After reading and reviewing the wood diaphragm (rigid v. flexible)
discussions and especially the personal attack on the SEAOC seismology
(lateral force) committee and its members, it is time to pause a moment and
review some of the items that WERE NOT said.
First of all, the seismology committee like other major SEAOC committees is
comprised of very strong individuals who seldom allow any one individual
(let alone the chair) to dominate the provisions. Being the chair of a
major SEAOC committee is like being the President of SEAOC - it's like
herding cats. Thus, assigning the authorship of a specific provision to a
specific author is simply not correct. All proposed provisions are
discussed in depth with input from a broad spectrum of practicing
engineers. Corresponding memberships are offered on all SEAOC committees
for those who WISH to participate but are unable to attend in person.
Communication is the key issue - if you have concerns (and hopefully
solutions) to specific code or design issues, a letter or email to the
committee chair is probably the first step to determine the background and
reasoning behind the provisions. If you don't know which committee may be
responsible for a specific issue, you can always address your comments to
the seaocchair(--nospam--at)seaint.org listserver for immediate access to all SEAOC
committee chairs. A little homework and research can avoid a lot of
misunderstanding. In any event, abusive personal attacks is not appropriate
Second is the concept of what is correct to the best of structural
engineering knowledge. Specifically in regards to the issue of rigid v.
flexible diaphragms - let's review the basic doctrines of how lateral forces
are distributed. Lateral forces are distributed by the relative rigidity of
the resisting elements. Thus, if the horizontal diaphragm is significantly
rigid v. the vertical lateral force resisting elements (i.e. the shear
walls, frames), then the distribution of lateral forces will essentially
follow rigid diaphragm assumptions. If the horizontal diaphragm is
significantly flexible v. the vertical lateral force resisting elements then
the distribution of lateral forces will essentially follow flexible
diaphragm assumptions. This is true independent of any code provisions.
The reality of this is that there is no absolute system that will distribute
its forces entirely by either rigid or flexible diaphragm assumptions.
As engineers, we are expected to know this basic doctrine and design our
structures accordingly. It is not a unique experience for an engineer to
design a building using both rigid and flexible diaphragm assumptions and
then make an evaluation of the appropriate design loads for the specific
structural elements. Calculations for the stiffness of various wood framed
shear walls (plywood, drywall, stucco) are documented in the literature
and/or not that difficult to develop. Programs like Enercalc or simple
Excel spreadsheets are appropriate for a rigid diaphragm analysis.
Third, the legal motivation (negligence, negligence per se, defense by
standard of care/practice) should not govern over the obligation to provide
a well thought out design that statisfies basic structural engineer
principles and the governing building code. The lateral forces will be
distributed by basic mechanics not by wishing that they would be distributed
by a set of simplified assumptions. A better use of an engineer's time will
be to focus on a design that will perform instead of looking for an escape
clause for flawed assumptions.