Why wtc collapsed

For studying the impact on a story building by an actual Boeing aircraft, a full-scale test was not feasible. For a test to capture the response of the towers as a system, it would have been necessary to construct a test assembly that included the core columns, exterior columns, floors and hat truss.

Even to replicate experimentally the response of the floors near and above the impact zones would have required test assemblies of about 20 stories for WTC 1 and 30 stories for WTC 2. No facility exists to conduct such a test, either with fire or in the absence of fire; and, indeed, such tests are not conducted in current engineering practice. Therefore, NIST relied on high-fidelity finite element modeling of the aircraft impact event and subsequent fires.

The analyses were calibrated against the observed structural response of the towers upon impact videos, photographs, and physical evidence and the evolution of the ensuing fires. NIST did not conduct reduced-scale system-level tests because there are no generally accepted scaling laws that apply to fire propagation, temperature evolution, and structural response. Furthermore, fire test facilities with the capability to apply arbitrary fire exposures in contrast to the standard time-temperature exposure and arbitrary loads to structural components did not exist in the United States at the time of the investigation.

Even had such a facility existed, each large-scale structural fire test would have evaluated only a single set of conditions, e. Even a modest parametric series of such tests would have been prohibitively expensive. NIST did conduct full-scale fire tests of single and multiple workstations. These tests were of sufficient size to properly capture the combustion physics. These tests established burning histories, mass burning rates, and heat release rates.

NIST conducted a single workstation burn and a multiple workstation burn as a part of its investigation. Why did NIST only provide temperature data for one of these tests? Was the ventilation used in these tests representative of the ventilation that was present in the WTC towers on Sept.

The principal quantity measured was the rate of heat release. This quantity, combined with the ventilation, heat losses to walls, etc. In these tests, the window openings were close to the same size and layout as in the fire floors in the towers.

There was no glass in the windows, replicating the broken windows seen in the photographs of the vicinity of the tower fires. The report includes plots of the heat release rate and temperature histories in multiple locations for all of the tests. Why didn't NIST consider a "controlled demolition" hypothesis with matching computer modeling and explanation like it did for the "pancake theory" hypothesis?

NIST conducted an extremely thorough three-year investigation that included consideration of a number of hypotheses for the collapses of the WTC towers. Some technical experts—including about 85 career NIST experts and leading experts from the private sector and academia—reviewed tens of thousands of documents, interviewed more than 1, people, reviewed 7, segments of video footage and 7, photographs, analyzed pieces of steel from the wreckage, performed laboratory tests, and created sophisticated computer simulations of the sequence of events that occurred from the moment the aircraft struck the towers until they began to collapse.

Based on its comprehensive investigation, NIST concluded that the WTC towers collapsed according to the scenario detailed in the response to Question NIST's findings do not support the "pancake theory" of collapse, which is premised on a progressive failure of the floor systems in the WTC towers the composite floor system—that connected the core columns and the perimeter columns—consisted of a grid of steel "trusses" integrated with a concrete slab; see diagram.

Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards.

Thus, the floors did not fail progressively to cause a pancaking phenomenon. NIST's findings also do not support the "controlled demolition" theory since there is conclusive evidence that:. Video evidence also showed unambiguously that the collapse progressed from the top to the bottom, and there was no evidence collected by NIST or by the New York City Police Department, the Port Authority Police Department, or the Fire Department of New York of any blast or explosions in the region below the impact and fire floors as the top building sections including and above the 98th floor in WTC 1 and the 82nd floor in WTC 2 began their downward movement upon collapse initiation.

In summary, NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives.

NIST also did not find any evidence that missiles were fired at or hit the towers. Instead, photographs and videos from several angles clearly show that the collapse initiated at the fire and impact floors and that the collapse progressed from the initiating floors downward until the dust clouds obscured the view. Was the steel tested for explosives or thermite residues? The responses to previous questions demonstrate why NIST concluded that there were no explosives or controlled demolition involved in the collapses of the WTC towers.

As for thermite a mixture of powdered or granular aluminum metal and powdered iron oxide that burns at extremely high temperatures when ignited , it burns slowly relative to explosive materials and would require several minutes in contact with a massive steel section to heat it to a temperature that would result in substantial weakening.

Therefore, while a thermite reaction can cut through large steel columns, many thousands of pounds of thermite would need to have been placed inconspicuously ahead of time, remotely ignited, and somehow held in direct contact with the surface of hundreds of massive structural components to weaken the building.

This makes it an unlikely substance for achieving a controlled demolition. The metal compounds also would have been present in the construction materials making up the WTC towers, and sulfur is present in the gypsum wallboard that was prevalent in the interior partitions. Why were two distinct spikes—one for each tower—seen in seismic records before the towers collapsed?

