Subject: Re: Seismic Upgrade ..... Appeal to those who created the code
Date: Wed, 12 May 1999 01:59:16 EDT
The topic of flexible versus rigid wood diaphragm analysis in light frame
construction, which has initiated quite an interesting discussion, may merit
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
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.
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