Random vibrations - Several PSD accelerations - (ANSYS WB)

[leecheng]

Hi everybody,
I have a problem with a random vibration analysis.

My structure have 2 boundaries conditions :
- area 1 : translation blocked in Y,Z directions. Rotations free.
- area 2 : fixed. All DOF blocked.

I apply, in Z directions, 2 PSD accelerations, one in displacement and one for the fixed area. For information, the PSD accelerations are the same.
At low frequencies, regardless of the boundary conditions and the considered node, I should logically recover at least the acceleration PSD imposed. But this is not the case, I get half.

I think that ANSYS WB consider the 2 PSD accelerations as 2 different PSD spectrum but they are the same. So, I insert this command :

psdfrq,1,2,10.,100.,200.,300.,400.,499.
psdfrq,1,2,500.,2000.
coval,1,2,0.1185, 0.1185, 0.1185, 0.1185, 0.1185, 0.1185
coval,1,2,0.1185,7.57e-003

But I get the same result.
Do you have any idea to solve this problem, I thought about a command that apply the PSD acceleration only on the nodes where the Z direction is blocked ?
Can I have a help, please.