Throughout most of modern human history, building demolitions were relatively simple matters -- shatter a few support columns, and the weight of the building will do the rest of the work. The only trick lies in getting the building to land where you want it.
The weight of the HTRN building is about what you'd expect, if you're the sort of person who goes around weighing buildings, but the materials are so very, very strong that even with key supports cut, and the entire interior gutted, the building won't develop more than a very unhealthy slouch. With materials like these the famous Leaning Tower of Pisa could be 100 times taller and could be made to "lean" a good eighty degrees from vertical.
Regardless, it is still quite disconcerting to watch the skyscraper above you shluump into a slouch.