The crux of the question is "what counts as 'can support it'"?
The item must appear on a surface or in a liquid that can support it.
How is a bard supposed to know the maximum load a structure can support?
Taking this 'real physics' idea further, can the item be summoned on a patch of grass, knowing that the grass can't support it? But the ground underneath it is not available either, since it lacks empty space (there's grass in the way).
Pushing this argument: where is the limit?
If I summon a ton of lead on a wooden table, the table breaks, but the ground underneath holds: is it valid?
What if it is on a slab of stone on the ground? What if it is heavy enough to compact the ground on which I summon it? Technically, anything with even some weight will do so.
Can you summon an empty barrel made of potassium in a pond of water? The water could 'support it', it would float, but potassium instantly and violently reacts with water.
I mainly bring this forth because I did the math and you could theoretically summon (1.35+[cha] x 0.45)10^44 Joules worth of matter/antimatter mix as a 14th level creation Bard. This mass (1.5210^27 + [cha]x5.07*10^26kg ) would most likely not be supported by anything, as it would likely be the greatest gravitational well around (more like 'it' would be the one doing the supporting). At the same time, its constituent parts could be made unstable enough to decay faster than they could gravitationally or electromagnetically interact with the 'support' medium (top quarks, for example). I.E. they don't have time to 'touch' the ground before exploding with literally the power of a supernova, likely atomising everything within a light-year of the object.
The game is not a physics emulation, which renders the question impractical for the scope and scale of the game as it is made; while pushing it to an extreme can be fun, what problem in play are you trying to solve?
Apologies for going overboard with the logic in a fantasy roleplaying game, but hopefully this serves to illustrate that taking the ability to a 'logical' extreme goes beyond the limits of bounded accuracy, which leaves the "does it crush the guard shack" in the province of a DM ruling. (Most DMs would likely rule that "yes, the shack is crushed" and then play would carry on).