# Are the participation factor and effective mass calculations correct for a model with a doubly symmetric cross-section?

 In ANSYS Release 10.0, the participation factor and effective mass calculations may be misleading for a model with a doubly symmetric cross-section. See the example below. If the cross-section of the beam is very slightly non-symmetric, the effective mass for mode 1 acts entirely along the Y axis and the effective mass for mode 2 acts entirely along the X axis. However, if the cross-section is exactly symmetric, the effective mass for modes 1 and 2 are distributed between the X and Y axes.This behavior is not an issue in the modal analysis, because the calculated frequencies and total effective masses are correct for both models. However, it could be an issue in analyses that use the effective mass in specific directions (e.g., SPECTRUM). The distribution of the effective mass between the modes is different between the models. To avoid any potential problems, you should include all the appropriate modes that have effective masses in a particular direction in any subsequent analysis.del = 1e-6/prep7et,1,185,,2mp,ex,1,10e6mp,prxy,1,.3mp,dens,1,1e-4block,-1,1,-1,1,0,100esize,.5vmesh,allnsel,s,loc,z,-1e-6,1e-6d,all,allalls/soluantype,modalmodopt,lanb,2mxpandsolvefini/cleardel = 1e-6/prep7et,1,185,,2mp,ex,1,10e6mp,prxy,1,.3mp,dens,1,1e-4block,-1-del,1+del,-1,1,0,100esize,.5vmesh,allnsel,s,loc,z,-1e-6,1e-6d,all,allalls/soluantype,modalmodopt,lanb,2mxpandsolve

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