Break of a material/spring

[dgale]

Hello everyone,
I would like to simulate a break of an element by torsion. To do it I think a good way wuold be to create a material like this:

force/stress
|
|...................................o o o o o o o o
|.................................o...................o
|...............................o......................o
|..............o o o o o o o ...........................o
|...........o..............................................o
|.........o..................................................o
|......o
|....o
|.o
-------------------------------------------------------------> Displacement/strain

code:
set Tcr 0.861e6; #[Nmm] Torsor máximo que aguanta la viga de borde
set Tu 1.75e6; #[Nmm] Torsor máximo que aguanta la viga de borde
set phi1 5.6e-6; #[1/mm] Curvatura 1
set phi2 4.72e-5; #[1/mm] Curvatura 2
set phi3 6.6e-5; #[1/mm] Curvatura 3
set phi4 9.9e-5; #[1/mm] Curvatura 4
set K1 [expr $Tcr/$phi1];
set K2 [expr ($Tu-$Tcr)/($phi3-$phi2)];
set F1 $Tcr;
puts "F1= $F1"
set F2 [expr $Tu-$Tcr]
puts "F2= $F2"
set gap $phi2
set b 0.00001; # Eh/E1
uniaxialMaterial Steel01 [expr $Sv1-2801] $F1 $K1 $b
uniaxialMaterial ElasticPPGap [expr $Sv1-2802] $K2 $F2 $gap $b; # Torsor positivo
uniaxialMaterial ElasticPPGap [expr $Sv1-2803] $K2 [expr -$F2] [expr -$gap] $b; # Torsor negativo
uniaxialMaterial Parallel [expr $Sv1-2800] [expr $Sv1-2801] [expr $Sv1-2802] [expr $Sv1-2803];

However, I don't know how to simulate the falling. I created a paralel material with uniaxialMaterial ElasticPPGap to simulate the 2 first steps but if I use the same idea to simulate the falling a warring message comes up. Becouse the slope can't be negative.

How would you do it?

Thanks for your tips in advance.