Tree Bridge

I've been making a map and wanted to add in the doodad under bridges and ramps called tree bridge and I guess some how your suppose to make it fall in the game, and it turns into a bridge but I can't seem to get it to work, when I test the map ,I can't even select it to attack it so basically I'm asking if any one knows how to get a tree bridge to work?

use a trigger that kill it,
if you want attack it by meele you will just attack the air in the middle of the doodad not the tree that fall