Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Envelope Method

[Subject Prev][Subject Next][Thread Prev][Thread Next]
You are, of course, correct Doug. The code does not require the design
engineer to envelope results obtained from a flexible and a rigid analysis.
The problem comes when the design engineer attempts to use that skill which
is becoming a dinosaur called "engineering judgement". As you know, a wood
structure (or probably any other structure) is not totally rigid nor totally
flexible. What further complicates the problem is the inability to calculate
the rigidity of a wood framed structure (much more so than a metal deck with
concrete fill laterally supported by masonry walls). Even if we talk about
the structural elements, we have to decide how stiff an unblocked diaphragm
is or a diaphragm that is unblocked over the middle half of the span. We
then have to calculate the stiffness of the shear walls only to discover
that the shear force now will not be distributed with the same unit shear
(PLF) in a line containing a 10 ft. shear wall and a 4 ft. shear wall. What
to do? What happens if the entire wall is sheathed? Do we/could we use the
methods described in APA 157? I dunno. What about the stiffness provided by
a GWB ceiling? Then, what happens when the contractor uses 1/2" plywood
instead of 7/16" =8O !! Instead of applying the deliberation, many engineers
faced with meeting a budget come to the conclusion that the only way to
analyze the structure and have a defensible position five years down the
line (when facing an expert witness in court - forget about the plan
checker) is to eliminate all of the subjectivity that could possibly be
imposed upon the design engineer by either the plan checker or expert
witness and thereby kill the most valuable skill of all, engineering

So, you are correct in that enveloping is not a code requirement. It is just
becoming a practical thing to do. What is interesting about this issue, is
that the language of "considering the stiffness..." has been in the code
since at least 1988 while most of us continued to use the flexible diaphragm
method to design wood framed structures. As far as I know, only subsequent
to the SEAOSC seminar in February, 1998 titled "1997 Uniform Building Code
(UBC) Wood Provisions" presented by yourself and Bill Nelson has this
methodology picked up steam. One of the more aggravating points of this
seminar is that the flexible vs. rigid methodology is NOT a new provision of
the 1997 UBC but the bulk of the seminar (count the sample calculation
sheets) revolved around this topic. As far as I'm concerned, the historical
point of where rigid vs. flexible became a real issue was February 21, 1998
not with the adoption of the 1988 or 1997 UBC.

No, Doug, enveloping is not in the Code, but it might as well be.

I would really be interested in seeing a structure that was damaged during
Northridge or Loma Prieta that would not have been if a rigid diaphragm
analysis was used. It wouldn't happen on those open front structures; they
would have been worse off. It would HAVE to be a structure where the back
wall of a garage failed where the front (open part) did not. Please,
someone, somewhere provide me an example so I can get off this damn
soapbox!! Of course, if someone only presented one, this could be an
anomaly, right? But, just provide one, please.

It would be *nice* if SEAOC could prepare and publish a position paper
(maybe not a one or two pager but a comprehensive treatise on "real world"
structures including examples) on this topic where we practicing structural
engineers could comfortably avoid enveloping the design. It's getting so
that "engineering judgement" just won't cut it anymore. Neither will our old
friend "design based on a rational analysis". I recently had a shear wall
where I had to design it considering the openings. With the experience I
have, the samples I have seen, and the tools I have used, I felt I prepared
a (more than) adequate design to strap and nail the corners of the openings
to get a compliant shear wall /wall frame/perforated shear wall or whatever
you want to call it. The plan checker (an outside consultant) disputed my
analysis. Apparently, my method was a "design based on HIS rationale" but he
could not provide a sample of what he wanted. Today, like it or not, we are
getting closer to having to cite a code section or a reputable technical
source. I can only hope that, with the diminished role SEAOC will be playing
in the code preparation process, they can focus on producing clarifying
documents that not only will help us interpret the code, but back us up at
plan check and in court.

Those believing this is just an "old dog" syndrome has forgotten that we
used to do our best work when we didn't have a calculator in our hands. Now
we are judged only by how fast we can generate numbers. No wonder our value
is declining.


Bill Allen, S.E.
Laguna Niguel, CA

|| Dennis:
|| Using the "envelope" method (taking worst case of flexible and rigid
|| diaphragms analysis) is NOT a code requirement.  The code
|| only recognizes
|| "flexible" and "rigid" diaphragms.  Using the envelope
|| method would be above
|| code requirements and is an engineering judgement that the
|| diaphragm may be
|| somewhere in between flexible and rigid.
|| Doug Thompson