Subject: Re: Code Created Malpractice Opportunity-Rigid v. Flexible Diaphragms
From: Lynn H <lhoward(--nospam--at)silcom.com>
Date: Sat, 29 May 1999 20:29:28 -0700
I agree 100% with our comments on personal attacks.
They have no place on this list.
In regards to plywood diaphragms being flexible or
rigid, you are again correct in stating that the
diaphragm will not act as a truly rigid or flexible
element. So REGARDLESS of the assumption we make,
the actual performance of the diaphragm will not
match our calculations.
If one desires, an analysis of the diaphragm based
on both methods could be made, and a conservative
design could be made based on a worst case basis.
However, this is a lot of WORK for design engineers
who do small (under 5,000 square feet), and light,
one and two story wood framed buildings. Fees do
not generally justify this type of an analysis.
In both the Northridge and Loma Pietra earthquake,
smaller wood framed structures built with plywood
diaphragms and plywood shear walls performed very
well from a life-safety basis. These structures
were designed based on the flexible diaphragm
Therefore, you HAVE to conclude that assuming a
plywood diaphragm is flexible in the lateral
analysis of buildings of this nature will provide
What really "IRKS" me is that the new Code seems to
be very clear on issue that you should not base your
design of plywood diaphragms as flexible unless you
check the the deflection of the diaphragm and walls
and pass some arbitrary ratio.
If the old tried and true assumptions about plywood
diaphragms give the desired results, then WHY make
designs based on these assumptions a CODE violation?
I know, some professor at (insert your favorite
campus here) is having a "spaz" attack because the
data from his 15 year study on diaphragms do not
match current design standards.
If someone has the time and inclination to analyze
diaphragms as rigid, rigid and flexible, etc., that
is fine with me. But I have a MAJOR problem with
the Code making the old method not longer
acceptable. The old method has proven itself in
real earthquake situations. You just can't argue
I am right and the Code is wrong :) (hehehe, even I
am laughing at my own words!).
> 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
> or professional.
> 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.
> JOHN SHIPP