question about the zero-length end springs

[2015James]

I read this website about LimitStateMaterial , http://opensees.berkeley.edu/wiki/index ... ynamic.tcl
but I can not understand why a soft spring is used here? Thank you.

#soft spring to take axial load after failure
uniaxialMaterial Elastic $softMatTag 99.9
element zeroLength 4 4 5 -mat $softMatTag -dir 2

####################################
# Define the zero-length end springs
####################################
#rigid material
uniaxialMaterial Elastic $rigidMatTag 9.9e9

#bottom of column slip spring
element zeroLength 1 1 2 -mat $rigidMatTag $rigidMatTag $centerSlipTag -dir 1 2 6

#top of column springs incl shear and axial
element zeroLength 3 3 4 -mat $shearTag $axialFailTag $centerSlipTag -dir 1 2 6

#soft spring to take axial load after failure
uniaxialMaterial Elastic $softMatTag 99.9
element zeroLength 4 4 5 -mat $softMatTag -dir 2