I've been looking into this myself. The rules are obviously written with things like Doors in mind.
I am Society for Creative Anachronism heavy fighter who fights in steel (Pathfinder) "Half-Plate". I can tell you, my plate armor is not even a 1/4" thick (If it was, I wouldn't be able to move!)
Typically steel body armor is made from 20gauge steel 3/80" (.0375) thickness. But since it's heroic fantasy and we're dealing with medieval steel we'll jack the thickness up to 16gauge 1/16" (.0625) thickness.
(1/4" would be 3gauge (.25))
Armor hit points is: 5xAC bonus for each INCH of thickness
This would mean that 16gauge Full Plate Armor would be 5x9x.0625"=3HP rounded up.
1/4" Full Plate (which btw is heavy enough to stop a .45caliber bullet at 20', we tested it :D ) would have a whopping 11hp but only a Dwarf could fight in it.
Plate Armor is very complicated. It's put together with rivets, leather straps, precisely articulated joints and sliding services and is designed to distribute weight across the body so it doesn't unduly hamper movement, or pull swings with momentum. (mine still does, its a huge disadvantage in speed to fight in steel vs. lighter materials)
So Heavy Full Plate Armor, the kind a Jousting Knight would wear, which weighs around 90lbs (from Claude Blair's European Armour Circa 1066-Circa 1700.) but since we've added thickness we'll just say 100lbs. Would be rocking 3HP and a Hardness of 10.
This is more accurate. It reflects the historical movement in tactics, once heavy plate armor was introduced, of attacking a knight's Armor instead of trying to kill the knight himself. Footmen employed Heavy Maces, Picks and Bec de Corbin type weapons specially designed to break the armor of these battlefield juggernauts. Once you get past the Hardness of the armor and damage it the Knight would have to retire from the field to repair his armor.
Keep in mind that a Heavy Mace does 1d8dmg, a Heavy Pick does 1d6dmg and even the highly specialized 2-handed Bec de Corbin does 1d10. That means only an attacker's STR modifier would have a chance of dealing damage to Full Plate, and only if Max Dmg was rolled on the die.