COMBIN14 for FULL TRANSIENT ANALYSIS

[Babar2000]

Hi guys!
I'm trying to learn how to master transient analysis in ANSYS.
I ve decided to test a simple analysis on a mass-spring-damper system undergoing an harmonic force of the shape: 4sin(20t).
I used COMBIN14 ans MASS21 elements.
I've computed on excel the exact solution and compared it to the result given by ANSYS.
There is big differences between the exact solution and the results that i m not able to explain.

Here is the lines I used:

/prep7
n,1,0,0,0
n,2,0,0,1
et,1,COMBIN14,0,0,0
et,2,MASS21
r,1,1421,5.27
r,2,,,1
MP,DAMP,1,0.07
nsel,s,node,,1
d,all,all,0
cm,CL,node
nsel,s,node,,2
cm,M,node
alls
real,1
TYPE,1
e,1,2
real,2
TYPE,2
e,2
*dim,accz,table,101,1,1
*tread,accz,Acel,txt
/SOLU
ANTYPE,TRANS
TRNOPT,FULL
*do,t_time,0.0314,3.14,0.0314
f,M,fz,%accz(t_time)%
TIME,t_time
SOLVE
*enddo
/POST26
nsol,2,2,u,Z

I also tried with COMBIN40 elements without more success.

Thanks for your help.