ansys 多层板模态分析

利用ansys的shell281单元建立多层板模型,为什么模态分析结果与实体186单元建模差别很大?感觉应该是shell281单元模型计算的问题,为什么??
建模代码如下:
/filname,shell281
/units,si
lx=0.6 !!!!板长
lz=0.4 !!!!板宽
/prep7
et,1,shell281
sect,1,shell,,Myshell
secdata, 0.01,1,0.0,3
secdata, 0.015,2,0.0,3
secdata, 0.02,3,0.0,3
secoffset,MID
seccontrol,0,0,0, 0, 1, 1, 1
MP,EX,1,7.1e10
MP,PRXY,1,0.33
MP,DENS,1,2700
MP,DMPR,1,0.005
MP,EX,2,5e7
MP,PRXY,2,0.33
MP,DENS,2,3500
MP,DMPR,2,0.005
MP,EX,3,7.1e10
MP,PRXY,3,0.33
MP,DENS,3,4500
MP,DMPR,3,0.005
!!!!!!!
rectng,0,lx,0,lz
esize,0.01
aatt,1,,1,,1
amesh,1
save