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RE: Report on Wood Diaphragm Issues

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I believe, with respect to light wood framed structures, and given what I
know today, as opposed to what I knew last week or will know next week, that
there are times when a horizontal diaphragm will act relatively rigid with
respect to the shear walls and other times when it will be relatively
flexible.   Jeff Smith's post 1-4-00 hits on some of the variables.  It is,
in our opinion,  appropriate to distribute loads with due consideration to

The second question is how accurate do calculations need to be.  In the case
of many typical (standard looking) homes distributing loads by tributary
area is likely going to produce a safe structure.  As safe as the practice I
have seen of taking the seismic mass and distributing it across the sum of
the lengths of all shear walls in a given direction.  Given either approach
or using the prescriptive design method or doing something in-between will
each likely assure that the structure will not fail. Jeff's comment about
detailing, workmanship and inspection is correct in that these tasks will go
much further in producing safer buildings than 50 pages of calculations for
rigidity.  In the case of a non-typical home, I feel we need a better grasp
on the true load path.  Just as the prescriptive design method may not be
appropriate neither may a simple flexible analysis.

A rigidity analysis where we calculate shear wall stiffness is very
sensitive to your assumptions.  To the point where the moisture content of
the floor joists at time of construction relative to the time of year when
the EQ hits after the structure has been occupied will change hold down
sizes and shear wall nailing.  And when you are all done with your rigid
analysis you may doubt what your calculations tell you.  You should.  But
until there are more answers out there, CUREe, SEAOC seismology, etc. We are
making our best effort to advise our clients how to build safe and
economical structures.  People need houses today, construction will not stop
until engineers get it all figured out.  If Ken Wilkinson's client in Santa
Monica is like many of ours, their interests are not in our issues but only
in moving into their new home.

The first question, of course, is one of liability.  We have our
professional liability.  For those of us who have developer clients those
clients also have absolute liability.  Even if I get the calculations wrong
they may still have to pay for it.  Given the cancerous individuals in our
profession even if rigidity analysis is overkill, as a business decision I
may still chose to make such an analysis both cover my own ass and to cover
those of our clients. 

A few closing thoughts.  Should we be doing rigidity analysis for wind
loads?  Why should the analysis be different than for seismic?  Ron O.
Hamburger commented about the Simplified Method in Section 1630.2.  We did
not read that as overriding section 1630.6.  What about section 1630.10.2
exception 1 with respect to light framed structures?  Why do some engineers
what to make distinctions between one and two family residences of wood and
other types of occupancies?  I do not think the earthquakes will make such a
distinction.  Don't rag on the software developers for how much they charge,
only if the cost is worth it to you.  Try writing a program that has a VERY
limited market, supporting it, and making a profit.  

Finally increase increase increase your fees. A newspaper (WSJ) reported the
other day that some guys carving punch ice for parties pull down $250,000 a