『求助』瞬态分析的问题

哪位高手可以告诉我，为什么做瞬态分析的时候在零时刻的响应不是零呢？很困惑。
命令流如下
/PREP7
!*
ET,1,BEAM3
!*
R,1,0.000513,0.000000012, , , , ,
!*
!*
MPTEMP,,,,,,,,
MPTEMP,1,0
MPDATA,EX,1,,2.08e11
MPDATA,PRXY,1,,0.3
MPTEMP,,,,,,,,
MPTEMP,1,0
MPDATA,DENS,1,,7850
K,1,0,0,0,
K,2,2,0,0,
LSTR, 1, 2
FLST,5,1,4,ORDE,1
FITEM,5,1
CM,_Y,LINE
LSEL, , , ,P51X
CM,_Y1,LINE
CMSEL,,_Y
!*
LESIZE,_Y1,0.04, , , , , , ,1
!*
LMESH, 1
/UI,MESH,OFF
FINISH

/SOL
D,1,all
D,2,all
antype,modal
MODOPT,LANB,20
EQSLV,SPAR
MXPAND,20, , ,0
LUMPM,0
PSTRES,0
MODOPT,LANB,18,15,10000, ,OFF
SOLVE
FINISH

/SOLU
ANTYPE,TRANS
TRNOPT,MSUP
ntime=0.01
myf=1
f,21,fy,0
kbc,0
DELTIM,1e-4
!OUTPR,none
OUTRES,nsol,1
alphad,2.9768
betad,1.01897e-6
LSWRITE
*do,i,1,100
time,i*ntime
myf=-20*(2-1/50*2)*1/50*2*(SIN(i*0.35*62.66)+SIN(i*2*0.35*62.66)+SIN(i*3*0.35*62.66)+SIN(i*4*0.35*62.66)+SIN(i*5*0.35*62.66)+SIN(i*6*0.35*62.66)+SIN(i*7*0.35*62.66)+SIN(i*8*0.35*62.66)+SIN(i*9*0.35*62.66)+SIN(i*10*0.35*62.66))
f,21,fy,myf
kbc,1
!lswrite
SOLVE
*enddo
SAVE
fini

/POST26
numvar,200
FILE,,RDSP
nsol,2,36,u,y,36y