33Hz Fundamental freqnency 2

[RubenOrihuela]

Hi all,

i need to perform a Dynamic anaysis using FEA, with Nastran.

i have question cause, I heard the next. If we will perform an analysis on a model with a fundamental frequency greater than 33 HZ, we can consider this structure as Hiperstatic, therefore, we can make an static analysis instead a dynamic analysis. For a FEA analyst is easier and faster perform a static analysis, if the assumption is true, we can make the analysis with knowledge that the results obtained are good.

But this assumption it's true? which is the theoretical explanation? this are considered in a code?

Thanks and Best Regards.

Rubén Orihuela

 

[PicoStruc]

Number of defree of freedom (is Hyperstatic or determinate) have no relation with dynamic analysis or static analysis performed.

 

[rb1957]

i think the point of the question is linear or dynamic FEA. the the structure's response is high then the structure behaves as though it is "rigid" and not "flexible" and you can do a linear FEA. if the structure's fundamental frequency is low, then even low loading rates can "excite" the structure so you need to do dynamic FEA.

another day in paradise, or is paradise one day closer ?

 

[JoshPlumSE]

You might want to look at ASCE 4 - 1998 (Seismic Analysis of Safety Related Nuclear Structures). Section 3.2.3 has a section concerning including rigid modes in the response. This may be the code reference that you were searching for.

I should point out that this reference really only applies for seismic analysis. If you have dynamic equipment or such, the rationale for how to include the effects of these rigid modes (or even the definition of rigid modes) would not necessarily be valid.

 

[StoyanAndreev]

This depends on the type of loading, its frequency content, the structure you are analyzing and the type of analysis. If you have a seismic loading on a dam or a levee you need to take into account maximum frequencies up to about 10-15 Hz. If you have a standard building without valuable equipment you may stop at about 20 Hz (rule of thumb), but when you have a Nuclear Structure, to assess the equipment you may need to calculate response up to 33 Hz (as it is in the aforementioned ASCE4-98) or up to 50 Hz as it is in NRC's NUREG800. However if you have some high-frequency loading - blast, impact or vibration from high-velocity rotors you may need to consider much higher frequencies - up to 150 or 250 Hz. If you are perfoming a direct integration dynamic analysis you may achieve this high frequencies with appropriate time steps - the Nyquist rule says that you need dt<1/2f to have frequency "f". Also for implicit dynamics you'll need fine enough mesh - 5 to 10 elements in one wave length. If you are performing a modal integration dynamic analysis you need to calculate sufficient vibration modes - up to the frequency of interest "f" and also to include the rigid body modes.

 

[Bagman2524]

Not sure of the reason behind it, but 33 Hz was always the dividing line between a rigid and flexible structure at the nuclear plant I was at.

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