I disagree with you. The glass isn't structural. You could take out the superficial parts of the structure pretty easily, but the ability to carry a load is embedded in the steel columns, and those are much sturdier. Look at 9/11 - a freaking jetliner hit a high-rise, with kinetic energy probably thousands of times as much as any BT weapon, and it still stood until it was gutted by fire.
That said, a glass-and-steel building probably couldn't be used as armour the way BT allows, so that part is fairly unrealistic.
You are very much correct. However, those steel beams can be picked out pretty easy with advanced 25th+ century sensors, and a few heat rays will cut right through them with ease. They don't have the same protection that you'd get with something that's got a thick concrete outer skin. My mind goes back to the Man of Steel, and I can imagine even a small laser sweeping through a portion of a Metropolis building very much like Cryptonian heat vision. Those buildings fell sideways, and not the clean demo-drops we see when explosives are properly placed.
But, I really agree more with your latter assessment - the idea of a certain amount of protection for units on the inside being the same across all buildings of a type is erroneous.
I can imagine a tiny, hardened bunker that doesn't take up a full hex, or have full proper elevations, including basement being able to support the same 100 ton Atlas as the heavy-duty high-rise that has steel supports holding up reinforced concrete floors. But, the glass that encloses those floors from the weather, let alone the drywall and mere wooden wall-frames that divide up the interior of that building provide next to no protection compared to the super-thick reinforced ferrocrete wall surrounding what amounts to a small room with some firing slits.
To do this proper, I guess there would have to be a distinction made between knocking out the walls or trying to take the building down. Some of the old stone buildings, the walls
are the structure. Churches and buildings on downtown main street in po-dunk small town don't have any real internal support except for a couple pillars, maybe. A lot of wooden houses are the same way. But, some of the more modern buildings, especially high-rises, are built on a framework, an internal structure.
Like BT buildings in standard rules, I guess there will have to be a secondary value which determines Protection Rating.
It will be in the same values as used for the Building Class, but is not necessarily required to match.
So, you could have Hardened Class buildings with Light Protection, or even Light Class buildings with Hardened Protection.
The Building Class determines how many Demolition Attempts are required to knock it down, and is a quick indicator of what kind of weight it can support per level. A Demolition Attempt remains being an act of trying to knock out support structures to send the building crumbling to the ground.
Let's just use Building Class as an indicator of what weigh classes it can support, topping off at the highest tonnage. So:
Light = 35 tons
Medium = 55
Heavy = 75
Hardened = 100
Aside: This means, however, that we've redefined the classes, and that most urban buildings, like houses, don't even rate placement on the map, although they take up a fair amount of space. Because, I can't even imagine my cinderblock house holding up a 2 ton car on its roof, let alone a 14 ton semi or a 20 ton Wasp standing on both feet.
Something like that needs a new terrain type or treat it as rough/rubble for cover and MP purposes. They simply aren't an obstacle.
Protection Rating determines what kind of divisor is applied to weapons attacks against units inside the building. The PR can potentially be reduced with each attack against units inside being resolved like a Demolition Attempt in addition to the damage inflicted on the target unit. Hit or miss on the targeted unit, like accidental fires, the attempt is tracked and rolled if there's a chance for success.
Example: A Wasp is trying to fire at a squad of infantry in a bunker, firing its machine-guns through the slits. It fails both rolls, being at long range and having moved to be a difficult target for the grunts. The bunker is only a medium class building, but it's got hardened protection, so both machine guns don't even have a chance to put real holes in the cover, so the Wasp's controller ignores each attack for potentially knocking down the cover rating by one level.
Protection Rating damage only effects the level being attacked, and it won't knock the building down if the PR drops below light. In fact, it probably shouldn't go below light.
An attacker may also declare a concerted effort to simply reduce a level's protection rating with weapon attacks. Any non-armored units will still take damage in the reduced manner, simply to reflect the way weapons will be hosed across the wall and plenty of firepower spilling through. Armored units won't be effected, because it's not a concerted attack trying to defeat their armor.
Combined attacks against a level's PR can be declared, much like the standard Demolition Attempt.
Versus Protection Rating only, Phyiscal attacks do double damage.
PR is reduced by one level when the building class is reduced by one level from a successful demolition attempt.
Now with that last thought, you could have fun with rubble hexes still having some damage protection even though the building itself is 'hollowed out'. I'm leaving this as an advanced option.
I've never been keen on how Anti-Infantry damage is worked under the standard rules, so I'm thinking that AnPers weapons get to roll their Anti-Infantry damage before having it modified by Protection Rating.
But, this would still be far simpler than HP Tracking. You have 2 stats to keep track of, and both can be done on a hand with four fingers if you needed. Tokens could be dropped on buildings to track damage level.
If you wanted to be very realistic, you could have the Protection Rating apply to individual hex faces, if you wished.