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RE: steel - vibration analysis

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Regarding the vibration analysis performed by FloorVibe, Josh Plummer's
response overlooked some fundamental concepts, although that is not an
uncommon mistake. Careful study of Design Guide #11 shows that the
information contained therein is only valid for essentially regular,
orthogonal framing schemes, and the FloorVibe program, which was
developed by one of the authors of Design Guide #11, Dr Thomas Murray of
Virginia Tech, closely implements that design guide. The fundamental
issue here is this: the Design Guide, and hence FloorVibe, only
considers orthogonal framing conditions not because it is an
oversimplification of a more complex problem, but rather because except
in very rare circumstances only the regular orthogonal framing systems
have a realistic potential of having a vibration problem. In the case of
irregular framing, resonance doesn't occur between the various members
with different frequencies, and hence the vibration of the system is not
perceptible. Even seemingly minor variations have often been found to be
sufficient to prevent vibrations from achieving perceptible levels - the
vibration is attenuated before it reaches perceptible levels. Thus we
see that by only addressing generally regular, orthogonal framing
systems, Design Guide #11 not only defines how to investigate the
vibration problem, but also defines the limit of the problem that needs
to be investigated. And if an irregular system did have a vibration
problem it wouldn't be revealed through any application of Design Guide
#11. So from a practical standpoint it is not necessary for the engineer
to investigate and correct the vibration characteristics of every beam;
that would be a waste of time, and of questionable value. If vibration
is going to be a problem, it is going to be because of the vibration of
the entire bay, not because of the vibration of an individual beam. Thus
only the regular, or nearly regular bays need to be investigated.
Although the Design Guide isn't completely clear on this, it is implied,
and this view is supported by an author of the Guide and others.
Furthermore, the sensitivity criteria have been calibrated to correspond
to the methodology of analyzing orthogonal framing described by the
design guide. It is improper to apply those same sensitivity criteria to
framing systems that have merely been analyzed using a general finite
element approach. There may be sensitivity criteria appropriate for such
an analysis, but Design Guide #11's sensitivity criteria are not.
Thus FloorVibe has the "limitations" that it has because that is the
limit of the problem that needs to be considered. Recognizing that there
is already software available for the task - and from the premiere
vibration expert, no less - we chose to integrate FloorVibe with the RAM
Structural System, and arranged with Dr. Murray for all of our clients
to receive a free copy of FloorVibe. I highly recommend FloorVibe to
everyone, whether you have the RAM software or not.
But none of this answered David Topete's original question. In Section
4.3 of Design Guide 11 there are special requirements for "Interior
floor edges, as in mezzanine areas or atria." FloorVibe has an option to
analyze the bay as a "Mezzanine". Although the framing that he was
analyzing was a "mezzanine" in the classical sense (a small partial
floor level), after discussing this with David we concluded that it was
not a mezzanine in the Design Guide sense (edge framing free of any
partitions or curtain walls - e.g., just a handrail). Once he deselected
that option the FloorVibe results matched his other results.
Allen Adams, S.E.
Chief Structural Engineer
RAM International
(800) 726-7789 x113
Allen.Adams(--nospam--at)bentley.com
http://www.kcba-architects.com/

From: Josh Plummer [mailto:josh.plummer(--nospam--at)cox.net] 
Sent: Thursday, May 31, 2007 10:48 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: steel - vibration analysis
David -
I've used the FloorVibe software, but only the share ware version that
AISC had on their website for awhile. Not sure if they have a better /
commercial version or not. Anyway, the problem that I see with this
software is that everything has to be nice and orthogonal. I didn't see
a way to enter in the types of skewed project grids that I really see on
design projects. My impression was that it was even more limited than
that, but it's been awhile so I can't quite remember what seemed so
limiting about it. 
I don't have any experience with RAM, but keep in mind that the values
handed to FloorVibe from RAM would have to be simplified and
approximated to fit into the very rigid constraints of what FloorVibe is
allowed to do. The assumptions and simplifications that RAM makes may
not be all that good. If the guys at RAM really new how to to floor
vibrations, then I'd think that they would just do the calculation
themselves? This may not be simple for the average engineer who does
this calculations once every few years. However, it's shouldn't be all
that difficult for a development team whose job it is to get it right
once when they're programming it. 
Disclaimer: I work for a structural engeineering software company (RISA
Technologies) that competes against RAM in the Floor design market and
which also brags about our ability to do DG-11 floor vibration
calculations. I'm ridiculously busy these days, but if you were to
e-mail me a good description of your floor system or some drawings and
such, then I can try to get one of our engineers to enter this into our
Floor program to demonstrate our ability to do the DG-11 calculations
accurately.
Sincerely,
Josh Plummer, SE
RISA Technologies

From: David Topete [mailto:dtopete(--nospam--at)gfdseng.com] 
Sent: Thursday, May 31, 2007 4:44 PM
To: seaint(--nospam--at)seaint.org
Subject: steel - vibration analysis
Checking vibration on steel framing for a mezzanine, I got 2 similar
answers by following example 4.4 of AISC Design Guide 11 (Designing for
Floor Vibrations) and the Floor Framing software from the
www.AISC.org/steeltools downloadable software. While those answers were
similar, the analysis provided by FloorVibe via Ram Structural System
indicated members being increased about 60% (conservatively). Has anyone
else encountered such a discrepancy?
David A. Topete, SE
Structural Engineer
GFDS Engineers
543 Howard St., First Floor
San Francisco, CA 94105
v : (415) 512-1301 x21
f : (415) 512-1302
dtopete(--nospam--at)gfdseng.com
www.gfdseng.com


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