Subject: Re: Charles Greenlaw's comments regarding Rigid v.Flexible Diaphragm
Date: Tue, 1 Jun 1999 14:36:20 EDT
Charles Greenlaw wrote:
"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."
that was a delicious virtual pie. I am sure glad the crust was semi-flexible!
Mr. Greenlaw, I have been reading your postings with interest since this
issue started appearing on the server. Your insight and in-depth knowledge of
the history of how seismic provisions have evolved are commendable. I even
like your sense of humor (as long as the pies being thrown remain virtual).
The fact that you spent the Memorial Day researching code change proposals
shows your dedication and love of this profession, which I admire. However,
there seems to be a bit of jumping to conclusion in your latest posting
regarding my position on this issue.
Charles Greenlaw wrote:
"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!"
There were two reasons given for the code change proposal to the final draft
of the 2000 IBC. One was that, at that time the ONLY provision for
distribution of horizontal shear was the based on the diaphragm rigidity. So
the intent was to insert a provision for flexible diaphragms consistent with
the current practice of distribution based on tributary area. So, unless one
believes the only way to distribute horizontal shear is by doing a rigidity
analysis, this was an improvement. The other reason had to with the
clarification of the line of resistance so that we don't get bogged down by
strict interpretation of what a line of resistance is.
"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 treatment of diaphragm as flexible or rigid in the code is not new, nor
is it unprecedentedly mandatory. The provisions for the two types of
diaphragm have not changed since the 1988 UBC.
"The same Ben who let the rigid vs. flexible diaphragm debate rage the last
eight months, and said
nothing to settle it."
>From the start of this discussion I have clearly stated my opinion on this
subject. I recall that this issue was first posted by an engineer who was
asked by a building official to do a rigidity analysis for a wood diaphragm
in an existing building that he was working on. And he was given the reason
for this requirement as the new 1997 provisions. At that time many of us on
the list server questioned the reason for this, since the 97 UBC provisions
for this item have not changed from the previous codes.
It turns out that some engineers, with possibly good intentions, are sold on
the idea that even wood diaphragms in light frame construction have to go
through this rigorous analysis to determine flexibility. They have apparently
been beating the drums on this issue and that information has been
disseminated to the engineering community through various mediums. I do not
agree with this interpretation (until proven otherwise) and my position has
remained consistent on that.
I was recently looking a draft the Seismic Design Manual Vol. II dated
6/9/98. There were 38 pages of calculations for the lateral design of a Wood
Light Frame Residence. Page after page of diaphragm deflection cacls and
rigidity analysis. Do I think this is a practical and absolutely needed
approach? Definitely not. Especially in light of the fact that many
improvement were incorporated to the 97 UBC such as inclusion of redundancy
and reduced allowable height to width ratios on shear walls. If one designs a
well laid out and detailed building according to the 97 UBC, for this type of
a structure, it would make little difference in the overall performance of
the structure, whether you assumed the diaphragm as rigid or flexible.
My final comment is regarding code language and how we interpret it. Codes
are written as general guidelines that set basic criteria for how things
should be designed. Thus the "shall" language addresses only the clear cut
situations without considering other relevant facts. It is not intended for
the code, nor is it possible, to address every possible condition in real
life design and analysis of a building.
Therefore, the judgment of the engineer of the record in conjunction with the
code regulating authority is of utmost importance in order to implement a
sound application of the code. I agree wholeheartedly that all things are not
black and white and there are many shades of gray. But we can't expect the
code to address all those shades. There are clear mechanisms in the code that
allow for application of sound engineering judgment in those cases. Please
refer to sections 104.2.7 and 104.2.8 regarding Modifications and Alternate
material, design and methods of construction.
Having worked in a building department environment for the past 12 years, I
have sat on numerous preliminary design meetings with architects and even
electrical and mechanical engineers to discuss interpretation of code issues
to their unique design. And on numerous occasions, applications are filed for
the alternate methods and materials used. Conspicuously absent from these
meetings are structural engineers. This has always baffled me. I urge the
design community to consider discussing issues with the building officials,
and others in a regulatory environment, when there are gray areas and work
together to come of with an interpretation that is mutually acceptable and
that "meets the intent of the code". In the meantime those who are not
satisfied with the current code requirements, such as in the diaphragm
flexibility determination, are encouraged to put their ideas forward and
hopefully come up with better language in the future codes.
PS the contents of my comments above reflect only my own point of view and
not necessarily my employer's nor other organizations that I am affiliated