On the technical side, the system is quite simple. The beam part is a small #Godot scene consisting of a rigid body with a box collider and box mesh (placeholder graphics). You can then instantiate this small scene at will.
To attach two parts together, I don't use joints (would be bad for performances and physics engine stability). Instead, I merge the rigid bodies, and add all the colliders as child nodes (and also update the total mass). So, the final bridge is a single rigid body.