An image tells thousands words, right, so here it goes:
Both ground and bridge are set as walkable nav mesh statics.
The bridge have box collider? if not.
Create an box collider in at same size of the bridge and bake again. That replace the walk bridge models., and of course hide the box collider.
Well i think i figured it out, so if any one having a similar problem, checking for invisible meshes could help. Because unity is using meshes to create mavmesh it also takes invisible parts of meshes that may cause such promlems.