problem with collider

hi,
i am new in this program, i had problem with collider that i want to make a bridge solid object but i cant because it not 100% square
so i neet to make the bridge solid with out any un used spaces any idea

If you made the bridge with a modelling software: export it .fbx and on importing click on generate colliders and it will generate a collider for its unique shape.
If you made it with unity add collider to every component added