Forty percent of the weight of the towers was carried by their facades made of steel columns. The floors were made of 10 centimeters thick light concrete slabs resting on a steel lattice. The designers and structural engineers had taken into account the effects of a commercial airplane impact and believed that the structures would remain standing should such an event occur.
The tower remained standing longer before collapsing. The tower first resisted but its lightweight construction, the hollow nature of the structures and the fact that the thermal insulation was blown by the plane allowed the kerosene to penetrate to the interior of the towers, igniting numerous fires simultaneously over a large area of the affected floors.
The tower, therefore, collapsed in just 10 seconds. Despite the magnitude of the impacts, a lot of structural engineers and veteran disaster investigators were stunned that it had happened.
Immediately afterward there was widespread speculation that the buildings were structurally deficient, that the steel columns had melted, or that the failure of the fire suppression equipment had been an important factor.
Subsequent investigations, which clarified all of these issues, disproved many initial theories. Analyzing videos, eyewitness accounts and debris, the team published a hypothetical report on the causes in April This was followed by a far more thorough and better-funded investigation by the National Institute of Standards and Technology NIST , which took around three years to complete. There were many questions that needed to be answered, chief among them being how and why these buildings collapsed following the initial impact of the aircraft, and how we could minimize the chances of a similar event occurring in the future.
A total of more than staff — including about 85 career NIST experts and leading experts from the private sector and academia — poured over tens of thousands of documents. They interviewed more than 1, people, reviewed thousands of videos and photographs, analyzed pieces of steel from the wreckage, and performed laboratory tests involving large fires and the heating of structural components.
According to NIST, however, the reason for its collapse was due to uncontrolled fires on multiple floors, which were caused by debris dropping from WTC 1 and 2. This was the first time that a building of this kind had rapidly and completely collapsed due to fire. Most high-rise buildings since have used a similar structure. They conclude it was not caused by direct impact by the aircraft, or the use of explosives, but by fires that burned inside the buildings after impact.
But the answer becomes clear once you consider the details. Aircraft are made from lightweight materials, such as aluminium.
If you compare the mass of an aircraft with that of a skyscraper more than metres tall and built from steel and concrete, it makes sense the building would not topple over. That said, the aircraft did dislodge fireproofing material within the towers, which was coated on the steel columns and on the steel floor trusses underneath concrete slabs.
The lack of fireproofing left the steel unprotected. As such, the impact also structurally damaged the supporting steel columns. When a few columns become damaged, the load they carry is transferred to other columns.
These theories have developed from video footage showing the towers rapidly collapsing downwards some time after impact, similar to a controlled demolition. But it is possible for them to have collapsed this way without explosives.
It was fire that caused this. From other disasters and experiments carried out by the aluminium industry, we know that reactions of this sort lead to violent explosions. Such reactions are particularly powerful when rust or other catalysts are present, which can raise the temperature to more than o C. Alcoa Aluminium carried out an experiment under controlled conditions, in which 20 kilos of aluminium smelt were allowed to react with 20 kilos of water, to which some rust was added.
The explosion destroyed the entire laboratory and left a crater 30 metres in diameter. Film taken of the buildings also showed explosions in the floor below the impacts. Given that the amount of aluminium involved was large in comparison with the quantity of water, and since rust was probably also present, I believe that it is highly likely that the building collapsed as a result of a series of extremely energy-rich aluminium-water explosions.
They were probably powerful enough to blow out an entire section of each building. The top section would than fall down on top of the sections that remained below, and the sheer weight of the top floors would be enough to crush the lower part of the building.
When these materials and everything else fell some three or four hundred metres to the ground, they were squeezed between the upper and lower sections of the towers.
This led to the neighbouring buildings being bombarded by hot particles, fuel and probably also aluminium droplets. Both large and small clumps of particles have since been found embedded in the walls of these buildings. At any rate, the building caught fire, which got out of control. In this case, the structural steel may have reached a temperature of more than o C, over seven hours, and the 13th floor collapsed in the course of a minute.
In this case I do agree with the findings of the federal commission. Overheating of steel beams was probably the cause of the collapse. Experiments could also be carried out to find out whether fuel tanks are cut cleanly when they plough through a network of steel beams at a speed of kilometres an hour.
We could also test on model scale whether an object that ploughs through a room at extremely high speed becomes covered in debris from collapsed walls, ceilings and floors. Note: Content may be edited for style and length. Science News. The explosions fed the conspiracy theories that someone had placed explosives inside the towers. Explosive meeting of molten aluminium and water Simensen believes that it is overwhelmingly likely that the two aircraft were trapped inside an insulating layer of building debris within the skyscrapers.
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