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About the eneregy. The buildings fell with 0,7G acceleration, so 70% of the total energy went in just accelerating the fall of the mass. 30% was left to crush the parts below. But with safety factor of 3, you would need 300% of the potential energy to crush the building. That is why in CD's they blow up the lower 1/3 of the building to get that energy to crush a skyscraper.
But if you just use Gravity you can barely crumble concrete, not pulverize, you can crush objects like desks, not pulverize, you can move stuff down, not 100-200m sideways.
Yes, let's talk about energy!
You may be surprised, but these numbers have already been crunched before, and it is fairly easy (for a good structural engineer) to estimate the energy needed to break the
structural supports - it is much much less than then potential energy of gravity stored in a 415 meters tall building.
Acceleration 70% of G (that is a rough number, and it may be incorrect, but let's not quibble here, argument doesn't change if it's 60% or 80%) means that less than 30% of the potential energy suffices to break structural supports, and yes, that is fully in line with those calculations I talked about. The balance to 30% is available to do additional destruction, such as pulverizing gypsum wallboard, some concrete, and of course office contens, from desks to people.
But the collapse doesn't stop there! In fact in can be shown that later in the descent, acceleration goes down dramatically to the point where velocity is almost constand = acceleration near zero. That means that later in the fall, near 100% of the potential energy is available to do work on the structure and its contents.
But the collapse doesn't stop there!
When all the remaining structure hits the ground, all the kinetic energy is used to crush even more stuff (and a minor proportion of heat).
So in fact, almost 100% of the potential energy is
eventually converted into mechanical work to destroy steel conectors (minor proportion), crumble concrete (major proportion) and all the other stuff, including people.
The numbers have been crunched for example by the team of Bazant, Le, Benson and Greening, if memory serves. The latter, Frank Greening, has gone into detail to calculate how much energy is needed to get concrete down to the observed particle size distribution, and the result he comes up with is right there in the ballpark of available potential energy.
But now suppose the gravitational energy does not suffice, i.e. what is needed to crumble the towers is
much more than was available from gravity alone! How much additional energy would you need? Twice as much? Before I continue,
I want you Jbui, to commit to an answer here. If the potential energy from gravity stored in a tower does not suffice, in your opinion, how much more do you need then?
1% more?
5% more?
25% more?
100 % more?
500% more?
Pick the minimum with which you could live!