『求助』Anasys分析的误差为什么这么大？？

以下为我算梁最大剪力（YZ方向）的步骤；结构显示为100MPa左右，见图示位置；但实际应为200MPa左右。为什么会差距这么大？？？？？？
/PREP7
thick=10.1
height=30
length=18
fload=-49000
!*
ET,1,SOLID92
!*
MPTEMP,,,,,,,,
MPTEMP,1,0
MPDATA,EX,1,,210000
MPDATA,PRXY,1,,0.28
BLOCK,0,thick,0,height,0,length,
ESIZE,3,0,
MSHAPE,1,3D
MSHKEY,0
!*
CM,_Y,VOLU
VSEL, , , , 1
CM,_Y1,VOLU
CHKMSH,'VOLU'
CMSEL,S,_Y
!*
VMESH,_Y1
!*
CMDELE,_Y
CMDELE,_Y1
CMDELE,_Y2
!*
FINISH
/SOL
FLST,2,1,5,ORDE,1
FITEM,2,1
!*
/GO
DA,P51X,ALL,
FLST,2,1,5,ORDE,1
FITEM,2,2
!*
/GO
DA,P51X,UY,fload
/STATUS,SOLU
SOLVE
FINISH
/POST1
PLDISP,2
/EFACE,1
AVPRIN,0, ,
!*
PLNSOL,S,YZ,0,1
/USER, 1
/VIEW, 1, -0.999518681539 , 0.247048074979E-01, -0.187637347391E-01
/ANG, 1, 1.75616370213
/REPLO
FINISH
! /EXIT,ALL