"A poo caused an avalanche which destroyed the twin towers."
The twin towers in New York were destroyed by two commercial planes piloted by terrorist hijackers on September 11, 2001. So far, a completely convincing scientific explanation of how the towers collapsed was not given, leaving a lot of space for speculations. I believe my interpretation gives a contribution to the straight answer.
The twin towers were two identical steel construction skyscrapers with bases of 60x60 meters, and heights of 110 floors. They were standing on massive steel columns located in the core and perimeters of the building. Steel columns were the most substantial part of the twin towers. They were 3" thick at the bottom of the building and a ¼ inch at the top. The big span between the core and perimeter columns required floors to be carried by the trusses. The trusses were the much weaker part of the buildings. They transferred their load to the columns connected by bolts.
The planes' impact of the twin towers did significant damage. It made big holes in the towers which disturbed the equilibrium of the construction. According to NIST, the planes destroyed 14% of the columns. The remaining construction elements had to take over the pressure from the torn down elements. All constructions have a safety factor which can withstand at least double the weight than planned. But some of the columns next to the ruined part of the towers have certainly overpassed the safety factor and were in substantial danger to collapse.
However, the whole steel construction of the twin towers was rigidly connected. Steel has an elastic ability to pass the burden from critically pressed elements, which cannot carry the pressure anymore, to the next parts which were not critically pressed. The force deforms the steel to the best position, which reduces dangerous bending moments and finds the greatest possible balance. Big holes from the airplanes' impacts were bridged by the steel facade construction above, which prevented the immediate collapse of the building section above the hole. It also prevented the destruction of the over-pressed columns located next to the hole. The problem was that the facade was not constructed to carry such burden, neither were the "bases" of this "bridge." The whole construction was unstable and prone to collapse.
Then fire from the planes' kerosene further deteriorated the carrying capacities of the already damaged steel construction. If there were no fire, the towers would most likely not collapse. They would probably be fixed and still standing today.
The fire damaged the weakest part of the floor construction first. The tiny elements of the trusses expanded in high temperature and the load from above loosened them. The loosened floor construction pulled the columns they were connected with and bent them. Every move of the structure caused by external pressure brought a new bending moment which additionally pressed and weakened the construction. The more the column bent, the more significant bending moment by the weight of the building was produced. The more significant bending moment weakened the columns more. However, the strength of construction was still able to resist all the chaos of attacking forces and was strong enough to stand. But, it was in significant danger to collapse.
The whole situation of the towers reminds me of a cartoon when a bird lands on top of a heavily damaged building and then poops. The poo caused the building to collapse. Now I will simplify the cartoon building collapse by comparing it to breaking a wooden stick. A stick would resist the bending power to the moment when tiny additional pressure snaps it, after which the stick does not resist anymore. In physics, this process is called buckling, which in this case is a sudden break of a stick structure bent by a more significant force than the stick is capable of withstanding. This is how the bird poo destroyed the building from the cartoon.
A very similar thing happened to the twin towers. But what was the bird's poo in the twin towers situation? A large number of people heard a huge bang before the collapse of the tower started. They assumed it was an explosion which destroyed the tower. But it was not an explosion. This bang came from the collapse of one floor or its part. Most likely this floor was the one most exposed to fire. Fire weakened the bolts which held the trusses of floors to the columns. They were the weakest part of the floor construction, and they broke. That increased the pressure on the remaining bolts which broke under the floor pressure and the floor collapsed on top of the level below. I analyzed what could produce this huge bang and found nothing else.
The shake of the building caused by the collapsed floor would, in normal circumstance, barely shake coffee in a cup. But that was the bird's poo from the cartoon. This shake destroyed the weak equilibrium of the steel construction and started an avalanche effect which destroyed the tower.
The South tower collapsed first. The plane hit the right side of the tower more, which hardly damaged the right side columns. The stress of the vibration of the collapsed floor suddenly reached the buckling effect of these columns, which started the whole collapse. The construction of this part of the building immediately transferred from an 18-meter long beam supported by two sides (core and exterior columns) to an 18-meter console supported by only one side. The core columns were suddenly attacked by the bending moment nine times stronger than was the weight of console. The construction was never strong enough to carry even a tiny fraction of that pressure. The columns at that floor bent and become unable to carry the pressure from above. Then the part of the building above the impact started collapsing. The unharmed building below the impact resisted the hit of many floors from above before it started collapsing under the enormous kinetic power of the weight above. The columns were crushed like nails hit by heavy hammers to a solid base. This started the cascading crush of the tower down, floor by floor, with a "trampoline bouncing effect" coming from the resistance of the building construction. It was accelerated by gravity and the additional weight of each floor.
The North tower had a central impact from the plane, which hardly damaged the core columns. The core columns carried the same weight as the exterior columns through the lesser dimensioned profile. (The exterior columns were over-dimensioned to resist strong winds) This most likely made the core columns in the North tower start the collapse slightly faster than the outdoor columns. The bending moment attacked the outdoor columns. This produced the inward implosion of the tower, making the building fall symmetrically. The symmetrical collapse presented that the bottom part of the tower was strong and resisted the impact of around ten floors (40,000 tons) from above before it collapsed. But the falling process in both towers was practically the same.
There were small differences in the collapse of the twin towers. The South tower collapsed first even though it was hit second. Why? The South tower was hit lower than the North so that the load above the plane's impact of the South tower was more significant than the pressure above the North. The more massive load quickened the collapse of the South tower even though it was burned less in the fire. Also, the South tower was hit on the side, producing a more significant bending moment attacking the columns, causing a faster fall than the centrally hit North tower.
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