combin14单元

我要模拟悬臂梁端部加阻尼器的自由振动，为什么加了combin14单元算到的自振频率和不加时的一样？
fini
/clear
/prep7
et,1,beam3
mp,ex,1,2.1e11
mp,prxy,1,0.3
mp,dens,1,7836.7
r,1,0.1,0.0013333,0.4
*do,i,1,21,1
n,i,(i-1)*1
*enddo
*do,i,1,20,1
e,i,i+1
*enddo
d,1,all
et,2,combin14
r,2,0,1
keyopt,2,1,0
keyopt,2,2,6
n,22,20
n,23,20
type,2
real,2
e,22,23
d,23,all
cp,next,ux,21,22
cp,next,uy,21,22
cp,next,uz,21,22
cp,next,rotx,21,22
cp,next,roty,21,22