求助：箱涵计算中地基采用弹簧来模拟，只添加了垂直竖向力，但是计算结果中箱涵总是存在水平向右的变形，问题在哪里？

求助：箱涵计算中地基采用弹簧来模拟，只添加了垂直竖向力，但是计算结果中箱涵总是存在水平向右的变形，问题在哪里？命令流如下，请大神帮忙
FINISH
/CLEAR,NOSTART
/FILENAME,Submarine Box Culvert Caculation
/TITLE, Submarine Box Culvert under the ground
/prep7
k,1,
k,2,7.55,
k,3,15.1,
k,4,,5.6
k,5,7.55,5.6
k,6,15.1,5.6
et,1,beam3
r,1,1.6,0.133333,1.6,0.215e-16
r,2,1.6,0.133333,1.6,0.215e-16
r,3,1.6,0.133333,1.6,0.215e-16
mp,dens,1,2.5
mp,ex,1,3.25e7
mp,prxy,1,0.167
et,2,combin14,0,0,2
l,1,2
l,2,3
l,1,4
l,4,5
l,5,6
l,2,5
l,3,6
lsel,s,loc,y,0,
lsel,a,loc,y,5.6
latt,1,1,1,,,,
lesize,all,,,20
lsel,s,loc,x,0,
lsel,a,loc,x,15.1
latt,1,2,1,,,,
lesize,all,,,20
lsel,s,loc,x,7.55,
latt,1,3,1,,,,
lesize,all,,,20
lsel,all
lmesh,all
nsel,s,loc,y,0
*get,nnum,node,,count,
*get,nmin,node,,num,min
*dim,aa1,array,nnum
*dim,aa2,array,nnum
*do,i,1,nnum
aa1(i)=nmin
ngen,2,2000,aa1(i),,,,-1,
aa2(i)=aa1(i)+2000
e,aa1(i),aa2(i)
esys,0
type,2
r,i+nnum,6000000
real,i+nnum
nmin=ndnext(nmin)
*enddo
finish
/solu
nsel,s,loc,y,-1
d,all,all
allsel
acel,,9.8,
lsel,s,loc,y,5.6
esll,s
sfbeam,all,,pres,361.7
allsel,all
solve