9 lines (5 with data), 234 Bytes

AnyVar l0 = lr/(er+1);
AnyVar eps0 = 0.03;
AnyVar eps = (meas.Pos[0]-l0)/l0;

AnyVar Stiffness = k*l0;

AnyFloat Val = {iffun(gteqfun(eps,0.0),iffun(gtfun(eps,2*eps0),-Stiffness*(eps-eps0),-Stiffness*1/4*(eps^2/eps0)),0.0)};