When looking over the rules for beastmen in the Monster chapter, it appears that each type of beastman is given a standard AC that is largely dependent on the sort of armor they usually wear. I notice in the Giant entry, however, that all true giants have a base AC of 5 (storm giants excepted), despite the fact that their descriptions seem to allow for variances in the armors they wear.
So how much of a giant's AC is tied to his choice of armor? Does a naked giant have an AC lower than 5? How much of a boost to AC can a fire giant expect from being clad in a bronze cuirass? What sorts of armor do other types of giants usually wear?
It seems like giants would have a natural AC of 5 and generally don't wear armor. Those that do like fire giants (based on description) and possibly storm giants (based on AC 7, though could be their natural level), could have the barding rules applied.
This does bring up an interesting question regarding the effect of the spell shield on a creature with natural armor. Would the highest apply, or would they stack like barding on a horse?
Hill Giants and Stone Giants have a -1 penalty to AC due to size (they weigh less than 2,000lbs).
Frost, Fire, and Cloud Giants have a -2 penalty to AC due to size (they weigh less than 8,000lbs).
Storm Giants have a -4 penalty to AC due to size (they weigh over 8,000 lbs).
Hill giants wear thick heavy hides,so thick that it counts as scale armor (AC 3). They have naturally tough skin (+3 AC). Their total AC is therefore 5.
Stone giants wear lighter hides (AC 1), but they have naturally tough skin (AC 5). Their total AC is therefore 5.
Frost giants wear a mix of heavy fur and thick chainmail (AC 4). They have naturally tough skin (+3 AC). Their total AC is therefore 5.
Fire giants wear huge, heavy metal or dragonscale pauldrons (AC 4). They have naturally tough skin (+3 AC). Their total AC is therefore 5.
Cloud giants wear lighter-weight armor, the equivalent of elven chain for giants (AC 4). They have naturally tough skin (+3 AC). Their total AC is therefore 5.
Storm giants wear hoplite-style panoply (if mountain) or armor of whalebone, shell, and shark skin (if acquatic), in either case providing AC 6. They have naturally tough skin (+3 AC) and a measure of magical/divine protection (+2 AC). Their total AC is therefore 7.
There is thus plenty of room to up-armor your giants if desired, or to equip them with magic, etc. Giants are not particularly formidable for their HD, so feel free to buff 'em up!
Now in regards to the shield spell, if a Thrassian were built with arcane value allowing it to cast shield on itself, would the spell AC add to the Thrassian's natural AC or replace it? In the same respect, if a shaman were to research a divine copy of shield and cast it before shifting to his or her totem form, would it add to the animal's natural AC or replace it if the spell is higher. I would assume the highest value wins and the two values wouldn't stack, but there has been dispute among the players as it isn't explicitly stated either way.