Here's a bit of a new idea to try and find the answer to the question of the Twin
In my 1969 Brittannica is a nice little article on Thermite *. The last paragraph states:
"One of the greatest hazards of the thermite reaction is contamination with moisture. During the reaction water is reduced and hydrogen evolved, which can produce an explosive mixture with the surrounding air."
(It's talking about regular old plain vanilla "aluminum plus rust" thermite, which was patented back in 1897).)
Note, this is not the same as putting a rock from a stream into a campfire and getting a steam explosion from inside the rock.
It's not a steam pressure explosion, but a hydrogen-air combustion explosion.
If a whole lot of water-contaminated thermite was set off together in a closed space the right size, it could in effect produce a wide-area thermobaric device.
Large amounts of hydrogen would be quickly produced as the moisture-contaminated thermite reaction breaks down its contained water into pure hydrogen and pure oxygen ( the oxygen probably being immediately taken up by the aluminum as part of the creation of the white aluminum oxide vapor a thermite reaction produces).
What is happening here is rather like the electrolysis of water into its component gases. Thermite can do the same thing.
Then the heat recombines the liberated hydrogen with either the liberated (from water) oxygen, or more likely, as the encyclopedia says, with surrounding air.
1 So one reaction would be the aluminum powder with magnetite, (rust or iron oxide) and the products are pure iron and aluminum oxide with the evolution of great amounts of heat at temperatures at or above the melting point of steel or iron, i.e. 2400 degrees C.
2 The second reaction would be the reduction of water into hydrogen and oxygen.
3 The third reaction would be the explosive recombination of hydrogen with the oxygen in surrounding air. If there was enough hydrogen evolved for a given volume, an immensely powerful blast, just like the blast made by a fuel-air or a thermobaric bomb, could be produced.
Fuel-air devices, also called thermobarics,** are the most powerful blast weapons before nuclear weapons.
So suppose there was enough thermite in, say, around or under one floor somewhere.
It has a volume of 12 x 208 x 208, of about half a million cubic feet of air space.
Thus the wet thermite, or apparently dry thermite which is formulated with some kind of bound water, is set off and the reaction proceeds.
The moisture is reduced by the reaction as in electrolysis, into oxygen and hydrogen.
The oxygen reacts with the aluminum to make aluminum oxide, as in the thermite reaction itself, but the hydrogen reacts explosively with the air in the room.
If the right few percent of hydrogen was in that air, it would turn the room into a giant explosion waiting to happen. I don't know the proper mix ration. With gasoline in air, the explosive range in air is about 3 to 5 percent. Hydrogen would be somewhat different.
There are other ways hydrogen or other gases could have been released, from pipes or hidden tanks, if the timing was right, to create this same effect; maybe wet thermite was not the method.
In either case, the thermite reaction heat and flame would instantly ignite the fuel-air reaction and make a big, perhaps a "soft" but powerful, kaboom sufficient enough to break the floors up and with enough heat to melt or disintegrate the trusses***.
As the concrete is pulverized it acts as a high-velocity sandblaster which perforates and shreds everything in its path. The desks and computers and everything are blown to bits and the people are liquefied by a rain of sand and rock travelling about five miles per second close to the source of the explosions.
Such blasts going off next to each other in quick sequence could shatter the floors from a giant double handed kind of both-ears slap.
The debris- concrete, pulverized trusses and pans, and everything and everyone else- tries to escape the force, but there is nowhere to go except outward and over the edge, since the forces are above, below, and and behind: there is another blast or blasts emanating from points in the core- presuming shaped charges on columns, which I would say are probable or even essentially confirmed at this stage of the game.
Thermite reaction: Al + FeO = AlO + Fe
Formula: Reduction of water by thermite reaction: [ 2 H2O ] = [H2 + (2 O2) ]
Recombination with oxygen in air: 2H2 + O2 = 2 H2O
*NOTE Just a few pages before Thermometry, in which Sosman and Day are mentioned and it is said that for a generation optical methods (optical pyrometry) depended upon interpolation from their scale.
NOTE ** As I understand it a fuel-air means a fuel dispersed into a volume of air an then ignited when the percentage is is right so it burns almost all at once with an explosive effect. Just like blowing up aunt Nancy's kitchen with the gas stove, only a lot, lot bigger.
A thermobaric means something a bit different; it may (I am not positive yet) denote a a device like a fuel air but instead of using air it provides oxygen some other way.
That oxidizer could be LOX (liquid oxygen), or certain compounds like hydrogen peroxide or potassium permanganate.
This is related to basic solid-fuel and liquid-fuel rocket system technology, in which the rocket engine must carry its own air, so to speak, in the form of one of these chemicals, in order to maintain the necessary rate of combustion or, in outer space, to maintain any combustion.
Different mixes and compounds can be used for oxidizers in a propellant or explosive. It's basically giving the fuel a pre-packed charge of air or oxygen in a concentrated form, rather than have is use ambient atmospheric air as the simplest fuel-air devices do.
NOTE ***The trusses themselves could also have had detonating cord [detcord] on them but that would likely have left identifiable pieces. Perhaps that was the case, and the pieces in the great totla mas of rubble and dust were overlooked as having originally been part of the trusses, but that is a whole other explosives theory).