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RE: steel - vibration analysis

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Jordan - 

I feel much better about our discussion now.  Our disagreements were mostly
related to what we mean when we say non-orthogonal bays.  In my experience
these are usually pretty regular because that's just how we all WANT to
layout our steel.  It's just an angled girder here or there.  Maybe there's
an angled joist here or there as well.The chapter 7 example in the design
guide is perfect because it's so darned common to have framing like that. 

One slight correctly is that is that the chapter 7 example problems is not
TECHNICALLY a girder problem.... It would be more accurate to say that there
was a problem with the combined Joist-Girder panel modes. So, the joist
calculations whould end up showing the worst vibration accelerations.  Of
course, the reality of what you said is correct in that you absolutely had
to strengthen the girder to solve the problem.  But, that's because
stiffening the girder will stiffen up the combined joist-girder mode much
more than changing the joists themselves.

Sincerely, 

Josh Plummer, SE
RISA Technologies

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com] 
Sent: Tuesday, June 12, 2007 9:21 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: steel - vibration analysis

Well, I read non-orthogonal as more skewed than you did.  I did forget that
the 7.2 atrium girders got columns, but that's a girder problem - the beams
could have been just about anything and the girder still would have been an
issue.  None of the triangular areas, nor the beams around the atrium
appeared to require work.

Again, I read the previous comments as applying to areas with varying beam
lengths - a special, limited case of non-orthogonal framing. The
exaggeration of the problem was just that - non-prismatic would have been
sufficient to describe the difference.  Having spent most of my pre-building
career in aerospace, I find vibrations mostly intuitive now (I say that
knowing the analytical knowledge is locked somewhere in my mind, I just
can't remember where).  Most of my cases come in two flavors - simple enough
to apply the DG11, which for me means regular orthogonal bays, or a job for
a dynamic modeling package. Since I own neither RISA nor RAMSS, I can't
really comment on either's implementation. I've found most building software
analysis (limited though it may be) to be somewhat simplified for the user.
That's a good thing for what we do, for the most part (I've incurred the
wrath of the staff at RAM before for having a black box approach...I swear -
that's a good thing in this case).

We do agree that there are cases where some non-orthogonal framing can
result in a framing issue, but there are enough common conditions where it
precludes the beam participation in the problem (most triangular bays in an
otherwise normal grid, for example) I felt it necessary to chime in on
Allen's behalf.  For what it's worth, I think the discussion is good - if
for no other reason than to get more engineers (who read the
list) to remember the vibration issue during design.

Jordan







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