prestressed modal analysis of a plate

[jmc109]

Hi,
I'm trying to make a prestressed modal analysis of a plate but the results i'm getting (frequencies) are the same as the results from the modal analysis without the membrane forces i'm aplying. I think i've followed the correct procedure:

1.Build the model and obtain a static solution with prestress effects turned on.
2.Reenter SOLUTION and obtain the modal solution, also with prestress effects turned on (reissue PSTRES,ON).

I'm using the shell91 element with these boundary conditions

DK,1, , , ,0,UX,UZ
DK,4, , , ,0,UX

*DO,i,1,4
DL,i, ,UY,
*ENDDO

and these membrane pressure:

SFL,1,PRES,-3000,
SFL,3,PRES,-3000,
SFL,2,PRES,+3000,
SFL,4,PRES,+3000,



here's the relevant section of my logfile (feel free to point out any errors):


FINISH

/SOL
ANTYPE,0
PSTRES,ON
/STATUS,SOLU
SOLVE
FINISH

/SOLU
/SOL
!*
ANTYPE,2

MODOPT,LANB,5
EQSLV,SPAR
MXPAND,5, , ,0
LUMPM,0
PSTRES,1
!*
MODOPT,LANB,5,0,0, ,OFF
/STATUS,SOLU
SOLVE

FINISH


thanks,
J.