by Randy Carey:
Whatever your architect suggests. There are no guidelines. I don't even know why it would be steel?
by Ken Golliher:
A historical footnote only: At one time the regulations implementing Bank Protection Act contained specific structural requirements for vaults, lighting, etc., aka minimum security devices. They were removed as it became apparent that there were many different ways to provide acceptable levels of security and there was no need to standardize. There never was an official standard on this point.