The speed of air that will tear a plane apart at sea level densities is 500 mph.
At 500 mph air creates a force equal to 674 pounds per square foot.
If an aircraft is not carefully controlled in it's flight path, 500 mph air will rip it apart at sea level density 14.7 psi.
Okay, so that's how strong or weak an aircraft is. now let's see how much force it takes to break a 14 inch box steel column.
Explosives experts will use a charge designed to move air at between 6818.2 mph and 20455 mph, which translates into forces of 125,370 pounds per square foot of pressure, to 1,128,400 pounds per square foot of pressure.
So we have Aircraft 674.23 lb/sq.ft
Steel box column: 125,370 lb/sq.ft (low)
Steel box column: 1,128,400 lb/sq.ft (high)
Worse yet is the fact that the aircraft has to break several steel columns, not just one.
If the aircraft must break at any pressure above 674.23 lb/sq.ft. how can it possibly stay intact and exert a force of over 125 thousand pounds per square foot, to over 1 million pounds per square foot, on the steel columns?
As the plane contacts the building, both the buildings steel, backed by reinforced concrete slabs, experience the same force of impact. While the steel columns can withstand this force, the aircraft cannot. So, as the force of contact rises, the aircraft's soft aluminum skin begins to crumple, split and break apart, at forces far below what the steel can withstand. This crumpling, splitting and breaking absorbs the energy of the aircraft, so that the amount of force it can deliver begins to immediately decline.
Let's take it by nano seconds. In the first nano second of contact the plastic nose begins to press into the building, the pressure it exerts rises with each passing nano second until, past just a few dozen pounds per square foot of pressure, the strength of the plastic is exceeded and it shatters away.
In the next few nano seconds the aluminum skin of the nose of the aircraft begins pressing into the building. The aluminum cladding of the building holds until the pressure reaches it's maximum tolerance, then the aluminum of both the plane and the building begins to deform and break away.
Next the aluminum of the aircraft, that has survived contact with the buildings aluminum cladding, begins to press harder and harder against the steel columns. Again the pressure begins rising, but at only a few thousand pounds per square foot of pressure, the aluminum aircraft begins to deform, in the next nano seconds, as the aircraft is deforming, the plane is breaking apart, while the pressure it is exerting on the steel columns is just a few thousand pounds per square foot. But the steel will require 125,000 lb/sq.ft. before it will deform and break, the aluminum aircraft cannot maintain sufficient integrity to produce such pressure. Every part of the aircraft that touches the buildings steel columns, either deforms, cracks or splits and bounces away, when the force it exerts on the steel exceeds several thousand pounds of pressure per square foot. Meanwhile the steel columns stand waiting for pressures in excess of 125,000 /lb/sq./ft. to arrive, such pressures never arrive, they cannot, because the aluminum of the aircraft is too soft and weak to deliver it.
