hmm. gotta be said, I prefer my equation for damage. It incorporates the (in)capability of things to destroy doors, while also weighting destruction towards the big things.
A dragon could destroy all of the hp of the door because it just hurts that much that even the damage resistance of the door does next to nothing. A troll would have to beat and batter for a while, and yes, the goblin could bash all day and cause no damage at all because it doesn't go over the damage reduction of the door/wall
Ah, I think I understand what you are aiming at:
(creature + strength + weapon) - (mat + quality) = damage done
(mat + quality) represents your 'absorption', incorporating my idea of being incapable of doing any damage in the first place. Better yet, you incorporate the whole thing in one equation as opposed to my two. I would go further, and make it (building + mat + quality) though. The Base Strength of the building (a door, a bridge, a wall, a natural wall(*)) plus the material used in that construction plus the quality of that construction).
Possible Problem
I presume your + weapon means adding the weapon damage? If so, this could be circling back to the rediculous position of, say 'Kobold + Masterwork Maul beating down that Adamantine gate'. Need to think about this a bit more.
* I mention this so as to incorporate a miner with a pick mining natural stone - this drives determining how long it takes a miner to mine through a given square (natural wall base plus stone material plus no quality modifier). What is left behind (i.e. stone) is as currently determined. Also incorporate treefelling? (tree base plus wood material plus no quality modifier), with wood cutting labour required to leave something behind (log).