Subject: Re: For Urgent Review and Comment - Blue Book Commentary
From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Mon, 24 May 1999 17:47:11 -0700
PART ONE of two. Continuation separately posted.
It is clear from Ali Sadre's 5/22/99 gracious invitation to commenters, and
from his previous postings, that 97 UBC Code language and the Blue Book's
corresponding code text is not open to revision. The 97 UBC is already in
use as enforceable legal code. Thus there is no use replying in "wish list"
fashion to what it ought to say.
What is open to possible revision is ONE paragraph of a series of paragraphs
in the Commentary portion of the Blue Book. These Commentary paragraphs are
for explanation and information to assist code users in understanding the code.
The Commentary cannot change the code, it cannot make regretted code
language have no effect, and it cannot add requirements on users that code
language itself does not impose. It is essential to understand this. The
code says what it says, for better or worse, until a new code is officially
adopted as a replacement. Weep as necessary, and dry your tears with pages
from the Commentary, because it can't change what the code says, or
otherwise fix anything.
Most of the recent concern seems to be over the applicability of the
definition of "flexible diaphragm" (that first appeared in 1986 for the 1988
Blue Book and 1988UBC) to "Horizontal Distribution of Shear" among many
shear walls, frames, etc., especially when the diaphragm is of wood sheathing.
I don't really care what the Commentary says, because it isn't enforceable
on me, and I have my own answers to the recent concerns. My answers are
consistent with UBC code language, consistent with code formulation history,
and consistent with past Blue Book Commentary that was contemporaneous with
the first appearances of the code language in question.
Well, I do care, because others may rely on this new replacement Commentary
to misinterpret UBC text and cause a nuisance to me in getting past plan
checks, and it can be disastrous when an "expert" in a legal proceeding
against me uses badly written Commentary to wrongly say I violated Code or
didn't meet a supposed standard of care.
Committee members who have thin skins might think of these consequences to
others when issuing Commentary that is vague, unclear, inconsistent with
Code text, and naive of the text's origins.
Returning to the current concern, as I mentioned above and in an earlier
posting on 5-13, the Flexible Diaphragm definition was introduced by Ed
Zacher on behalf of SEAONC in 1986, TO MODIFY a particular ONE of the 1988
Blue Book Section 1E provisions accepted by SEAOC Seismology in 1983. The
1983 1E provisions were introduced by David L. Messinger, then representing
SEAONC, and were accompanied by a very good commentary paper that I have.
David Messinger's commentary notes that the 1988 Blue Book under preparation
in 1983 had as a prime purpose "to follow a modified ATC-3 format." The
primary modification was to retain "working strength" concepts. Sect 1E
covered the familiar "equivalent static lateral force procedure at working
stress levels." Mr Messinger compared the 1982 UBC to ATC-3 to 1988 Blue
Book draft as of 1983, and revealed that,
The 1988 Blue Book as of 1983 (and as later adopted into the 1988 UBC) DID
NOT CHANGE the 1982 UBC provision that, for horizontal distribution of
shear, horizontal forces are distributed out of ANY diaphragm "to the
various elements of the lateral force resisting system, CONSIDERING the
rigidity of the diaphragm."
This same language appears, without substantive change, from the 1970 UBC at
Sect 2314(e), through the current, 1997 UBC at Sect 1630.6, first paragraph.
Look it up for yourself.
The 1973 Blue Book Commentary covers the origins of this "distribution...
considering relative rigidity of the diaphragm" provision that is so
misunderstood in 1999 now:
"Some thought was given to requiring design of vertical resisting
elements for lateral forces tributary to the element, when the diaphragm is
a "flexible" type. This has been done in some codes. However it was felt
that consideration of the stiffness of the diaphragm in relation to the
stiffness of the vertical elements is preferable to assigning diaphragms
arbitrarily to "rigid" and "flexible" categories."
[This was written many years before the 1986 appearance of Ed Zacher's
definition of "flexible", which ONLY applied, AND STILL ONLY APPLIES, to
whether the 5% accidental shift of center of mass for distribution and
torsion purposes need be calculated-- if flexible, no. It was relief of a
burden, not the addition of one, that the 1986 "flexible diaphragm"
definition served. Unfortunately, obtuse wordsmithing in 1986 results now in
a grotesque mutation of that intent, which acts to improperly negate the
meaning of "considering".]
The 1973 Blue Book Commentary continues: "The goal in distributing shears
is to make a realistic assignment of shears to the resisting elements,
conforming to the action of the structure. ... It is not the purpose here to
go into details of rigidity determinations...these are difficult to evaluate
by any practicable method in complex, multi storied buildings and like
structures. Diaphragm stiffness and strengths must be considered concurrent
with vertical element stiffness. Approximations that must be made in
practical solutions introduce varying degrees of error. [etc.]"
END of PART ONE. see continuation separately posted