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Re: Charles Greenlaw's comments regarding Rigid v.Flexible Diaphragm

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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."

Yum!!!!!! 
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. 

Charles wrote:

"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"

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. 

Charles wrote:

"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.

Regards, 

Ben Yousefi

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 
with.