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RE: Vibration problems

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Gunnar:

References:
Steel Tips, Structural Steel Educational Council, September 1991, "Design
Practice to Prevent Floor Vibrations", Farzad Naeim.

American Institute of Steel Construction, Engineering Journal, 3rd Quarter
1991, "Building Floor Vibrations", Thomas Murray.

Dr. Murray is a professor at Virginia Polytechnic Institute, in Blacksburg,
VA.  
His e-mail address is: thmurray(--nospam--at)vt.edu
Dr. Murray has focused much of his academic efforts in the area of vibration
and human perceptibility.  

Human perceptibility of floor vibrations is an interesting problem because
it depends on both frequency and amplitude according to the Reiher -Meister
chart.  There is also a Canadian Standards Association scale devised by
Allen and Reiner to determine acceptability of vibrations.

Per Murray:
The average walking frequency is 2 hz.  If only casual pedestrian traffic on
a pedestrian bridge or platform is anticipated, the following formula is
recommended:
	D > 35 (A sub 0) f + 2.5 
		D = damping in percent of critical
		A sub 0 = maximum initial amplitude of the floor system due
to a heel-drop excitation (in)
		f = first natural frequency of the floor system (hz)

One of the more clever solutions to a vibration problem was a container that
was suspended from the floor structure with springs or mounted on
elastomeric pads on the bottom flange of the beams.  The container was
filled (tuned) with water until the vibration (heel drop) was no longer
perceptible.  

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Gunnar Hafsteinn Isleifsson [SMTP:gunnarhi(--nospam--at)post4.tele.dk]
> Sent:	Tuesday, October 02, 2001 5:19 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	RE: Vibration problems
> 
> The problem is, as I understand it, that the average step frequency of a
> person walking along the platform, lies very close to the eigenfrequency
> of the structure, whereby there is a risk of resonance effects. The access
> platform, suspension rods and the transverse beams are very stiff, so the
> problem lies within the roof girders, which span about 100 feet.
> 
> A sales representative for a company specializing in vibration dampers,
> suggested to install a three inch thick soft elastomeric pad in between
> the top of the suspension rods and the transverse beams, and stated that
> these pads had an eigenfrequency equal to 6 Hz, i.e. much higher than the
> 1.5 Hz eigenfrequency of the structural system (I don't understand how
> that fits together).
> 
> I attended a lecture this evening at The Danish Society for Structural
> Science and Engineering, where the subject was vibration problems in
> office building, and asked the lecturer about possible solutions. His
> response was almost identical to what Christopher Wright says in his
> reply, and suggested that the only feasible solution was installing tuned
> mass dampers, either on the platform or on the roof girder itself.
> This would require reasonably accurate frequency analysis in the design
> stage for the design of an appropriate mass damper and vibration
> measurements, for mass adjustments, after completion of the structure. 
> He added that one could avoid the frequency analysis and just measure the
> vibration characteristics after completion of the structure, but that
> could delay the project, since it takes about a month to measure, analyze,
> design, produce and deliver a tuned mass damper.
> Is this solution like "shooting sparrows with a shotgun" (does that phrase
> exist in English?).
> 
> With best regards and thanks to all who responded. Further comments are
> most welcome and appreciated.
> 
> Gunnar Hafsteinn Isleifsson
> Denmark
> 
> 
> -----Original Message-----
> From: Daryl Richardson [mailto:h.d.richardson(--nospam--at)shaw.ca]
> Sent: Tuesday, October 02, 2001 18:16
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Vibration problems
> 
> 
> Gunnar,
> 
> 	It's not completely clear to me what you are analyzing, however, you
> appear to have three options.
> 
> 1.)	Revise the analysis.  Review your analysis to be sure you are really
> analyzing the problem you think you are analyzing.  Dynamic analyses can
> be tricky and the results might not mean what you think they mean.
> 
> 2.)	Revise the requirements.  Review the requirements to see if they can
> be changed to be compatible with what you can produce.
> 
> 3.)	Revise the design.  Even if you could stiffen the girders it
> probably wouldn't work; to double the frequency you have to increase the
> stiffness-to-mass, k/m, ratio 4 times.  You probably need to remove the
> girders from the load path altogether; dispense with the transverse
> beams; and install new beams or trusses (which do not share in the roof
> loading) parallel to the existing girders.
> 
> 	Hope this is helpful.
> 
> 				Regards,
> 
> 				H. Daryl Richardson
> 
> Gunnar Hafsteinn Isleifsson wrote:
> > 
> > This isn't actually my problem but one of my colleague's, but here it
> goes:
> > 
> > In an existing structure, consisting of steel roof girders, some
> transverse beams are to be installed in between the girders. From these
> secondary beams, an access platform is to be suspended by steel rods.
> Analysis indicates that the lowest eigenfrequency, for the whole assembly,
> is 1.5 Hz, due almost entirely to vertical vibrations of the roof girders.
> The frequency of the access platform has to be raised to at least 3 Hz,
> and stiffening the girders is not an option.
> > 
> > What possibilities are there. What effect, if at all possible, will it
> have on the eigenfrequency to install vibration dampers in the suspension
> rods. Does this make the roof girders and the transverse beams "think"
> that they are statically loaded, and thus the eigenfrequency is solely
> dependent on the mass and stiffness of the access platform itself. Or am I
> totally missing something essential here.
> > 
> > With best regards,
> > 
> > Gunnar Hafsteinn Isleifsson
> > Denmark
> > 
> 
> 

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