These steel columns were incredibly thick - each wall measuring 2.5 inch (6.35 cm), so the entire thickness of either of the columns was 5 inch (12.7 cm).
To imagine how thick this is, here is a good example to compare with: front armor of the best tank of the WWII period - T-34 - was only 1.8 inch (4.5 cm) and it was single-walled. Yet there were practically no armor-piercing artillery shell available that time that would be capable of penetrating such front armor.
Of course, no explosives whatsoever would ever be able to tear throw such front armor of a tank either (except only a hollow-charge shell which would still not be able to tear a complete piece of such armor, but only to burn some narrow hole through an armor plate).
Considering that the Twin Towers' steel frames consisted of double-walled steel columns that were almost trice as thick compare to the T-34 tanks' front amour, it would not be possible to find any solution to break such columns simultaneously in many spots in order to achieve an "implosion" effect - the basic goal of any 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 solution would not help 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-raised structure would fall strictly down to its foot print - it might as well scatter its debris as far as a quarter of a mile, considering its mere height.
So, it was impossible to bring the WTC Towers down by any kind of traditional controlled demolition.
The same thing could be said about the WTC building # 7 and of the Sears Tower in Chicago. Either of them was constructed using similar thick double-walled steel frame that was impossible to break at once due to reasons described above.
However, in accordance with the US laws governing construction of skyscrapers buildings designers had to submit some satisfactorily demolition project before their construction project could be approved by the Department of Buildings. No one could be allowed to build a skyscraper that can't be demolished in the future.
This is the main point of the skyscrapers' in-built nuclear demolition features.
Ironically, such a nuclear demolition scheme of a skyscraper is not meant to actually demolish the respective skyscraper, especially considering that no one has any practical experience in demolishing skyscrapers by such means - it is only intended to convince the Department of Buildings to permit the skyscraper's construction whatsoever.
It appears that all designers and proponents of such nuclear demolition schemes sincerely hope that their ideas would not be put to use during their life-time.
Anyhow, "Controlled Demolition Inc." began to study possibilities of demolishing modern skyscrapers by underground nuclear explosions at the end of 60s, at request of the then New York Sate Governor Nelson Rockefeller - when it became necessary to get a legal approval from the New York Department of Buildings for the WTC Twin Towers construction.
After some research, a final solution was found and approved by the Department of Buildings and "Controlled Demolition Inc." got its nuclear demolition know-how patented.Part 3. How does a modern nuclear demolition work? http://www.bibliotecapleyades.net/ciencia/ciencia_uranium26.htm