Perimeter columns are being inspected at “ground zero”. These steel columns were incredibly thick - each wall measuring 2.5 inches (6.35 cm), so the entire thickness of each column was 5 inches (12.7 cm). To imagine how thick this is, here is a good example to compare to: imagine the front armor of the best tank from the WWII period - the T-34 – whose steel was only 1.8 inches (4.5 cm) thick and was just single-walled. The T-34 tank and its armor are in the pictures below:
Yet there were practically no armor-piercing artillery shells available at the time capable of penetrating such front armor.
Of course, no explosives whatsoever could ever tear through the front armor of a tank (except for hollow-charge shells which would still not even be able to tear through the armor completely, but would merely just burn a narrow hole through the armor plate).
Considering that the Twin Towers' steel frames consisted of double-walled steel columns that were almost three times as thick as the front armor of a T-34 tank, it would not be possible to come up with a solution to break these columns simultaneously and do so in many spots simultaneously in order to achieve the "implosion" effect – which is, of course, the basic goal of controlled demolition.
It was, of course, technically possible to break some of these columns in certain spots, using exceptionally huge amounts of hollow-charges attached to each individual column, but even such an incredible amount of explosives wouldn’t be enough to achieve the desired "implosion effect". The Towers were simply too high and too rigid - their steel cores would have been simultaneously broken in too many spots on every floor, which no one could afford, and even if they could, still, such a solution would not lead to the desired effect - there would not be any guarantee that such a high-rise structure would fall straight down into to its foot print. It would most likely just scatter its debris over the course of a quarter mile given its mere height alone. So, it was impossible to bring the WTC Towers down by any form of traditional controlled demolition.
The same thing could be said about WTC building # 7 and the Sears Tower in Chicago. Both of them were constructed using similarly thick double-walled steel frames which were impossible to break at once due to the same reasons described above.
However, in accordance with US laws governing the construction of skyscrapers, designers had to submit a satisfactory demolition schema before construction would be approved by the Department of Buildings. No one would be allowed to build a skyscraper that could not be demolished in future.
This is the main reason for having a built-in nuclear demolition feature. Ironically, the nuclear demolition schema of a skyscraper is not actually meant to demolish the skyscraper, especially considering that no one has any practical experience in demolishing skyscrapers by such means - it is merely intended to convince the Department of Buildings to give permission to build the skyscraper.
It appears that all designers and proponents of such nuclear demolition schemas sincerely hope their ideas not be put to use during their life-time.