Variable length of the boundary load

Dear all,

I have a simple 2D model, that is a cantilever beam. I would apply on this beam a boundary load, but the LENGTH of the boundary load should change over time (increasing or decreasing).
Just to be more clear, I have the length of the cantilever beam equal to 100 mm. At time t=0, the length upon which the boundary load should be =0 mm, and then it should increase with time increasing, for instance at t=40 the length it's 40 mm etc. Of course, the load per unit length remains constant.

I hope I've been clear
Thank you for any suggestions

Fabio