"One World Trade Center and Two World Trade Center, commonly the Twin Towers, the idea of which was brought up by Minoru Yamasaki, were designed as framed tube structures, which provided tenants with open floor plans, uninterrupted by columns or walls. They were the main buildings of the World Trade Center."
And a link to a Google search. Surprisingly, most of the images are misleading, suggesting the load on the outer columns was minimal. They were some interior columns mainly for elevators and air shafts, taking maybe last than a quart of the load.
In a way is similar to the design of unibody cars which have no chassis but a frame made of sheet metal. There are advantages and disadvantages of cars without a chassis. I've once been hit from behind waiting in my truck at an intersection by a small but heavy Nissan Z built similarly to Porsches that is with a heavy chassis. Chassis hitting another chassis both with massive, steel bumpers made my mandible and bones in my spine move a bit from their normal positions which hurt for weeks after. Luckily though i had my foot pressing pretty hard on the break pedal when it happened and i was not pushed in traffic at that intersection. Also, minimal, almost invisible damage to both cars.
If i was in a unibody car, the frame would have bent and suffered damage while the shock would have been minimal.
As i said yesterday as i vaguely remembered reading it a long time ago, the WTC twin towers where designed by the American architect of Japanese descent Minoru Yamasaki in an innovative fashion for skyscrapers at that time. He was given the task after winning a contest against more famous architects. Today i checked and my memory was right.
Towers, instead of having massive columns inside, they were built more like a unibody car or wide bodied plane or most expressively said, like a can. All the resistance was at the outer walls, leaving
more freedom for floor plans.
How innovative this type of design was?
"Tube structures cut down costs, at the same time allow buildings to reach greater heights. Tube-frame construction was first used in the DeWitt-Chestnut Apartment Building, designed by Khan and completed in Chicago in 1963.[4] It was used soon after for the John Hancock Center and in the construction of the World Trade Center."
Among the requirements the towers should have withstood 80 mph winds and an impact of the biggest plane at the time. Probably like all the others.
But there is one difference between the previous and even following tubular structure design of tall building making the WTC design even more innovative. The size of the outer beams. On the same cross section, the load was divided on thinner, more numerous beams, 1 meter apart each. That made them more prone to breaking when the plane crashed through them and more vulnerable to heating by fire.
On September 11 2001 two planes full of kerosene after just taking off hit each tower.
What freedom means for floor plans? Lack of walls allows installing vast areas with cubicles which allowed the fuel from the shredded planes to soak the carpet and furniture and air go get in all the way to the core to feed the fires where about 1/4 of the vertical load of the building lays upon.
Here is a diagram out of the web of continuous oil distillation process.
Heated oil is pumped continuously inside the column and fuels separate by weight and come out continuously from those pipes. The temperature inside the distillation column is higher at the bottom and lower at top to keep the heavier fractions flowing. The fuel fractions separate by weight of the fraction, the more hydrogen and less carbon content of each hydrocarbon fuel fraction distilled, the higher in the distillation tower.
I brought this up because burning carbon gives you more energy per each individual molecules combined than hydrogen. The heavier the fuel, also the more energy per weight it packs. From this point of view kerosene is close to diesel, that is a more energy carrying fuel than gasoline.
I has been speculated a lot on the web about burning temperature of kerosene being lower than melting temperature of iron.
But does kerosene have a precise burning temperature?
Everybody who has ever seen somebody welding with a torch fed with acetylene knows that temperature or volume of flame can both be adjusted. The more oxygen you give to the flame, the higher the temperature.
Now that i think i started to realize there is not a precise burning temperature for anything. What is burning. Combining oxygen and hydrocarbons. Each hydrogen molecule that combines with carbon and hydrogen gives a precise amount of energy, not temperature. Temperature in a fire is dictated by the volume of the burning involved. The more oxygen you have or better said the closer to ideal the mix, the higher the temperature.
"It is unfortunately not too rare to find that fire investigators estimate flame temperatures by looking up a handbook value, which turns out to the adiabatic flame temperature. Statements are then made about whether some materials could have melted, softened, lost strength, etc., based on comparing such a flame temperature against the material's melting point, etc. The purpose of this short paper is to point out the fallacies of doing this, and to present some more appropriate information for a more realistic assessment."
https://www.doctorfire.com/flametmp.html
In a typical open fire you will have burning only at surface surrounding the fuel. That is because oxygen is all being consumed at the surface and cannot reach inside because of the dynamics of hot gas or plasma moving and preventing it from reaching inside and because of being spent there. That's why all the blacksmiths since the beginning of iron age invented and use the bellows.
They also use porous carbon rich coal made of wood that allows air to flow inside their volume after eliminating hydrogen and water from it usually by partially burning it in a low oxygen environment like in sand at high temperature.
They blow air with the bellows like in this video for reaching high temperatures necessary to forge the iron, usually above 771 Celsius or 1420 Farenheit when the iron starts to soften by changing to a different allotropic state, from body-centered cubic (BCC), most resistant (same with diamond) to a face-centered cubic (FCC) which allows movement or slip of crystals on slip planes.
https://en.wikipedia.org/wiki/Slip_(materials_science)
https://www.google.com/search?q=iron+beta+allotropic+temperature
But way before that happens there is another phenomenon happening.
During fabrication of beams through cold roling the steel like any metal hardens (before breaking). This hardening through lamination done by stressing metal is an advantage of cold lamination. But this type of hardening is lost if metal is heated at re-crystallization temperature, which is lower than the temperature stated above.
Hot rolled beams are also hardened during the controlled cooling process.
Both type can loose strength if heated above 260 degrees Celsius through a process called re-growth (of crystals).
With all these being said i think it's easy now for everybody to figure that if the steel towers were designed in a more "classical" fashion, with vertical beams way inside, the air and fire could not have reached to create near the beams the temperatures necessary to weaken them being consumed by the fire in the windows area.
Once the breakage started at one floor, the rest of the building above will start falling reaching enough moving energy to break he floor under. Simply because the outer beams where thicker towards the bottom of the tower, breaking first in the weakest area which was always at the floor under the breakage front. In a way, it was like a controlled demolition, only by design.
Inside the floors they were enough materials like concrete that would put out vast amounts of harmful dust that would cover significant areas around for a long time.
