Not to be a spoilsport, but I don't recall this type of training ever being done in any of the older dojo or recommended by any of the old masters. I do remember my sensei warning me about hitting anything in training that didn't have "give" to it.
Reminds me of Mas Oyama pounding his knuckles on an old TV show (you as for it) with a large hammer.
Unfortunately, many youngsters and a few oldsters I'm sure, will believe they have just discovered an ancient and venerable training mechanism and will promptly add kicking and punching steel girders in their dojo instead of a heavy bag!
Side note: One of my students here in Florida was the recipient of such training many years ago. (kicking steel girders with his toes) Today he has a difficult time standing because of the constant pain in his feet. (but of course, years ago he could impress kids and girls with his tough feet)