Azazal:
Well, according to
http://www.ce.berkeley.edu/~astaneh/1-Publications/Astaneh-9ASEC-WTC%20Paper%202003.pdf the exterior columns were about 46cm x 46cm exterior, with varying thickness. The interior columns were either 2" steel or 5" steel, so I'll assume the exterior columns are 2" steel as well.
So, 46cm is 18.11 inches. We'll round to 19 for argument's sake. That gives a cross section of a solid column as (19x19=) 361 in
2. The interior "hole" would be 15" x 15", taking up 225in
2. That leaves (361-225) 136in
2 for our A value.
So, we take P = 3/8A and end up with P = 3/8 * 136 = 51 pounds of TNT, or 23.13kg. And that's an upper estimate.
Realistically, it's 18.11 inch columns likely to be about 1 inch to 1.5 inches thick, which gives a mid-limit A of 68.44 and a P of 25.665 lbs, or 11.64kg.