Trying to build new log bridge

I’m not happy with the current swamp maple log bridge for a particular map I’m working on, so I thought I would create a new one by pointing to another existing mesh.

I see that walkable surfaces have a blue hit box in the Editor, but those meshes with yellow hit boxes in the editor are apparently unwalkable.
When I copy a DBR of tree_giantmaple_log01a, rename it and put it in my mod folder using the same folder hierarchy, then go in with the DBREditor and change the mesh variable to level art/flora/trees/treeoakdead_log02a.msh (the same as for the nonbridge oak tree log), and then build the new DBR, the editor shows it and allows me to place it. The hit box however is yellow, and my test character wont walk on it.

Is there something else i need to do to make a mesh walkable?
(I did rebuild the map in AM.)