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RE: steel - vibration analysis

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Allen,

Thank you for your well reasoned response, but I have to disagree with some
of your assertions.

1) The concepts in DG-11 are NOT specifically limited to orthogonal framing.

I spent a good part of the afternoon / evening perusing 10 or so Engineering
Journal articles on Floor Vibrations as well as 5 articles from less
academic publicatios (Modern Steel Construction and such). None of those
articles suggest to me that these concepts are limited to orthogonal
framing. The closest you can get is the 1993 Allen and Murray paper which
acknowleges that one of their equations were based on orthotropic plate
action. Yes, those equations were originally based on rectangular panel
modes of vibrations.  But, nothing says that you cannot extend those
concepts towards non-orthogonal systems. The Blodget equations for base
plate design were based on square an I beam columns.  But, we extend those
concepts out to consider pipe shapes don't we.  Why can't we do the same
thing here?  I don't think that there is a mention in any of the articles
that limit or prohibit this line of thinkig.  

2) You seem to be saying that is nearly impossible to have a system where
the non-orthogonal portions have a realistic potential for vibration
problems. 
To refute this, I'd like to point you towards chapter 7 of the AISC Design
Guide.  They give an example (figure 7-2) which has non-orthogonal framing.
The text specifically talks about the section of the floor where the
vibration problems were most severe / dectectble. When you look at their
figure, it's the section of the floor with non-orthogonal / skewed framing!

If you can find ANY reference from an engineering journal article which says
that vibrations can be ignored when you have non-orthogonal or skewed bays,
I'd love to see it.  I want to read it because I've got about 200 pages of
techncial papers and such about vibrations and I've never read that.  I feel
like I've read every substantial article that AISC has published on Floor
vibrations.  But, maybe I'm wrong.  If I'm missing something important, then
I'd like to know about it.  Please point me in the right direction.


Now, I'd like to make a couple of points of my own.  
3)If the angle between my beams and girder is 89 degrees, I would want to do
a vibration calcualtion in that area.  That's just my engineering judgement,
and yours may be different.  However, I'll assume that we agree that it
would be a good idea to do the DG-11 calculations in that bay which is so
very, very close to orthogonal framing.  If it's necessary to do it for this
89 degree bay, then when do we decide that the framing is not orthogonal
enough?  At 85 degrees, at 75 degrees?  The real world of structures just
doesn't consist of perfect, orthogonally framed structures. It just makes
sense to do these checks for the types of non-orthogonal structures that we
all work on. Maybe we don't work on them every day.  But, we all see them
over the course of a given year.  

4)It's absurd for you to say that it's a "waste of time" to do these
calculations for non-orthogonal systems. 
You and I work for software companies!  Who's time are we talking about
wasting? It's certainly not the users time.  We'd be saving them time.  If
I'm a user of the RISAFloor program, it didn't take me ANY time to get this
information because the program did all these calculations automatically. No
time was wasted at all. 

Yes, it took some time for the DEVELOPERS of the program to write the code
that could handle these non-orthogonal systems.  But, why should the design
engineer care about that?  At the very least, these results will give the
design engineer some ideas about the level of vibration.  Therefore, it will
take a lot LESS of the engineer's time when the program does this kind of
work for him.  

Calling this a waste of time is like saying, "deflection isn't likely to
control for this type of floor system.  Therefore, it's a waste of time to
have a program that checks deflection ratios".  So, the program took another
4 nanoseconds to calculate these results.  


Eagerly awaiting your reply.... 

Josh Plummer 
RISATechnologies 


-----Original Message-----
From: Allen.Adams(--nospam--at)bentley.com [mailto:Allen.Adams(--nospam--at)bentley.com] 
Sent: Monday, June 11, 2007 7:46 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: steel - vibration analysis

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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