What's the easiest way to cut this beam?

Hello, I’m new to Rhino. I need to cut this beam by this horizontal line that will be the floor and I’ve tried with cut and split commands but they aren’t working. I don’t know what’s the way to do it. Thank you

go to an orthogonal side view, select the line then use trim for the overlapping parts then use the command cap, alternatively you can also use boolean split that will result in a capped object but you would have to delete the residual elements manually. just see that you have apparent intersections checked in the options then it does not matter if the line is way behind or in front it does not have to touch it at all.

I think I’ve managed to do it but everytime this weird line shows up and I cannot delete it