Galaxy class hull

http://www.youtube.com/watch?v=fyNm8EWM5K0&NR=1

Here we se that hull thickness of Enterprise-D is 70 centimeters. Hull of galaxy class starship is made of tritanium, and was shown here to be able to absorb rather impressive damage.
Both from Q Who and Best of Both Worlds, Part II it is clear that Enterprise-D has inner and outer hull. That means that, while total hull thickness might be 70 centimeters, it is actually lower due to space between inner and outer hull plates, which is around 6 times thickness of single hull plate. So Galaxy class has double hull, with each hull being 12 centimeters thick.

"Hand phaser can put 100 megajoules of disintegration energy in a beam a centimeter wide, and yet can't even melt - let alone disintegrate - a tritanium plate a few centimeters thick? If 100 MJ/cm^2 were sufficient to punch a hole through several centimeters of tritanium, disintegrating several hundred square meters of tritanium cladding several tens of centimeters thick would require close to a gigaton." - from Starfleet Jedi

So it seems possible that GCS hull might be able to withstand kiloton- to megaton- level weaponry. Enterprise-D saucer was able to survive crash-landing without any apparent damage to hull - althought damage to inner parts was heavy, outer hull "shell" seemed intact.

That is reinforced by TNG episode "Wounded" - in that episode, both Galaxy-class USS Enterprise-D and Nebula class USS Phoenix were able to withstand hits from Cardassian warship's main beam weapons as result of unshielded attack - Enterprise D was hit while its shields were still down (with only damage reports being "minor damage to secondary hull" and "starboard power coupling down" reports by Worf and LaForge), and Phoenix was hit with its shields disabled. Also, explosion caused by warp core breach of "bug" that collided with USS Odissey produced relatively minor damage to secondary hull - however, due to sensitivity of that area, ship was destroyed by warp core breach. USS Galaxy, lead ship of class, withstood significant pounding in First battle of Chin'toka.

Analyzing Arsenal of Freedom

Hand phasers have 5 to 12 GW power output, with 73 GW high end. In "Arsenal of Freedom" Tasha Yar states that Federation weapons cannot melt centimeter thick plate of tritanium. If we take that hand phaser can melt 5 cm^3 of tritanium per second (tritanium is able to withstand 12 000 ° C without showing any signs of melting) then heat capacity of tritanium is around  300 000 J/(cm^3 x K) to 720 000 J/(cm^3 x K).

Generations incident

D 12 was able to destroy 1 058 968 052 to 2 117 936 103 cubic centimeters of Enterprise-D's hull with each shot, depending if only outer hull was damaged or inner too. Actual value is between these two. I will take that shots heated hull 15 000 K. So we have 4.76 x 10e18 to 1.143 x 10e19 joules per shot for our outdated D-12. That is 1.138 to 2.73 gigatons per shot.