shock response spectrum analysis in NX.Nastran

[cavva78]

thread825-258197
Hello,
I'm performing a SRS analysis with FEMAP 11.1 and NXN 9.0. I'm following the FEMAP tutorial no.19 doing a modal analysis applying a SRS (acceleration vs. frequency)on a node with a large mass and a SUPORT entry in the excited DOF.
My question is: is it mandatory to use a large mass? What is the relation between the large mass value (tutorial just says 1e+3 to 1e+6 times model mass) and the SRS function? I see that if I delete the mass my results change with a factor about 3.5, but if I change mass value to 1000, 2000 or 10000 results are exactly the same. Is the result independent from mass, and still in standard units (that is m/s/s for accelerations, Pa for stresses, etc.)?

Thanks
Marco

 

[BlasMolero]

Dear Marco,
Do not confuse Response Spectrum Analysis Application with Modal Frequency Response Analysis (SOL111), is a different type of FE analysis at all. If you need to perform a Response Spectrum Application using FEMAP & NX NASTRAN take a look to this tutorial:

http://www.iberisa.com/soporte/femap/dinamico/response_spectrum_4story.htm

Bes tegards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[cavva78]

Dear Blas,
thanks for the answer. I already read your tutorial, and I just have to perform a shock response spectrum analysis on an electronics box. I have my SRS input curve (acc vs frequency) from customer, to be applied to a base node connected to the box constraint points. My question is just on the analysis setup: you (and also NXN manual) say the large mass method must be used with a mass 1e+3 to 1e+6 times the model mass, but in any case the acc. vs freq. curve remains unchanged independently from the mass used? This is my doubt in a few words... I tried removing the mass and results changed, but if I change the mass results are always the same.

Thanks and regards
Marco