Need a book? Engineering books recommendations...

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

RE: steel -vibration analysis

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dr. Murray -

Gosh, it's certainly a pleasure to have someone of your stature participate
in this thread!

There are, however, a couple of things that I'm not fully understanding
about your response. In particular, related to the example in chapter 7 of
your design guide.

1) You say that the DG11 procedures could "certainly be used to evaluate the
interior 45 degree beams, but not the triangular areas themselves".  Can I
take that to mean that the beam panel (or is this considered a girder
panel?) equations from the design guide will give some reasonable prediction
of the vibration problems of this girder.... Even though this region has
significant non-orthogonality between the Joist to Girder framing?

2) In your experience, would a transient analysis (perhaps one which varies
the LOCATION of the excitation along the walking path) also be able to give
some reasonable prediction of floor excitations.  That's a slight variation
of some of the work I've done in the past... Excitation of steel
(non-building) structures due to rotating equipment.  A bit more rigorous
than a simple modal analysis, but still workable.

Would it be possible for you to give me some more detailed information
(dimensions, member sizes and such) about the structure in that chapter 7
example (Figure 7.2)?  I would love to use that example to further explore
the DG11 calculations for a irregular structure like that....especially one
with a proven history of vibration problems.  

Sincerely,

Josh Plummer, SE
RISA Technologies 

-----Original Message-----
From: Tom Murray [mailto:thmurray(--nospam--at)vt.edu] 
Sent: Sunday, June 17, 2007 10:47 AM
To: seaint(--nospam--at)seaint.org
Subject: steel -vibration analysis 

The recent postings concerning floor vibration analysis have been
interesting.  I am the first author of the AISC/CISC Design Guide 11, "Floor
Vibrations due to Human Activity" and the author or co-author of a number of
paper and articles in AISC publications.  I admit that I never stated that
the procedures in these publications apply only to rectangular or nearly
rectangular bays but they do.  It never occurred to me or my co-authors that
someone would want to apply the procedures to triangular or trapezoidal
bays.  All of the examples are for rectangular bays, which I guess we
thought would define the limits of the DG11 or earlier procedures.  If they
could be applied to other bay configurations, we would certainly have
provided examples for such.

I have been involved with floor vibration evaluation for over 40 years.  I
get questions on a daily basis and reports of floor problems at least
monthly.  There has never been a report of a vibration problem that did not
involve rectangular or nearly rectangular bays.  Once I tried to measure the
response of a triangular bay and was not able to excite it sufficiently to
get a good measurement.  Whenever anyone asks about the vibration of
significantly non-rectangular framing, I tell them that I would not be
concerned and that there is no need to do an analysis.  Now if the bay is
slightly non-rectangular, I recommend, as an approximation, to use the
average span length in the DG11 procedures. What's slightly?  Engineering
judgment is required.

On the subject of using FE analysis for walking vibrations.  We at Virginia
Tech have been working on this problem for several years.  If one "walks" a
standard footstep pulse like the one shown in Figure 6.3 of DG11 at a
sub-harmonic of the predicted floor frequency, a very high acceleration will
be predicted by the FE model.  Actual walking will not result in an
acceleration nearly that high.  The reason for the difference is that the FE
"walking" will be at a precise sub-harmonic frequency whereas actual walking
will rarely exactly match the sub-harmonic frequency.  Further, I have yet
to read a paper where FE analysis was successfully used to predict measured
floor response due to walking. (Rhythmic excitation is a different
matter.) There have been a few papers on footbridges, but none on floors to
my knowledge.  One other comment, I get questions periodically from someone
who is trying to do a FE analysis for walking.  The most common question is
"what I do with all of the frequencies that I get?"

Concerning Figure 7.2, this floor was actually "fixed" by Dave Allen, one of
the DG11 authors.  Dave provided the following: "The two interior beams at
45degreesadjacent to the atrium support a busy walkway shared with an
office. These beams supported a small mass and had a low natural frequency
(about 4 Hz), hence the biggest walking acceleration (about 1%g) throughout
the floor. The girders in the bay to the left were stiffened for the same
reason as for many other bays. The whole floor was unacceptable based on
Chapter 4 (about 0.8%g). However it was acceptable based on the CSA S16
Appendix. The girders at 45 degrees at the exterior walls were not a
problem."  Note that it was not any of the triangular bay framing that
caused the problem.  Certainly, DG11 procedures could be used to evaluate
the interior 45 degrees beams, but not the triangular areas themselves.

Hope this helps.

Tom Murray





Thomas M. Murray, Ph.D., P.E.
Montague-Betts Professor of Structural Steel Design Department of Civil
Engineering Virginia Tech Blacksburg, VA 24061
540-231-6074
540-231-7532 Fax


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********