Experiment: Determining the size required for a creature to be immune to bridges.
The next experiment conducted was to determine how large a creature could be before it could not be flung or crushed by a bridge. For the purposes of this experiment I created a creature with a series of castes in gradiated sizes, the scalebeast:
[CREATURE:SCALEBEAST]
[DESCRIPTION:A standard-sized animal used for calibrating scales.]
[NAME:scalebeast:scalebeasts:scalebeast]
[CASTE_NAME:scalebeast:scalebeasts:scalebeast]
[CREATURE_TILE:'W'][COLOR:6:0:0]
[PETVALUE:1]
[PREFSTRING:precise weight]
[FREQUENCY:100]
[LARGE_ROAMING]
[COMMON_DOMESTIC]
[BENIGN][MEANDERER][PET]
[BODY:QUADRUPED_HOOF:TAIL:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:SKULL:MOUTH:GENERIC_TEETH:RIBCAGE:2HEAD_HORN]
[NATURAL]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[USE_MATERIAL_TEMPLATE:HOOF:HOOF_TEMPLATE]
[USE_MATERIAL_TEMPLATE:HORN:HORN_TEMPLATE]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[USE_TISSUE_TEMPLATE:HOOF:HOOF_TEMPLATE]
[USE_TISSUE_TEMPLATE:HORN:HORN_TEMPLATE]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:BODY_HAIR_TISSUE_LAYERS:HAIR]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SKIN:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:STANDARD_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RIBCAGE_POSITIONS]
[USE_MATERIAL_TEMPLATE:SINEW:SINEW_TEMPLATE]
[TENDONS:LOCAL_CREATURE_MAT:SINEW:200]
[LIGAMENTS:LOCAL_CREATURE_MAT:SINEW:200]
[HAS_NERVES]
[USE_MATERIAL_TEMPLATE:BLOOD:BLOOD_TEMPLATE]
[BLOOD:LOCAL_CREATURE_MAT:BLOOD:LIQUID]
[CREATURE_CLASS:GENERAL_POISON]
[GETS_WOUND_INFECTIONS]
[GETS_INFECTIONS_FROM_ROT]
[USE_MATERIAL_TEMPLATE:PUS:PUS_TEMPLATE]
[PUS:LOCAL_CREATURE_MAT:PUS:LIQUID]
[BODY_SIZE:0:0:500000]
[DIURNAL]
[HOMEOTHERM:10070]
[SWIMS_INNATE][SWIM_SPEED:2500]
[MUNDANE]
[CASTE:SIZE500000]
[CASTE_NAME:wb500000:wb500000s:wb500000]
[BODY_SIZE:0:0:500000]
[CASTE:SIZE600000]
[CASTE_NAME:wb600000:wb600000s:wb600000]
[BODY_SIZE:0:0:600000]
[CASTE:SIZE700000]
[CASTE_NAME:wb700000:wb700000s:wb700000]
[BODY_SIZE:0:0:700000]
[CASTE:SIZE800000]
[CASTE_NAME:wb800000:wb800000s:wb800000]
[BODY_SIZE:0:0:800000]
[CASTE:SIZE900000]
[CASTE_NAME:wb900000:wb900000s:wb900000]
[BODY_SIZE:0:0:900000]
[CASTE:SIZE1000000]
[CASTE_NAME:wb1000000:wb1000000s:wb1000000]
[BODY_SIZE:0:0:1000000]
[CASTE:SIZE1100000]
[CASTE_NAME:wb1100000:wb1100000s:wb1100000]
[BODY_SIZE:0:0:1100000]
[CASTE:SIZE1200000]
[CASTE_NAME:wb1200000:wb1200000s:wb1200000]
[BODY_SIZE:0:0:1200000]
[CASTE:SIZE1300000]
[CASTE_NAME:wb1300000:wb1300000s:wb1300000]
[BODY_SIZE:0:0:1300000]
[CASTE:SIZE1400000]
[CASTE_NAME:wb1400000:wb1400000s:wb1400000]
[BODY_SIZE:0:0:1400000]
[CASTE:SIZE1500000]
[CASTE_NAME:wb1500000:wb1500000s:wb1500000]
[BODY_SIZE:0:0:1500000]
[CASTE:SIZE1600000]
[CASTE_NAME:wb1600000:wb1600000s:wb1600000]
[BODY_SIZE:0:0:1600000]
[CASTE:SIZE1700000]
[CASTE_NAME:wb1700000:wb1700000s:wb1700000]
[BODY_SIZE:0:0:1700000]
[CASTE:SIZE1800000]
[CASTE_NAME:wb1800000:wb1800000s:wb1800000]
[BODY_SIZE:0:0:1800000]
[CASTE:SIZE1900000]
[CASTE_NAME:wb1900000:wb1900000s:wb1900000]
[BODY_SIZE:0:0:1900000]
[CASTE:SIZE2000000]
[CASTE_NAME:wb2000000:wb2000000s:wb2000000]
[BODY_SIZE:0:0:2000000]
[CASTE:SIZE3000000]
[CASTE_NAME:wb3000000:wb3000000s:wb3000000]
[BODY_SIZE:0:0:3000000]
[CASTE:SIZE4000000]
[CASTE_NAME:wb4000000:wb400000s:wb4000000]
[BODY_SIZE:0:0:4000000]
[CASTE:SIZE5000000]
[CASTE_NAME:wb5000000:wb5000000s:wb5000000]
[BODY_SIZE:0:0:5000000]
