I think Calc_em is right, did you check the eigenfrequencies of the beam? and if you are creating large strains in the beam then the system is no longer linear…in Kraska example is stated
the masses are represented by beam elements which are rigidized by an equation linking their nodes.
that makes the system linear again probably.
I plotted the input load (StartAndStop) but not sure about the units so didn’t checked the resulting strains
