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Re: Residential Flexible/Rigid Diaphragm Analysis

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It is amazing how much attention this issue is getting.  From one who
participates in the code developement process and currently have several
residential projects under design, I am also juggling all the issues
associated with this "code improvement".

don't forget to divide 97 UBC base shear by 1.4 for working stress.

Bill Nelson

----- Original Message -----
From: <Parkerres(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Wednesday, March 31, 1999 7:50 AM
Subject: Re: Residential Flexible/Rigid Diaphragm Analysis

> Dennis -
> The following is clipped from my earlier post regarding this issue in
> 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
> 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
> factors)!  This is a 40% increase in load.  Couple this with the 25%
> in plywood shear wall values following Northridge, and we have almost
> 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
> weaker!  Our fancy analysis will help to create a soft story in the
> B)  The code prohibits (or at least frowns on) rotation in wood
> This is justified because rotating wood diaphragms tend to perform poorly
> Northridge Meadows).  The rigid analysis encourages rotation.
> C)  Wood buildings are not like commercial steel and concrete buildings
> the architectural finishes have little effect on the structural system.
> 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
> Where does that go in the model?  The same is true for the ceilings and
> floors.  Is your floor diaphragm stiffer with hardwood or tile or carpet
> 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
> life and even the original construction period.  Does the addition of a
> 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
> 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!!
> 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
> have severe structural problems in an earthquake?  Let's not be afraid to
> off the analysis horse and focus on the design issues.