I’m going to start this off by saying that I long ago decided that I didn’t want to write an article on the 9/11 conspiracy theory. It was a terrible tragedy that resulted in thousands of innocent deaths. Most conspiracies I write about are distant enough to not carry a lot of emotional weight.
This is an exception.
I decided to write an explanation of events as a result of seeing a lot amount of memes on social media in a short time. I think the best way to explain why all the conspiracies are absurd is to just stick to the facts in this case. This article aims to establish a basic understanding of structures and metallurgy associated with the World Trade Center terrorist attacks in order to debunk the conspiracy theories surrounding it.
8:46 am – First airplane strike
9:03 am – Second airplane strike
Both the north and south towers were struck by jetliners that were heavily loaded with fuel. The attack was calculated by an engineer in such a way as to inflict the maximum amount of damage.
Both airplane strikes resulted in structural columns being severed. Buildings are designed to withstand load bearing columns being severed; however, this reduces your effective safety factor of the design.
At each strike, at least 10,000 gallons of jet fuel were introduced into each respective building. This fuel traveled across the impacted floors and ignited. The jet fuel burned for several minutes causing a massive structure fire consuming multiple levels of each building.
9:59 am – South tower collapses after 56 minutes.
10:28 am – North tower collapses after 102 minutes.
In both cases, load bearing columns were destroyed. In both cases, high intensity fires were started by ~10,000 gallons of jet fuel. In both cases, a major structure fire was started by each respective fuel fire.
While structural engineering is a somewhat arcane subject that I do not claim expertise in, I’ll try to break down some key points:
Structural members carry load. Load is distributed among structural members. If a structural member is severed, the remaining members must assume that load. In the case of the WTC attack, multiple structural members were severed by the airplane strikes. This would substantially increase the loading on the remaining beams. Since skyscrapers are built with huge safety factors in mind, this is not enough to destroy the structure.
However, that’s not the only thing going on in this equation.
Structural steel undergoes a process known as “heat treatment.” Various elements are used in the production of structural members. Structural steel mainly makes use of carbon infused iron. Structural steel is created with two things in mind: carbon distribution and grain formation. The details of this are not terribly important beyond stating that the heating of a particular structural steel I beam can cause changes that drastically impact the strength, rigidity, elasticity, and many other characteristics of that beam.
In the case of the WTC attacks, the remaining structural members were subjected to extremely high temperatures. After about 500F, you start to see a marked decrease in the strength of steel. If we assume the fire was at 1500F, that’s enough to drive structure steel to form different grain phases which comes with a subsequent change in material properties.
The net result of the softening of the structural steel combined with loss of supporting structural members was sufficient to cause collapse.