Those beams were all perfectly cut at 45 degrees, which is why the demo was so successful. The south tower plane was supposed to hit in the center. It missed, and it sped up the need to chop steel beams/girders because there was not enough time for the planted thermite to melt the joints. Indeed, it was the miss wide by the plane which put the flaming jet fuel on thermite ready to go, and that is what produced the river of molten steel 2200F
There was no need to chop steel in the north tower, since the variable of the "plane" was never there.... There were planted charges inside to blow out what looked like the shape of a plane.....