Subject: Re: Seismic Upgrade ..... Appeal to those who created the code
From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Thu, 13 May 1999 16:33:40 -0700
[My comments in rebuttal, correction, and complaint are added at
numbered intervals, 1 through 6, in this earlier, Ron Hamburger-acclaimed
message by Ali Sadre. It makes for a long, long posting, but there's no
alternative way to do it-- sorry. Some sorry conclusions, too. Sorry for my
SE Association in particular. -- C.G.]
At 01:59 AM 5/12/99 EDT, Ali Sadre, SE, SEAOC Seismology Committee Past
Chair [in recent times-- 1996-97] wrote:
*** (1) ***
>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].
[C.G.] My version of this history is quite different, and suggests a
much different spin on this matter with respect to wood diaphragms on wood
buildings and the flexibility analysis supposedly required now.
I was on Central Section of SEAOC's Seismology Committee for the
entire time the 1988 SEAOC "Blue Book"/ 1988 UBC Seismic Provisions were
written and debated, 1981-87. Additionally, I was on the State SEAOC
Seismology Committee in 1985-86 when the "flexible diaphragm" definition,
and what it was to be used for, was introduced by Ed Zacher in the summer of
1986 as Agenda item 86-4-11-B.1. It was adopted right away, which is
significant to this story.
Ed Zacher's diaphragm flexibility offerings became parts of Sections
1E5 and 1E6 in the 1988 SEAOC Recommended Lateral Force Requirements (the
"Blue Book") and Section 2312(e)5 and 6 in the 1988 UBC. They continue as
Sections 1630.6 and 1630.7 in the 1997 UBC, without substantive change in
language, but with a subtle relocation in 1994 of the flexibility definition
from the second to the first of these two subdivisions. (The effect of this
relocation is discussed below.)
Significantly, Blue Book Chapter 1 was titled "General Requirements
for the Design and Construction of Earthquake Resistive Structures." And its
Section 1E was titled "Minimum Lateral Forces and Related Effects." Nothing
in these parts of the code purported to distinguish between one type of
building material and another. Construction materials and their
peculiarities were dealt with in later chapters, and by different authors
than those who did Chapter 1.
Subsections 5 and 6 in both 1988 documents deal with "Horizontal
Distribution of Shear", and "Horizontal Torsional Moments", respectively. It
is clear from Ed's draft, and just as clear in the context of these two Blue
Book subsections: The concern regarding "flexible" and "NOT flexible" was
for multi-story buildings, and for the effects of horizontal eccentricities
of loading, and of horizontal eccentricities of resistance from one story
level to the next. It was separate from distribution of horizontal shear
among resisting elements "in proportion" to their rigidities, while merely
"considering" rigidity of the diaphragm, for which a provision was already
established in the draft 1E5 three years earlier.
Anyone who was involved in the rewrite of the Blue Book for 1988
knows that the "model building" in mind for virtually every provision that
came up, especially in the "General" Section 1, was an office-type building
of at least three floors and more typically five to twenty floors.
Residential woodframe construction was not under consideration at all. Period.
At the October 1986 SEAOC Seismology Committee meeting at the
Convention, Chairman-to-be Allan Porusch joked privately to me that "We
shouldn't allow this code to be used for anything over 15 stories." I
replied back, to his obvious understanding and agreement, "No, we shouldn't
allow it for anything under three stories."
***** (2) ******
[Ali Sadre continues:]
>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.
[C.G.] Well, sorry, but there wasn't any argument from Allan or
anybody else about residential construction features such as these, not at
SEAOC Seismology in connection with Ed Zacher's flexible diaphragm
definition and what happens where diaphragms are "not flexible", as Ed
called it. Nobody talked about residential construction. It simply was not
of concern, except a very little bit for Chapters 5 and 6 of the Blue Book
--"Wood Construction", and "Other Material" (like Stucco and gypsum drywall
board) respectively-- that were handled by Central Section of SEAOC's
committee on which I served throughout. Residential woodframe was no more of
a concern at the time to the SEAOC Seismology Committee than was radio
towers or highway bridges. Allan Porusch was occupied in quantifying
vertical and horizontal irregularities for major buildings in Sect 1D; that
was his baby.
****** (3) ********
[Ali Sadre resumes:]
>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.
[C.G.] Three things: First, if is desired to interpret something that
really hasn't changed, why not look first to the 1988 Blue Book Commentary
that accompanied the flexible diaphragm provisions as they first appeared,
and when the intent was still fresh: [note no mention of wood diaphragms or
buildings in commentary on Chap 1]
1988 Commentary for 1E5, Horizontal Distribution of Shear:
"Consideration should be given in the modeling of structures where
vertical bracing elements are tall enough to have significant flexural
deformations. The rigidity of these elements can be substantially different
from rigidities calculated by the simple story-by-story method. Also,
considerable differences in the distribution of wall shears can occur when
the diaphragm deformations have been considered as compared to results for a
fully rigid diaphragm.
"The location and distribution of all of the weights required to be
considered for earthquake motion response cannot be determined with complete
certainty. The requirement for displacement of the mass at each level was
introduced to account for this condition, along with others listed in
commentary Section 1E6. The requirement applies to all structures, whether
the diaphragms are flexible or rigid." [Note: as of the 1994 UBC, this five
percent accidental eccentricity need not be done where the diaphragm is
"flexible". That is the effect of the revised location of the flexibility
definition mentioned above.]
