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Re: Rigid vs Flexible residential diaphragm discussion

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Mark, there is no doubt that i support all of the list subscribers who agree 
with your comments. Unfortunately, nothing will be done within SEA to insill 
change without following the ago old steps create for committee work. I know 
that seismology is aware of our discontent as I received numerous calls from 
members of the wood and seismology committee asking me to submit my comments.
I can't guarantee that Seismology or wood sub-committee will agree with us 
and that we will win this debate, BUT THE ONLY CHANNEL WE HAVE AT THIS TIME 

The biggest mistake that our list subscribers are making is the assumtions 
1. They can not change the policies which are created by politics within 
2. That the debate that is now occuring has no affect upon members outside 
seismic regions. This is the greatest underestimation of the ability of the 
professional community because what we are trying to accomplic in seismic 
zones will set a precedence for addressing changes to the code making process 
for all other areas of concern as they apply to every engineering around the 
world in time. Therefore, the prinicples of the issues are at stake since 
they will have a consequence to how the process develops in the future.
3. That if others have stated the positions, the individule might not 
contribute since the postion is already been stated. The wood and seismology 
committee (and board of directors) will not be likely to act upon the 
passionate arguments of a few if they do not believe that the concerns are 
universal. Therefore, it is very important to voice your support to the 
seismology committee for what you personally believe. This is the only way 
for them to gauge the importance of the argument.
4. There is no way to be heard. This is another opinon that allows apathy to 
reign. The fact is that members of these committees monitor our list. I 
personally believe that have a greater responsibility to come out of the 
shadows (from those who are not computer literate by representatives of the 
committees who can summarize the reasons for their support of the intentions 
of the code) and help those of us who are speculating as to the intention of 
the committee to understand their true motivation. Instead, we are forced to 
at times yield to our frustration which makes the committees believe that we 
only attacking them without constructive suggestions for resolving the 
situation that is more in keeping with the step in which they are used to 
making changes.

To raise awareness on this list is not enough until some time in the future 
when the lines of communication run in both directions and we are given 
access to all the information that committees consider in their 
determination. Until then, we must do more than voice our concerns here, we 
must foreward our concerns to the seismology listservice (or any other 
appropriate list) so that when they meet they will be forced to consider the 
growing number of the professions with concerns.

AT seaocchair(--nospam--at)

In a message dated 8/4/99 11:34:09 PM Pacific Daylight Time, 
mswingle(--nospam--at) writes:

<< Bill Allen wrote in part:
 ".... IMO, it would be prudent to include an
 exception for residential, wood framed structures
 three stories or less...."
 Hear! Hear! I applaud Bill's comments and I
 generally agree with the concept.  Where are the
 I would, however, offer one change to his
 proposal.  I think that there should still be the
 OPTION of considering the stiffness of the
 diaphragm, as would still be required for other
 buildings.  It is sometimes useful to use this
 approach if one wants to reduce the tributary load
 on a short shear wall by allowing the diaphragm to
 redistribute some of the tributary force to
 adjacent walls.
 Regarding horizontal distribution of shear in
 general, the amazing thing is how long this
 requiremet has been in the code (and applicable to
 wood all along), and yet has been so routinely
 ignored.  It is not just the 1997 code that has
 this requirement, and it didn't start in 1988
 either.  The 1988 UBC simply TRIED to define what
 a flexible diaphragm is, which had not been done
 in the UBC before then.  If the diaphragm doesn't
 calculate to be flexible, then one is REQUIRED to
 "consider" the rigidity of the diaphragm.  If the
 diaphragm does calculate to be flexible, then the
 consideration is finished, and the implication is
 that tributary loads are to be applied, since 97
 UBC Section 1630.5 says in part that "At each
 level....the force....shall be applied over the
 area of the building in accordance withe the mass
 disribution at that level."
 With no offense to the original author, the
 definition of "flexible" diaphragm (that first
 appeared in the 1988 UBC) is not well-defined (Ed
 Zacher already knows this), and therefore does
 VERY LITTLE to alleviate possible liability for
 those who distribute the forces based on tributary
 area.  There are many, many residential buildings
 where if one were to calculate the relative
 stiffnesses based on the definition, the
 diaphragms would NOT be deemed to be flexible.
 The problem is that almost no one had performed
 this "flexible diaphrgm" calculation until
 recently (even though it has been in the code for
 11 years), because nearly everyone relied on the
 assumption that "everyone else does it this way",
 and also relied on the numerous references (deemed
 reliable by the reputation of the authors) that
 say "wood diaphragms are flexible".  Relying on
 either of these, and ignoring the code, is to
 subject oneself to possible future litigation.
 It is time that the code is brought back to
 reality.  The code should be one step BEHIND the
 state of the art, but in this instance, the code
 has been AHEAD of the state of the art for some
 time, and I don't see the art progressing in any
 significant way with regard to this requirement,
 especially for wood structures.
 The calculations required to determine the
 stiffness of plywood shear walls are poorly
 understood and poorly defined.  In addition, the
 methodology is cumbersome to apply since it
 requires numerous assumptions to be made that are
 not defined in the code, which is compounded by
 the fact that iteration must be done to arrive at
 the final "solution."
 In my opinion, designing residential buildings
 with wood diaps and shear walls (of 3 stories or
 less) for seismic forces by the tributary force
 method ("flexible" diaphragms) will result in
 structures that are no less safe than if they were
 designed by the more rigorous procedure.  That's
 the bottom line, isn't it?
 Well I've blabbed enough already....
 Mark Swingle, SE
 Oakland, CA
 These are my own views and not necessarily those
 of my employer.
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