You need to use the quadratic version of beam188 like in the test below:
/prep7
et,1,188,,,2 ! Default beam188 only has one Mz, same at both ends
mp,ex,1,3e7
mp,nuxy,1,0.3
sectype,1,beam,rect
secdata,1,1
n,1
n,11,20
fill
e,1,2
*repeat,10,1,1
d,1,all
f,11,fy,100
fini
/solu
solve
fini
/post1
set,last
etab,mzi,smisc,3
etab,mzj,smisc,16
plls,mzi,mzj