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Re: Vibration problems

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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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