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Re: Seismic Upgrade ..... Appeal to those who created the code

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Dennis,

The topic of flexible versus rigid wood diaphragm analysis in light frame 
construction, which has initiated quite an interesting discussion, may merit 
some history. 

As early as 1985, when the State Seismology Committee was in the process of 
updating the Blue Book Provisions for the 1998 UBC, there were two views 
within SEAOC. 

One view, spearheaded by the Northern Section and led by Ed Zacher, believed 
that wood diaphragms depending on their finishes and application may have to 
be treated as rigid. [For example, consider 1/2" glued wood flooring over 
5/8" plywood sheathing, or tile with 1/2" mortar over 3/4" T&G, particulary 
when the floor is being glued and nailed/or screwed to the joists with 
frequent interior partitions below]. 

The other view, from the South, led by Allen Porsch, argued that the very 
fact that you have frequent interior partitions which are unaccounted for in 
design (e.g., closet, bathroom, bedroom wall, etc.) will dampen the response 
of the structure and provide added toughness. This should more than 
compensate for any amplified shear demand due to rigid diaphragm analysis.

While, the majority of the Committee did not necessarily disagree with the 
crux of Ed's argument, most of us felt that the introduction of this 
Provision into the 1988 UBC and the Blue Book would not change the state of 
practice with respect to this type of costruction.  During the last dozen 
years or so this issue has come up from time to time in litigation cases.  

Now that the Seismic Design Manual (Volume II) is being preparared, for the 
first time, we have to state the strict interpretation of the code on this 
issue.  So the examples, when published in summer, will present both rigid 
and flexible diaphragm analysis as required by the UBC. 

However,  we have discussed providing a statement in the Design Manual to 
indicate that the current state of practice is not to analyze untopped wood 
diaphragms in light frame construction for Rigid Diaphragm Provisions, unless 
the engineer of record choses to do so. This issue will also be referenced to 
the Blue Book and discussed in the 2000 Blue Book Commentary on Flexible 
Diaphragm and Wood Provisions.

This will be a simple statement of fact and calrification that the 1997 UBC 
has not changed the state of practice in this respect, nor did it intend to.  

The above clarification should support our members againt any possible threat 
of future litigation and minimize efforts to perform cumbersom rigid 
diaphragm analysis for conservatism that is more than compensated for by 
other factors built into the 97 UBC. Among these factors (for this type of 
construction) are: near source Provisions, aspect ratio limitation of 2:1 for 
shear walls using structural wood panels or particle boards, new sill plate 
anchorage requirements as well as calculation of redundancy or rho factor. 

I hope this short write-up sheds some light on the issue.

At this point, I want to take a moment and particulary thank you, Shafat, 
Mark and many other dedicated members of our EC Committee who have devoted so 
much of your time and have put forth such an incredible effort to keep this 
list server as a great resource for all of our members and other interested 
colleagues across the globe during the last 5-years. Keep up the good work.

P.S.
Either myself or Saif Hussain the current Chair of the Seismology Committee 
will keep our members abreast of any further developments via the server.


Ali Sadre, SE

SEAOC Seismology Committee, Past Chair