ANSYS - prestressed column with increasing load

[HStruct]

Dear FEA gurus,

I am wondering if somebody can help me with this situation.

1. I have a column (with 1000x1000x2000 units dimentions) compressed by a load of 100 units.
2. After the solution is done in ANSYS I want to keep the column in compression (prestressed) and add another compressive load of say 100 units.

It seems pretty simple but the the column is not holding the stresses due to first loading during second loading. I have tried Prestres option however doesnt seem to work.

Could anybody please suggest me how to do it?

-HMT

!--------------Code ----------------
/PREP7 $SMRT,OFF
ANTYPE,STATIC
ET,1,SOLID45 ! 3-D SOLID ELEMENTS
MP,EX,1,0.6
MP,NUXY,1,0.25

! Y direction is longest, and hence is the height of column
BLOCK,0,1000,0,2000,0,1000,
LESIZE,ALL,,,15

MAT,1
MSHK,1 ! MAPPED VOLUME MESH
MSHA,0,3D ! USING HEX
ESIZE,,5
VMESH,ALL

FLST,2,1,5,ORDE,1
FITEM,2,3
DA,P51X,ALL,

PrLoad = (1000/(1000*1000)) ! Pressure = (load/(XX*ZZ))
FLST,2,1,5,ORDE,1
FITEM,2,4
SFA,P51X,1,PRES,PrLoad

PSTRES,ON
FINISH

/SOLU
SOLVE
SAVE

PSTRES,ON
FLST,2,1,5,ORDE,1
FITEM,2,4
SFA,P51X,1,PRES,PrLoad

SOLVE
SAVE
FINISH