Hit points and damage thresholds for defensive structures

Question

The party is deep into my 5e-updated classic Greyhawk Giants series.

The giants are about to attack the outer walls of the Sterish city of Headwater in an attempt to take the city.

Many monsters, such as the Ogre Battering ram, have the Siege Monster property, which says "The [monster] deals double damage to objects and structures."

The DMG (255, 256) has rules for siege equipment, their attacks, and damage.

On hit points for objects, the DMG also says that stone objects have AC17 (246) and that (247):

An object's hit points measure how much damage it can take before losing structural integrity. Resilient objects have more hit points than fragile ones. Large objects also tend to have more hit points than small ones, unless breaking a small part of the object is just as effective as breaking the whole thing. The Object Hit Points table provides suggested hit points for fragile and resilient objects that are Large or smaller [Large, resilient objects have a suggested 27 hp]...Normal weapons are of little use against many Huge and Gargantuan objects, such as a...towering column of stone...You can track a Huge or Gargantuan object's hit points if you like, or you can simply decide how long the object can withstand whatever weapon or force is acting against it. If you track hit points for the object, divide it into Large or smaller sections, and track each section's hit points separately. Destroying one of those sections could ruin the entire object...Damage Threshold. Big objects such as castle walls often have extra resilience represented by a damage threshold. An object with a damage threshold has immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes damage as normal. Any damage that fails to meet or exceed the object's damage threshold is considered superficial and doesn't reduce the object's hit points.

So it seems like, RAW, I assign hit points to large (10' x 10') sections of a gargantuan (20' x 20'+) stone wall, assign a damage threshold for the wall, and then apply damage as rolled until the wall fails its structural integrity, at which point the attackers move through the rubble as difficult terrain.

I can't find any guidance in the DMG for what the appropriate damage threshold would be for a stone wall, or, for that matter, how hit points and damage threshold might vary between a stone wall, a tower (round or square), a gatehouse, etc. While I could, of course, just make these up, I wonder if there is any guidance in other official sources, or at least analogies that can be made with, for example, the vehicle rules (in BG:DiA) or the ship rules (in GoS).

Somewhat Related: How do I account for palisades and other similar defenses in an invasion?

Very related, possible duplicate, but all answers there predate publication of GoS and BG:DiA which this question is specifically soliciting: How should damage thresholds be assigned?

Answer 1

Strongholds & Followers by Matt Colville has some interesting bits about those two things, but it's more about the mechanics of how you roll for a unit's damage instead of "how much HP does a fort have"

Is there an official threshold? No. But that's great because, like you said, we can make one up!

What is HP?

HP is less of a pool, and more of a "how likely is this event going to kill you?" 1 point of damage is very significant! It means you're more likely to die. 10HP commoners aren't blown about by winds, they're farmers and such, and yet they only have 10HP. You get more as a player because you get better at combat. A strike that would kill an inexperienced person won't kill you because you can take a punch, or dodge a knife better, or tend to your own wounds more effectively, and so on and so on. That being said, let's say 1HP is something VERY significant, but doesn't mean certain death. I like to think of it as breaking an important, but not incredebly strong, bone. Like a rib! Apparently 3-4kN of force is required to break a bone like a rib. Gonna say 5kN, makes it more certain that it will break, definitely gonna remove 1hp but hopefully not more (2 ribs splitting 5kN, about 2.5kN < 3kN, not certain to break). If 5kN is enough to do the smallest amount of definnitely-significant damage, then what about a stone wall of a fortress? Ten lots of 5kN would probably be enough to kill an average person (10 lots of 1hp). I mean, broken bones, arteries, ruptured lungs at that point? Probably enough. And while cummulative damage could take out a person (death by 1000 cuts) I like to think that this is kinda the damage threshold. Ten strikes until destruction.

What is damage threshold

You've already done research into this, but it's the lower limit of damage causing, well, damage. A 1kN strike is DEFINITELY gonna bruise you, but it won't put you at mortal danger. I like to think the damage threshold of a humanoid is 1HP. If 1kN is 0.2 of 5kN, then you could argue that if 5kN causes 1HP of damage, 1kN causes 0.2HP, yet 50 of them probably won't kill you. Hurt like hell, though.

Structure HP

I think we could apply the same rule of "Ten 'till destruction" to a stone wall, or maybe any other kind of construction. Take this YouTube video. Sure, they hit it quite a bit, but it looks like around only 10 strikes actually caused structural damage. Again, could break it in 1000 small hits, but you can pickaxe through a stone wall with about 1000 strikes too if the wall was bad enough.

Structure Damage Threshold

Imagine a fragile stone wall, the whole 27HP, AC 17, 10ft high. A raging barbarian, first level, max stats, could technically destroy the entire thing, have the whole thing fall down, in 2 hits if they get lucky. It's more likely with a warhammer that they take it down in 5ish hits. Thin stone wall of a bar, that feels right. Taking the thickness of an average brick, a castle wall is 50ish times thicker. 1350hp if we're just multiplying here. A person whackin' away at the wall? Not gonna do anything. But a trebuche? Well, apparently it takes a LOT of strikes from one and is mostly used for psychological reasons, but Meteor Swarm is a lot like a beefed-up version of that, enough for a rock to probably start damaging the wall to a fortress. An attack whos average damage PER ROCK STRIKE is 120HP, almost 135HP. In otherwords, 1/10th of the total HP of that wall.

Proposed Solution

My approach with all of this would be the following: Calculate the total HP for the structure/part of the structure. You seem to have a method of doing this, so that's all good. Then the threshold should probably be about 1/10th of the total HP. Combine this with an AC for the material, I think that's your best bet: A bard won't chip through a wall, but a barbarian with a hammer could probably take a decent house down. Again, there are MANY different methods of calculating this. But if you're looking for an algorithm that doesn't set the bar too high, I think you could do worse than this! Lemmie know your thoughts!