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Effective Mass output in FEMAP 11.1.1


Effective Mass output requests in FEMAP 11.1.1 (Sol 103) seems to be disappeared !!!

Please let me aware about a solution.


Thank you



Re: Effective Mass output in FEMAP 11.1.1

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Dear Marco,

No, simply is hidden in the regular options when defining the normal modes eigenvalue analysis (SOL103). After creation of the analysis expand the analysis set manager and locate the branch MODAL XYPLOT, click there and set your options!!.




Probably FEMAP developers believed that this option complicates the modal analysis interface, making more complex with the many options we have in the analysis tree, but in my opinion this is an error: to have information of the MODAL MASS PARTICIPATION FACTOR is critical in any normal modes eigenvalue (SOL103) analysis!!.


Most FEMAP users do not perform dynamic analysis, but there are many reasons to compute the natural frequencies and mode shapes of a structure. One reason is to assess the dynamic interaction between a component and its supporting structure. For example, if a rotating machine, such as an air conditioner fan, is to be installed on the roof of a building, it is necessary to determine if the operating frequency of the rotating fan is close to one of the natural frequencies of the building. If the frequencies are close, the operation of the fan may lead to structural damage or failure.


Activating both the SUMMARY and the MODAL EFFECTIVE MASS FRACTION will give you insight information of the dynamic behaviour of the model:




After finishing the normal mode eigenvalue analysis using NX NASTRAN (SOL103), the user in FEMAP can plot the MODAL EFFECTIVE MASS FRACTION to see the amount of mass captured by each mode in each direction:




Another interesting XYplot is the SUM OF MODAL MASS PARTICIPATION FACTOR, this value shows the % of sum of modal mass for each mode.




With the above information the user can see how mass is distributed along the computed modes. Also, is critical to decide the number of modes to use when performing either a modal frequency response dynamic analysis (SOL111) or a modal transient frequency response (SOL112) analysis, here a minimum of at least the 80% of the total mass participation factor of the system mass structure is required in the direction of the excitation, then the above values will help the user to estimate the number of retained modes to obtain accurate results.


Best regards,



Blas Molero Hidalgo, Ingeniero Industrial, Director
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