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Re: Effects of the New Code on Wood design - good or bad?????

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Charles Greenlaw wrote:

	....
	By the above method, the preponderance of your Fri posting advocated
	as code policy use of extreme-limit bracketing for both rigid and
flexible,
	and used that rationale to explain away the interpretation that Lynn

	said he got from DSA/SSS. But a minor element in it indeed floated 
	the bracketing gambit as merely an escape method when one (as 
	usual) can't accurately calculate shear wall and diaphragm
deflections.
	Harmonizing your points (absent your later-posted clarifications) 
	remains difficult, and I stand by what I posted. 

Lynn Howard wrote:

	....
	Prior to our starting on the project, I met with the DSA and told
them I
	was considering designing the diaphragms as rigid, because that is
what
	the Code clearly said.  With full knowledge of what the code said,
the
	DSA plan checker said that if I wanted to do a rigid analysis on the
	diaphragm, that was okay, but that I would have to also do a
flexible
	diaphragm analysis and the building would have to fully comply with
the
	results of that analysis.  They basically were not going to require
the
	rigid diaphragm analysis, and in fact WOULD NOT ACCEPT that type of
	analysis alone.

Mark Swingle responds:

Charles:  
Re: advocating code policy:

Apparently you haven't read Dennis' original posting on this subject, and
you haven't read my two posts of last Thursday.  There were two different
items I was trying to present:  1) the fact that the code has not changed
regarding horizontal distribution of shear,  and 2) a method whereby one
could find the worst possible load to each shear wall without doing a
complicated analysis of the diaphragm.  I admit that mixing the two concepts
may have lead to confusion, and I apologize for the limitations of my
writing skills.  There were some concepts that were left unsaid due to lack
of time.  I admit that I am not good at writing compactly.

My intent was not to advocate that the code required using the worst case
from the two methods.  Rather I was proposing that if one wanted to do more
than only a tributary load analysis, that by doing a rigidity analysis as
well, one can see the two extremes for each wall line, and then, after
considering the rigidity of the diaphragm relative to the walls, one can
choose the design forces based on professional judgement.

My point about NOT considering the diaphragm is that one does not need to
know or consider the diaphragm WHEN FINDING THE TWO EXTREMES.  The diaphragm
rigidity need only be considered when deciding the final design forces.  If
one wants to be conservative, one can choose the worst load from the two
methods, however, it is not required by code.

In buildings where the mass distribution is fairly uniform, the spacing
between wall lines is nearly equal, and the length of walls in each line is
nearly equal, the difference between the two methods will be negligible.  In
buildings that are more "irregular" or "unsymmetrical" with respect to these
properties, the differences in some wall lines between the two methods will
be greater.

If the wall lines are spaced far apart, then the diaphragm will be
relatively more "flexible", and the designer can choose a force for each
line that is closer to the tributary load case than for the infinitely rigid
case.

If the building has diaphragm spans that are relatively small (such as many
residences), one may choose a force for each wall that is closer to the
infinitely rigid case, since the diaphragm in this case will redistribute
forces based on the shear wall stiffnesses to a greater degree.

However, straying too far from the tributary load case for wood buildings
with wood shear walls will raise red flags with plan checkers, "expert"
witnesses etc, regardless of the merits of your approach.  You have spoken
eloquently in the past about this subject and the inherent legal and
professional pitfalls.  It is unfortunate that the "wood diaphragms are
always flexible" mentality is so pervasive.

Re: the DSA plan checker:

Lynn stated that he "told them I was considering designing the diaphragms as
rigid, because that is what the Code clearly said."  That statement implies
that he wanted to consider the diaphragm to be infinitely rigid, unless I am
misinterpreting his intent.  That means that he was planning to go to the
opposite extreme of what most designers do.  This approach may lead to some
shorter walls with relatively large tributary forces to be underdesigned.

I did not intend to use my method "to explain away the interpretation that
Lynn said he got from DSA/SSS."  The code does not "clearly require" that
the diaphragms be considered as rigid.  I was simply pointing out that using
either extreme is not correct.

Charles, I think we are actually very much in agreement about most of these
points, but we have such different ways of expressing them, have different
professional experiences, and are both so stubborn, that we are having a
difficult time understanding each other.  Let's keep the dialogue going.

Mark Swingle, SE
Oakland, CA

These views are my own and are not necessarily those of my employer.