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. Sisson, Jr. The collapse of the World Trade Center WTC towers on September 11, , was as sudden as it was dramatic; the complete destruction of such massive buildings shocked nearly everyone.
Immediately afterward and even today, there is widespread speculation that the buildings were structurally deficient, that the steel columns melted, or that the fire suppression equipment failed to operate. In order to separate the fact from the fiction, we have attempted to quantify various details of the collapse. The major events include the following:. Each will be discussed separately, but initially it is useful to review the overall design of the towers.
The towers were designed and built in the mids through the early s. They represented a new approach to skyscrapers in that they were to be very lightweight and involved modular construction methods in order to accelerate the schedule and to reduce the costs. To a structural engineer, a skyscraper is modeled as a large cantilever vertical column. Each tower was 64 m square, standing m above street level and 21 m below grade.
This produces a height-to-width ratio of 6. The total weight of the structure was roughly , t, but wind load, rather than the gravity load, dominated the design. This permitted windows more than one-half meter wide. It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story.
Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction that is about 95 percent air, explaining why the rubble after the collapse was only a few stories high. The egg-crate construction made a redundant structure i. Prior to the World Trade Center with its lightweight perimeter tube design, most tall buildings contained huge columns on 5 m centers and contained massive amounts of masonry carrying some of the structural load.
The early news reports noted how well the towers withstood the initial impact of the aircraft; however, when one recognizes that the buildings had more than 1, times the mass of the aircraft and had been designed to resist steady wind loads of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising.
The only individual metal component of the aircraft that is comparable in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraft fuselage. While the aircraft impact undoubtedly destroyed several columns in the WTC perimeter wall, the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure.
The ensuing fire was clearly the principal cause of the collapse Figure 4. The fire is the most misunderstood part of the WTC collapse. Even today, the media report and many scientists believe that the steel melted. It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true. Part of the problem is that people including engineers often confuse temperature and heat.
While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density or mass. Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material.
Traditional skyscrapers owed their stability to a system of large vertical columns running through each floor at intervals of feet, with the exterior walls providing little support on their own. The builders of the World Trade Center put extensive research into the effect of wind on the towers, commissioning one of the earliest wind tunnel studies for a skyscraper and performing perceptual tests disguised as eye exams on unsuspecting subjects to figure how much the building could sway in high winds without people noticing.
With this cutting-edge shock absorbing system in place, the towers were designed to be able to sway up to three feet in either direction on a windy day. New York City, circa v iew from the Hudson River of the first tower of the World Trade Center under construction, with 'kangaroo' cranes. In all, workers used some , tons of steel to construct the twin towers of the World Trade Center.
This system enabled engineers to use up far less space for elevator shafts, greatly increasing the amount of rentable space in each tower. But as an interstate agency, the Port Authority was exempt from such codes. 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.
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