Isn't this indicative of an explosion occurring in each tower? The seismic spikes for the collapse of the WTC towers are the result of debris from the collapsing towers impacting the ground. The spikes began approximately 10 seconds after the times for the start of each building's collapse and continued for approximately 15 seconds.

There were no seismic signals that occurred prior to the initiation of the collapse of either tower. The seismic record contains no evidence that would indicate explosions occurring prior to the collapse of the towers.

How could the WTC towers collapse in only 11 seconds WTC 1 and 9 seconds WTC 2 —speeds that approximate that of a ball dropped from similar height in a vacuum with no air resistance? NIST estimated the elapsed times for the first exterior panels to strike the ground after the collapse initiated in each of the towers to be approximately 11 seconds for WTC 1 and approximately 9 seconds for WTC 2.

These elapsed times were based on: 1 precise timing of the initiation of collapse from video evidence, and 2 ground motion seismic signals recorded at Palisades, N. As documented in Section 6. The potential energy released by the downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that energy through energy of deformation. Since the stories below the level of collapse initiation provided little resistance to the tremendous energy released by the falling building mass, the building section above came down essentially in free fall, as seen in videos.

As the stories below sequentially failed, the falling mass increased, further increasing the demand on the floors below, which were unable to arrest the moving mass. In other words, the momentum which equals mass times velocity of the 12 to 28 stories WTC 1 and WTC 2, respectively falling on the supporting structure below which was designed to support only the static weight of the floors above and not any dynamic effects due to the downward momentum so greatly exceeded the strength capacity of the structure below that it the structure below was unable to stop or even to slow the falling mass.

The downward momentum felt by each successive lower floor was even larger due to the increasing mass. From video evidence, significant portions of the cores of both buildings roughly 60 stories of WTC 1 and 40 stories of WTC 2 are known to have stood 15 to 25 seconds after collapse initiation before they, too, began to collapse.

Neither the duration of the seismic records nor video evidence due to obstruction of view caused by debris clouds are reliable indicators of the total time it took for each building to collapse completely. Weren't the puffs of smoke that were seen, as the collapse of each WTC tower starts, evidence of controlled demolition explosions?

As stated in Section 6. These puffs were observed at many locations as the towers collapsed. In all cases, they had the appearance of jets of gas being pushed from the building through windows or between columns on the mechanical floors. Such jets are expected since the air inside the building is compressed as the tower falls and must flow somewhere as the pressure builds. It is significant that similar "puffs" were observed numerous times on the fire floors in both towers prior to their collapses, perhaps due to falling walls or portions of a floor.

These observations confirm that even minor overpressures were transmitted through the towers and forced smoke and debris from the building. Why does NIST state that a yellow stream of molten metal seen in some photographs pouring down the side of WTC2 was aluminum from the crashed plane, even though aluminum burns with a white glow? This flow lasted approximately four seconds before subsiding. Many such liquid flows were observed from near this location in the seven minutes leading up to the collapse of this tower.

There is no evidence of similar molten liquid pouring out from another location in WTC 2 or from anywhere within WTC 1. Photographs, as well as NIST simulations of the aircraft impact, show large piles of debris in the 80th and 81st floors of WTC 2 near the site where the glowing liquid eventually appeared.

Much of this debris came from the aircraft itself and from the office furnishings that the aircraft pushed forward as it tunneled to this far end of the building.

I believe that some of the aircraft's fuel tanks must have suffered major damage, but that most of them would have been cut in two when they met the steel beams in the buildings, and that the development of the fire was therefore fairly constant. The entire internal basin must have been heated by the burning fuel.

Outside of the basin, the temperature would have been much lower. Experience gained from the aluminium industry suggests that it may have taken between half and three-quarters of an hour to reach such a temperature.

If molten aluminium is heated further to a temperature of o C, it becomes just as liquid as water. I presume that this is what happened within the Twin Towers, and that the molten aluminium then began to run down into the floors below.

All the water above the hot aircraft bodies must have turned to steam. If my theory is correct, tonnes of aluminium ran down through the towers, where the smelt came into contact with a few hundred litres of water. 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 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. One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles.

Thus, the fact that there were 90, L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel. In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame. A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber.

Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet. This is what occurs in a jet engine, and this is the flame type that generates the most intense heat. In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions.

It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame. A fireplace flame is a diffuse flame burning in air, as was the WTC fire. Diffuse flames generate the lowest heat intensities of the three flame types.

If the fuel and the oxidant start at ambient temperature, a maximum flame temperature can be defined.

This maximum flame temperature is reduced by two-thirds if air is used rather than pure oxygen.