If you open a terrain offline and go into the objects settings and hit save, it will produce a terrname.dod file with the following about down times in minutes and hardness(lbs of 1000lb bombs to destroy). It has not changed much aside from Hitech's adjustment for the HQ fiasco at the end of AH2 and some AH3 objects.
30, 0.0313,VOID
30, 125.0000,Tower
1, 125.0000,Runway
30, 0.3125,Hanger
30, 0.3125,Rearm
30, 0.0063,Auto Gun
30, 0.3125,Bunk Radar
30, 0.3125,Bombable
30, 0.3125,Ftr Entry
30, 0.0015,Map Room
30, 0.3125,Officer Club
30, 0.3125,Bunk Ammo
30, 0.3125,Bunk Fuel
30, 0.3125,Bunk Barr
180, 0.8438,City
5, 15.6250,HQ
180, 0.3125,Fact Ammo
180, 0.3125,Fact Fuel
180, 0.3125,Fact Radar
180, 0.3125,Fact AAA
180, 0.3125,Fact Barr
30, 0.3125,Bomb Ent
30, 0.3125,Veh Ent
15, 2.7813,Ftr Hgr
15, 2.7813,Bomb Hgr
15, 2.7813,Veh Hgr
60, 1.5625,Ship
15, 2.7813,Boat Entry
15, 0.0188,Gun Bat
600, 0.1563,Train
600, 0.0781,Convoy
600, 0.1563,Barge
15, 0.3125,Station
30, 0.3125,Town
45, 0.3125,Barrier
45, 0.3125,Structure
45, 0.3125,Group Master
15, 0.0063,Gun Bat Soft
15, 0.3906,Gun Bat Armor
30, 0.0188,Hard Auto Gun
30, 0.3906,Arm Auto Gun
15, 3.9063,Hard Gun Bat
30, 125.0000,Brief room
30, 125.0000,Debrief room
1800, 125.0000,Flag
1800, 125.0000,Pennant
1800, 125.0000,Sheep
1800, 125.0000,S Pen
30, 0.3125,M Entry