Random loads (PSD) + Static Loads (inertial acceleration)

j.marotta over 4 years ago

Hello,

I'm concerning with Random analysis using either MSC Random Tool either Nastran.

I have a PSD table (Hz vs g^2/Hz) on wich I need to superimpose a static load expressed in g.

So I have a random "load" plus a static load, how can I evaluate their effects when acting together?

Thank you

0 Don.Graff over 4 years ago

Please take a look at the following article:

simcompanion.mscsoftware.com/.../index

Is the static G loading defined at discrete frequencies? If so, it might be that doing a 'sine burst' is what you are looking for... in which case a spreadsheet is supplied to help convert the sine burst at a specific frequency into a psd to add in with your existing psd.
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0 j.marotta over 4 years ago

Hallo, thank you. Unfortunatelly my G load is not defined at discrete frequencies or like a sine. Typically I've a PSD

Hz G^2/Hz

20 0.05

100 0.2

2000 0.05

plus a static load expressed as:

nx 10g

ny 10g

nz 10g

It is quite different from a sine.
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0 Don.Graff over 4 years ago

What specifically is this meant to represent? If you are looking for the static load to act as a preload, then that is done using STATSUB.

I did a quick search and found where a coworker answered a similar question in the past... here is that comment: "Normally one would not combine the static result with the random result since this may lead to misinterpretation of the stress data. The static stress is the average static stress value and the RMS is the mean vibratory stress value. I don’t know what this would represent in a physical sense although one could add the static stress value to the RMS stress value. I think you will have to add the static and RMS stress values outside of either Patran or MSC Nastran to obtain a sum."
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