1988 Commentary for 1E6, Horizontal Torsional Moments:
"When the diaphragm deformation is more than two times the
associated story drift then the diaphragm is considered to be flexible. Here
the term "flexible" implies that the diaphragm may be modeled as a simple
beam between vertical resisting elements. When the diaphragm deformation is
less than or equal to twice the story drift, then the provisions of this
section apply. In most cases, the diaphragm may be modeled as "fully rigid"
without in-plane deformability. However, there are structural configurations
such as: vertical resisting elements having large differences in stiffness
or offsets between stories, and diaphragms with irregular shapes and/or
openings, where the designer should investigate the effects of diaphragm
deformability. The use of the most critical results obtained from the "fully
rigid" and the "flexible" models would be acceptable." [Note the
permissive "may" on modeling as a series of simple beams a "flexible"
diaphragm crossing multiple supports, which appears to pertain more to the
previous section, 1E5, distribution, than 1E6, torsional moment. And note
the imprecision of the whole exercise that's implied by the permissive last
Second, what is meant by, "both rigid and flexible diaphragm
analysis *as required* by the UBC." ?? As of the 1988 edition, and
continuing in the 1997 edition, there are prescriptive burdens imposed by
code ONLY if the diaphragm is "NOT flexible". The code requires nothing
special if the diaphragm IS "flexible" as defined. And the Code only
requires a "consideration" of relative rigidities in assigning horizontal
distribution of shear, without requiring that EITHER rigid or flexible be
decided on for the diaphragm. The code doesn't use the word "rigid" at all.
Is the Design Manual going to create a code requirement that the code itself
hasn't got, and is it going to explain words the code doesn't use?
Third, this idea of "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."
Is the manual going to say WITH APPROVAL that the current state of
practice is to ignore a code provision that you also say the code requires
be used, and has required since the 1988 edition, and with respect to which
"the 1997 UBC has not changed the state of practice"?? Where is '97 UBC
code language found that exempts flexible and NON-flexible determinations
for engineered wood diaphragms in engineered light frame construction? How
do you figure SEAOC Seismology Committee can let a code provision remain in
code that isn't followed and that you could have had removed, and "cure"
that problem with a Design Manual that has no regulatory effect?
**** (4) ****
[Ali Sadre resumes:]
>The above clarification should support our members againt any possible threat
>of future litigation....
[C.G.] How is this known? Has the Seismology Committee retained
competent legal counsel to verify this supposition? My own experiences
defending engineers in woodframe residential disputes suggests that
practitioners' risks of facing legal action will be compounded, and their
prospects for defense impaired, if these "clarifications" are relied on.
**** (5) *****
[A.S. carries on]
>......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.
[C.G.] These points suggest that the code should have deleted
outright the diaphragm flexible/ non-flexible determination requirement for
woodframe designs, or alternatively eliminated some of those other
burdensome requirements. There is a confession of both unnecessary
conservatism and imposition of unwarranted burdens on the designer, plus
inattention to design practice realities as to woodframe construction, if
these points are what Seismology Committee actually believes.
**** (6) ****
[and Ali Sadre concludes]
>I hope this short write-up sheds some light on the issue.
[C.G.] Well it does shed some light, but not the kind I find at all
reassuring. There's the screwy notions as to what the Code the Committee
wrote actually says, never mind the plain meaning of its words, and about
how a mere user's manual can add code provisions that aren't in code, and
negate code provisions that are. It is a bit dim how the manual will act to
ward off litigation. And there is light shined on surplus conservatism via
multiple and confusing burdens, rather than a simple fat factor anyone can
get right and design to.
But the really dismaying part of Mr Sadre's account is his attempt
to mollify unhappy code users with a facile and convenient version of
history that won't square with what this Committee participant knows from
being there at the time.
But what can I know? I have a crippling handicap: Back in 1986 when
the flexible diaphragm determination provision happened, and I was on the
SEAOC Committee, I was also on the SEAOC Board of Directors, by virtue of
having just been president of Central Section of SEAOC, an elected position.
Ron Gallagher of the Seismology Committee recently wrote of SEAOC Directors:
"Board Members come up [from] the ranks from the local associations,
and many have not served on, or been involved with, the Seismology
Committee. Consequently, I feel the Board as a group does not fully
comprehend the amount of work the Committee does."
Well, perhaps not. But as SE's, they sure were Committee product
USERS by then. Doesn't that count for something? And what of the other way
around? Who charges this unelected Committee of volunteer regulatory
enthusiasts with representing the interests of the SEAOC membership? Who
holds them to a decent standard of due care about what they adopt and foist
Back to this version of history Mr. Sadre disarms us with: Where
was he at the time the events he tells of happened? He says, as though he
was there in the thick of things, "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
I wonder...Besides having Ed Zacher's 1986-accepted draft in hand, I
have rosters of all four SEAOC Sections from that whole era. I don't find
Mr. Sadre a member of any seismology committee in 1986. I don't even find
him a member of SEAOC until a listing appeared in the 1987-88 SEAOSD Roster.
Nothing anywhere for 1986-87 and earlier, when the decisions in question
Very interesting. Is there something chronically bizarre in how
the top levels of SEAOC Seismology Committee and other SEAOC officialdom
represent themselves and their doings to the rest of us these days, or did
Dennis Wish and myself and many more bewildered, incredulous list
contributors just fall down a rabbit hole in a dream?
Charles O. Greenlaw, SE Sacramento CA