Wrong Mode shapes??

[pumpkinseed]

I did a modal analysis of a cabinet,its a big cabinet 72" tall,30" wide and 35" deep. Its an electrical cabinet with various electrical components in it. When I ran my modal analysis I am seeing my modes in components and not the entire assembly?The first freq is around 9Hz.I dont see how the whole assembly would deform ..like deformation from front to back or side to side..but i see maximum deformation in some structural components inside the cabinet instead of the whole unit.Can that be a concern? Static structural doesnt show stresses higher than yield stress.

 

[Drej]

Sounds like you have probably requested to output modes in your model within a particular frequency range (ie 0 - 100 Hz) or else a certain number of modes only (ie 20), and therefore the only modes you see are these internal components. Increase the number of modes to be output or the frequency range.


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[pumpkinseed]

Thanks for your suggestion.I tried it but it didnt work,can you suggest anything else? can it be those components are not rigid enough?

 

[seymours2571]

You could very well have the wrong material properties set for the internal components, but as Drej indicated increasing the output or frequency range should have shown you some results, unless your material properties are way out of whack.

Could you also have the wrong material properties for the cabinet itself? Could it be too stiff in your model?

Steve

 

[pumpkinseed]

Hi Thanks for your answer.Material properties are correct.What i read from other articles,typically a telecommunication cabinet or other electrical cabinets that big,when tested for seismic analysis have modal frequency around 10Hz.Actually 10Hz is desirable,usually its less than that.In the mode shapes i am getting, for eg the first mode shape,it shows the door having maximum dispalcement rather than the whole cabinet moving and the frquency is around 10Hz.But sheet metal parts(door),should they not have really high frequency?why is it showing in that small freq range?the way i am reading it is the door is showing 10Hz freq rather than the cabinet,is that correct?

 

[seymours2571]

It all depends on the overall stiffness of the door or any other part in your assembly, in which the total stiffness is a function of material stiffness, geometrical stiffness, and any stress stiffening effects.

Are you able to go through the modal results and find the first mode that pertains to the cabinet as a whole? I know that in past analyses of my own sometimes you have to wade through some of the low freqency mode shapes that pertain to non-critical components.

Also, with respect to the door, is the first mode shape that you are seeing a displacement normal to the front surface of the door? Reason being, is that if the door is just a simple sheetmetal door with folded edges and hinges I would expect a low freqency mode with a shape that resembles a deflection at the center of the door. Basically, this mode would be expected to be at a low freqnecy and I would be more concerned with the mode shape and freqency associated with displacments of the overal cabinet structure.