[CASTE:SIZE10000000]
[CASTE_NAME:wb10000000:wb10000000s:wb10000000]
[BODY_SIZE:0:0:10000000]
[SELECT_CASTE:ALL]
In the first experiment, I embarked with two of each size of this creature, and enough stone to build a retracting bridge and mechanisms to build a lever and connect it to the bridge. I then pastured the weightbeasts one at a time on the bridge and pulled the lever.
When the scalebeasts of size 500000 up to 1100000 were placed on the bridge, the bridge would still retract and extend, flinging the creatures. These creatures were too small to prevent the bridge from working.
When any scalebeast of size 1300000 or greater was placed on the bridge, the bridge would not retract. These creatures were large enough to prevent the bridge from working.
Curiously, of the two size 1200000 scalebeasts I brought, one would prevent the bridge from working and the other would not. This was consistent per-creature rather than being a random chance each time. It appears that even though all the creatures were specified to be the same size, there was still some creature-to-creature variation. I do not know how large this variation is, so I cannot put precise error bars on the measurement. At best all I can say is that the size limit to prevent a bridge from working is approximately 1200000.
In the default DF raws, the following creatures have an adult size greater than 1200000:
roc, blind cave ogre, cave dragon, bronze colossus, cyclops, dragon, draltha, elephant, ettin, manta ray, sturgeon, giant, hippo, hydra, jabberer, giant lion, ogre, rhinoceros, rutherer, sea monster, sea serpent, basking shark, great white shark, whale shark, giant tiger, walrus, whale
Any one of these creatures when on a bridge will prevent that bridge from operating. This only applies to the fully adult size - some of these creatures may have child stages small enough to not prevent a bridge from working.
In addition, most or all procedurally generated creatures (forgotten beasts, titans, demons, night creatures) are above the size limit.
Experiment #2: Testing if the weight limit is cumulative.
Using the same experimental setup as before, I took two size 1000000 scalebeasts, two size 1100000 scalebeasts, and the one size 1200000 scalebeast confirmed to be below the bridge limit, and placed them on the bridge. The combined sizes of these creatures was 5400000, well over the bridge size limit. The lever was then pulled. The bridge did retract and fling the creatures.
It appears that the size limit is not cumulative. A single creature of size greater than 1200000 will prevent a bridge from operating, but multiple creatures below that size will not. This agrees with experience I have had in my fortresses in which dozens of elves riding horses, bears, and moose all standing on one bridge did not prevent that bridge from retracting from under them.
Note that goblin sieges often arrive riding rutherers or jabberers, a single one of either which will prevent a bridge from working.
Experiment #3: Testing the limits of raising bridges (atomsmashers).
The prior experiment was repeated with raising rather than retracting bridges. The weight limit for raising bridges was found to be the same as for retracting bridges: approximately 1200000. Furthermore it was verified that multiple size 1000000 and 1100000 creatures could be crushed simultaneously, but a single size 1300000 creature would cause the bridge to deconstruct instead.