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RE: Vibration problems

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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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