Subject: Re: Code Created Malpractice Opportunity-Rigid v.Flexible Diaphragm
From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Tue, 01 Jun 1999 03:16:26 -0700
At 04:54 PM 5/29/99 -0700, JOHN SHIPP wrote:
"A little homework and research can avoid a lot of misunderstanding. In any
event, abusive personal attacks is not appropriate or professional."
Thanks, John. I spent Memorial Day doing a little homework, like you
suggested. Frank McClure appreciates that you "stuck your neck out", and so
do I. Since abusive personal attacks aren't appropriate, how about a pie
being thrown? One for you and another for a list member who already
responded in agreement with your post on this topic. Hope the "flavor" of
attack is OK.
You say, of the SEAOC seismology committee "All proposed provisions are
discussed in depth with input from a broad spectrum of practicing engineers..."
Surely you know that the Seismology Committee had moved on from the 97 UBC
to the 2000 IBC several years ago. Your reply below is about seismology, and
presumably as SEAOC President your advice to all of us about Rigid vs.
Flexible Diaphragms is consistent with what this committee agreed to after
all that in-depth discussion and input.
And you said, "Specifically in regards to the issue of rigid vs.flexible
diaphragms... if the horizontal diaphragm is significantly rigid v. the
vertical lateral force resisting elements (i.e. the shear walls, frames),
then the distribution of lateral forces will essentially follow rigid
diaphragm assumptions. If the horizontal diaphragm is significantly
flexible v. the vertical lateral force resisting elements then the
distribution of lateral forces will essentially follow flexible diaphragm
assumptions. This is true independent of any code provisions. The reality
of this is that there is no absolute system that will distribute its forces
entirely by either rigid or flexible diaphragm assumptions.
"...It is not a unique experience for an engineer to design a building
using both rigid and flexible diaphragm assumptions and then make an
evaluation of the appropriate design loads for the specific structural
elements. ....The lateral forces will be distributed by basic mechanics not
by wishing that they would be distributed by a set of simplified assumptions."
Well, John, certainly the structural behavior you describe is true, but your
very reasonable advice on how to proceed in design would only hold if there
were no 2000 IBC already 10 months beyond its Final Draft, the reading of
which was my homework Monday. (The 2000 IBC final draft has some
ambiguities, but the need to distribute horizontal shears and deal with
overturning per Sec 1617.4 appears not to be expressly waived even when the
low-rise, "simplified" analysis of Sec 1617.5 is used.) The 2000 IBC
thoroughly countermands your advice, presumably with SEAOC Seismology's
acceptance. Maybe that's why Seismology has been quiet.
In 2000 IBC, Rigid and Flexible diaphragms are both defined, at the very
beginning of Chap 16. That flexible diaphragm definition, so tentatively
advanced in 1986 by Ed Zacher on apparent guesswork solely to give relief
from designing for torsional shears, now is carved in "Codestone" so as to
eliminate all judgment on how Horizontal Story Shear may be distributed.
Further, that tagged-on optional method of determining whether lateral
deformation is or is not "two times the average story drift", namely by
comparing computed "midpoint" deflection of the diaphragm-- well, it's the
ONLY way allowed in the 2000 IBC. I wish I understood that method. The
language has always baffled me, and I'm not that bad with language. But up
through the 97 UBC, it didn't matter.
Why does it matter under the 2000 IBC? Because if the diaphragm is Flexible
by that one obtuse means of determination, you SHALL distribute seismic
design story shear "to various vertical elements based on the TRIBUTARY AREA
of the diaphragm to EACH LINE [as defined] of resistance." On the other
hand, if the diaphragm is Rigid (ie, other than flexible), you SHALL
distribute seismic design story shear to various vertical elements "based on
the relative lateral stiffness of the vertical resisting elements and the
Wow! Two completely different methods, with no gray area in between them. No
splitting a difference or using judgment; it's all one way or all the other.
And it didn't happen with Seismology Committee unaware; it was discussed "in
depth with input from a broad spectrum of practicing engineers." I guess
they didn't ask you, John, nor did they tell you how it came out. Enjoy your
Yet those who replied, including me, agree with you about shear distribution
being inexact and rarely being at the extremes that IBC 2000 always requires.
For example, Ben Yousefi, S.E., of San Jose CA, writes his firm agreement,
which a later piece of my homework puts in a very different light:
"Structural engineering for earthquakes is not exact science....with wood
diaphragm construction the degree of uncertainty multiplies compared to
steel moment connection. There are many elements that comprise the end
product and workmanship plays a big role in how it is all put together. So,
relying on some equations that try to predict all these features may not be
a prudent thing to do....
"The real life experience of these diaphragms though, has not indicated, at
least to my knowledge, that the assumption of flexibility in light framed
wood construction is flawed. As far as we know, this assumption has not
contributed adversely to structural, or nonstructural performance of any
buildings during the recent earthquakes. So, the better use of an engineer's
time may be to spend the time on ensuring that he/she is designing a well
thought out structure as whole, rather than performing diaphragm rigidity
calculations (for these types of building) that may have little to do with
actual behavior of the lateral system....
"should we follow the letter of the code or the intent of the code. I
personally rather do the latter."
The other part of my homework was looking up the origins of the 2000 IBC
Rigid and Flexible Diaphragm provisions as to the new, unprecedentedly
MANDATORY method of distributing horizontal story shear strictly one way or
the other, unlike how Ben Yousefi just said it should be done.
This strict new way wasn't in the First Draft. It was proposed as a change
to the First Draft in early 1998, and approved at the April 1998 ICC meeting.
And who proposed this provision that none of us repliers including Ben
Yousefi agree with? None other than Ben Yousefi of City of San Jose,
representing the ICBO Structural Review Committee. The same Ben who let the
rigid vs. flexible diaphragm debate rage the last eight months, and said
nothing to settle it.
And do you know what Ben's proffered code change "reason" was for newly
dividing story shear distribution into extremes, with none of the middle
ground we all say we believe in? There was NO reason given about this!
A pie for you too, Ben. Tell me if the crust was rigid or flexible, by the
LETTER of your own code provision, the same way laws are construed in court.
Charles O. Greenlaw SE Sacramento CA