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Re: Plywood rigid diaphragms

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The original question was for comments on "light frame" construction, and as
to "analysis approach". I do lots of design in this type of construction,
and in general I do not have a lot of use for fussy, high-blown analysis
approaches such as come from SEAOC-type code-influencing committees and
seminar presenters. I don't play at jigsaw puzzles as a hobby, and I don't
enjoy sifting through fragmented and scattered pieces of code provisions to
see if I can assemble the same elaborate "picture" some few persons
supposedly in the know got installed in code for reasons and on merits I am
skeptical about.

On the other hand, there have been many excellent and astute replies already
to this email topic, and lots of good tips. And no control or rescue agendas
showing.

Bill Cain reminded us that diaphragms and shear walls "don't know" what
human-written manuals declare them to be, whether flexible or rigid; "they
will just try to distribute loads according to their relative stiffness".
Amen to that. Others added that the stiffness itself in each element varies
according to stresses actually experienced, and according to myriad details
of construction and workmanship, that defy accurate understanding.

I would account for all this uncertainty by saying, with complete certainty,
that the structure behaves the way IT wants to. My task is to estimate that
behavior and design in features of strength so that the structure can
survive IT'S OWN pleasure as to how it behaves.

In doing that estimation I use a wide range of information and judgment, and
I "bracket" the resulting design to cover the spread of imprecision I
perceive in load distribution and sensitivity of construction to overload,
etc. Other engineers' methods are among the considerations, but are not
followed mindlessly, nor from a position of subordination to my "betters".

Obviously, this attitude requires a measure of confidence in oneself as a
competent and capable professional. Such confidence varies among
individuals; no problem about that. But I resent the growing tendency among
some in our profession to stuff the code full of their own doctrinaire
preferences, in disempowering detail, to the effect that other engineers
can't "engineer" anymore. Engineering as professional discretion within
broad limits gets replaced by micro-ministerial execution of code item
pre-programming. Unfortunately some of us appear awfully willing to practice
engineering the way trolley cars travel: strictly on tracks laid out by
others in advance. This makes for a dependency syndrome that those certain
codewriters cater to, sort of like welfare dependency. 

Jeff Smith mentioned the lawsuit peril inherent in the attempt to separate
plywood diaphragms unnaturally into two separate categories and then force a
particular theory of analysis on the engineer. I agree. At one time it was a
useful metaphor to explain diaphragms by using names for the extreme limits
of their range of behavior. But that was only a tool for instruction. Now
some people seem to have latched onto "flexible" and "rigid" as the only
possible descriptions. Try another example of extremes: in matters of
drinking alcohol in beverages, are all persons either teetotalers or alcoholics?
             
As has been said, the map is not the territory. Structural realities should
not have to be dealt with falsely just to conform to some human-inflicted
abstraction.

Charles O. Greenlaw, SE    Sacramento  CA
_______________________
>Just finished seaonc wood design seminar. An interesting issue was raised
>regarding the design of shear walls and diaphragms.
>
>According to the presenters, design engineers should consider the
>horizontal wood diaphragms as rigid elements, and distribute the horizontal
>shear to the shear walls based on a rigid analysis.
>
>Of course at this time no one has any published data on the relative
>rigidities of the shear walls other than longer walls are stiffer than
>shorter walls!!!!
>
>Does any one have any comments on this analysis approach for light frame
>wood construction?
>
>John Buchanan
>