GURPS Magic...Making and Breaking a bridge?

[Kelly Pedersen]

Originally Posted by rerednaw

Again, what single spell would do this in the Making and Breaking college?

None of them work perfectly. Probably your best bet is to assume that the engineer-mage used Shatter or Disintegrate and had enough levels of Magery above 3 that he could pour sufficient energy into the spell over its normal limits to break the keystone in one hit. Or assume there's a better spell that can break a bigger object more energy-efficiently, that's just not in the book. I don't think the vignettes in Magic are necessarily supposed to be illustrating a specific spell in every case, but rather how the type of magic from that college looks like in practice.

Originally Posted by rerednaw

And the keystone is NOT accessible from the top of the bridge.

In this case, it quite clearly is. The mage drives "a faintly glowing fist into the massive basalt keystone". You can argue that it shouldn't be accessible, perhaps. But in the vignette, it is.

[Refplace]

Originally Posted by rerednaw

Again, what single spell would do this in the Making and Breaking college? Going by the Basic rules if say the keystone was a measely 1 foot cube...it would have over 40 hp. And the keystone is NOT accessible from the top of the bridge. So unless the mage can cast through several feet of solids? How does this work?
Shatter does not effect the target if you don't equal or exceed the hp.

Those are the reasons I did not include Shatter in my list. Its great for destroying a vase or weapon, not so much anything massive.
Shatter does a max of 3d6 and the text describes a "massive basalt keystone" which seems to me to be to big to be bothered by Shatter. Weaken is not all or nothing so would do it eventually but I doubt one casting would be enough.
However if you HAVE to do this with a Making and Breaking spell I think Rive is your best bet.
1 per d6 damage for a 2 foot hole and no damage cap listed.
Use a powerstone and a lot of your FP and your talking serious damage.
Does not quite fit the description in the narration but its pretty close. And if the Keystone was enchanted breaking it would typically ruin the enchantment as well.


PS: The keystone could be accessible from the top of the bridge. The stress the keystone typically has to deal with is at the bottom where both arches are sort of leaning on it. However it could be a really ong pin that extends all the way through the bridge.
Shape Stone and other magics let you build things beyond or differently than typical for a given TL.

[GodBeastX]

How I would handle it.

If a spell targets the entire bridge, you treat is as a higher SM. This is ~SM 17 so you multiply energy times 18 to affect the bridge (See Regular Spells on P.11 of Magic).

But I would also multiply damage based on the object HP too since it would be relative to the bridges size for energy. Meaning a direct damage spell to an object with 1d for a human target should do that multiplied by the size of a bridge. This would be effectively (HP / 10) * 1d damage.

That's how I would handle it! If you get the bridge to a certain HP it's rolling HT as people walk across it or it is falling to pieces.

[Kelly Pedersen]

Originally Posted by GodBeastX

If a spell targets the entire bridge, you treat is as a higher SM.

That's not a bad approach, actually! I don't think you need to target the whole bridge in this case, either - the keystone should be just fine. If we assume the keystone is the size of a large car (SM +5, for 7 yards long or so, but boxy, so +2 to SM), that would multiply Shatter's cost by 6, which is expensive but doable. I'd suggest multiplying the damage based on the change in scale. So, SM +5 is about 7X the size of something SM +0, so you'd multiply the damage dice by that. 7d per energy spent, multiplied by 6, means that you'd need to spend about 12 energy to reliably break a keystone that size. That sounds about right to me.

Edited to add: Sorry, I realized after posting that that hypothetical SM +7 keystone would have more HP than I thought - I was still working off the "70 HP" numbers. I figure that a 7 X 5 X 5 yard chunk of obsidian would have about 620 HP, which needs about 25 levels of Shatter, or 175 energy, to break reliably. So we're back to "that engineer-mage has a lot of levels of Magery", or "he's using a custom spell".

[rerednaw]

I thought about Rive, but missed the part where the keystone was accessible from above. That makes sense then.

Sorry I've built bridges so didn't see that in the story... LOL.