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Harmonic response of a cylinder

Sir I am having a Cylindrical structure where, i want to run a harmonic response analysis to find the transfer function. I have run a free free condition Normal mode analysis to find out the axial mode (along the length) The frequency is at 2500 Hz and i have run another normal mode analysis where i have constrained 5 DOF (Releasing the axial DOF) hear also i have got the Axial frequency around 2500 Hz. But when i want to run them for response analysis, i have created a RBE2 element connecting at the bottom surface of the cylinder to a point which will simulate the base excitation, so I have constrained 5 DOF and I have Given 1 Enforced motion at the point, As soon i solve sol103 response analysis, the result is different from the normal mode analysis the axial frequency is shifted to 900Hz and when i plot the XY graph the resonance is in 900Hz not at 2500Hz plz help to solve this problem thank you karthik