How hard is it to break a window?
Roger has already discussed this at length, but noted the lack of stats for any form of strengthened glass - except specific cover DR figures, with no details of weight or HP or DR, when military-grade glass is used to protect pilots in certain vehicles. With reinforced glass (toughened, laminated, you name it) now the norm for doors and windscreens, and widely used in civilian applications, this is an irritating omission.
So let's find out: How tough is security glass? I chose this video at random.
The security glass here is two 4mm-thick sheets connected by laminate film. That's slightly thicker than the standard 1/5" glass from the Structural Damage Table (p. B558), which is plate glass. That has DR 1, pseudo-ablative, and 3 HP, and is Brittle. This glass... is rather different.
Three 4kg balls dropped from 9m is, in GURPS terms, three 9 lb. homogenous objects dropped from 10 yards. The Object Hit Point Table gives 16 HP for these objects. Consulting the Falling Velocity Table (p. B431) we get a velocity of 15. With the damage equation (HP x velocity)d/100, we get 240/100, or 2.4d of damage per ball. On average, each inflicts 10 damage.
The window survives these impacts, absorbing 30 HP of damage. It's clearly damaged, indicating that its DR doesn't resist 10 damage but actual injury (that is, loss of HP) has occurred. Nevertheless, it hasn't been destroyed as an object. You can still look through it, and solid objects can't pass through it, let alone a burglar. If it were typical plate glass, it would have made three HT rolls to avoid destruction, at HT-2, HT-5, and HT-8 respectively, the last nearly impossible.
We could say that HP have increased. We could also say that the security treatment has increased its HT and made it less likely to fail rolls and "die", though increased HP also does this by making it harder to penalize the HT roll. It has also removed the Brittle limitation from the glass, since the film stops it from shattering into tiny fragments the way that disadvantage works (p. B136). Instead, like a wooden door or a concrete wall, it's suffering damage but remaining broadly functional, and will eventually be too flimsy and broken-apart to act as a barrier.
The glass also takes ten axe blows to break through. This doesn't clear it, but after this it would be relatively easier to strip the glass out of the frame. An axe does swing+2 cutting damage, which for a typical human (the presenter doesn't look notably strong or weak) is 1d+2, or 5.5 damage on average. Assuming the same pseudo-ablative DR 1, which is gone after the first cutting attack, he does 51 damage by the time he breaks through the window, and like the ball test, it's still not destroyed.
If damage exceeding the object's HP is sufficient to create a hole, it looks like the security glass is roughly equivalent to a 3" wooden door (DR 3*, HP 33).
Honestly, composite material makes it complex to stat up - you'd probably want two separate sets of stats for the rigid glass and the flexible, rubbery laminate. Blunt objects shatter the glass easily but struggle to damage the laminate. Cutting attacks sever the laminate once they've got through the glass, though it's still hard work. Bullets and other hard piercing attacks can get through fairly quickly but do very little damage (effectively, they're dealing with the glass as cover DR, not really attacking it). And soft things bounce of it with minimum damage anyway. GURPS could in theory handle this, but it's below the necessary resolution for most use cases, which are "how difficult is it for me to get through this barrier" or "how long do I have before the zombies break through this glass door?".
Based on what I've seen here, it seems like an ordinary person punching at security glass should have no effect whatsoever unless they are phenomenally strong. A punch does thrust-1 crushing damage, and anyone below ST 15 tops out at 1d, so DR should probably be 3-5 (I don't have an assortment of friends of varying strength to test their punching abilities on security glass and break their knuckles, oddly enough). To be fair, I don't think I could punch hard enough to damage any modern window - perhaps some of those thin, single-glazed Tudor ones. I remain sceptical about pseudo-ablative DR for glass, too.
Yerwhat, you cry? Pseudo-ablative DR (p. B559, not using that term) is a special form of DR for... basically non-metal objects. Repeated pointy attacks that do 10 or more points of damage to the same spot reduce DR by one each. Repeated cutting, bashing, burning etc. attacks simply remove DR as though it were hit points!
There are lower limits for wood and stone (you can't reduce DR to less than 1 for wood, for example), but honestly, I am very sceptical that by repeatedly punching a door I could eventually smash it. Glass has no limit, so in theory anyone can slowly punch their way through glass of infinite thickness (you'll want gloves). I'm not sure why this exists except to make very bullets and knives slightly less effective against objects by effectively giving things more hit points against bullets and knives. I mean, they hadn't invented Damage Resistance (Piercing and Impaling Only) yet. It's weird and complicated and I'm not at all sure it's convincing. You get things like "it's easier to chip a hole through this stone wall by hitting it with an iron sphere in the same spot than by hitting it with a chisel in the same spot", which are nonsensical.
In fact, looking at it, that is very clearly exactly what it's for. It superficially looks like it must be to allow for repeatedly striking the same place to chip through, but in that case the results are exactly backwards: fire and hammers and acid are more effective drills than the actual tool you use for this job. Object DR scales linearly with thickness. You have a weird edge case of some materials having flat DR that scales, and others having non-scaling actual DR (wood, glass, stone) plus extra hit points against piercing and impaling weapons. A sheet of iron is much harder to damage at all if it's thicker, but nothing else is. This makes some sense for really thin metal, but surely you can say the same for any very thin substance vs. something thicker? I could punch through a sheet of 1/100" inch oak easily but a two-inch oak wall will break every bone in my arm without budging an inch.
The thing is, you can also drill through solid steel with a sufficiently hard tool, just like you can through stone, or wood, or earth.
I think any system that really handles very hard and tough materials, and dense materials, and things you can chip away at, and very thin and very thick materials, would be very complicated.
I dunno. I'm not here making GURPS: Breaking Things, am I? Let's move on.
I'm not here to overrule the rulebook, so let's stick with their concept. Considering that the blunt object is notably not very good at smashing the glass, I'd probably suggest that ordinary security glass (1/5" to 2/5", for GURPS purposes) has DR 5*, 30 HP, cover DR 4.
