low (less than one).  In a building with just a few wood shear walls (maybe a single family residence), you may have a shear wall that is taking a large percentage of the shear force, and because of the lack of redundancy in the building (few shear walls) the rho penalty should be applied.


When we spoke I was thinking about the lw/10 limit which has not been applied to the wood shear walls (SEAOC white paper that Ben Youseffi listed in his e-mail response), but was intended for the concrete and masonry walls.  I don't think any building official has been enforcing the lw/10 limit on wood walls...but don't know for a fact.


I would provide the rho factor if asked, and hopefully demonstrate that it is very low


I don't know what is happening in the IBC as of yet...haven't look into it there yet.


I apologize for steering you in the wrong direction..


Mike Cochran S.E.



-----Original Message-----
From: Suresh Acharya [mailto:SAcharya(--nospam--at)]
Sent: Friday, September 22, 2006 5:07 PM
To: seaint(--nospam--at)
Subject: Re: Redundancy/Reliablity Factor (ro) in Wood Frame BLDG


The redundancy/reliability factor is almost always less than 1.0 for

typical residential buildings unless you have a very scary-looking

layout, or the area is huge.

If your spreadsheet is not set up to calculate this factor, you could

just do one or two lines of hand calculations for the critical shear

walls and show that the factor will never be greater than 1.0. If it is

much greater than 1.0, then you should really be scared now than later

(think about the quality of construction around here!!!)


Suresh Acharya, S.E.