Are the participation factor and effective mass calculations correct for a model with a doubly symmetric crosssection?
In ANSYS Release 10.0, the participation factor and effective mass calculations may be misleading for a model with a doubly symmetric crosssection. See the example below. If the crosssection of the beam is very slightly nonsymmetric, 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 crosssection 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 = 1e6 /prep7 et,1,185,,2 mp,ex,1,10e6 mp,prxy,1,.3 mp,dens,1,1e4 block,1,1,1,1,0,100 esize,.5 vmesh,all nsel,s,loc,z,1e6,1e6 d,all,all alls /solu antype,modal modopt,lanb,2 mxpand solve fini /clear del = 1e6 /prep7 et,1,185,,2 mp,ex,1,10e6 mp,prxy,1,.3 mp,dens,1,1e4 block,1del,1+del,1,1,0,100 esize,.5 vmesh,all nsel,s,loc,z,1e6,1e6 d,all,all alls /solu antype,modal modopt,lanb,2 mxpand solve 

