Besides being a code violation to use structural steel as a means to grounding (bonding) a box, the steel itself is used as a grounding electrode as required by 250.52 (A) (2). The box needs to be bonded, not "earthed" as Mike Holt likes to say.
"This is why I like the newer handi-boxes and 4S's that have the raised area for the ground screw."
Those boxes work beautifully. We use them exclusively at our shop. Now there's no excuse for not installing the pretty little green hexhead screw and bonding the device properly.
