11 сентября 2001 - Мейссан Тьерри - Страница 57
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Although dramatic, these fireballs did not explode or generate a shock wave. If an explosion or detonation had occurred, the expansion of the burning gasses would have taken place in microseconds, not the 2 seconds observed. Therefore, although there were some overpressures, it is unlikely that the fireballs, being external to the buildings, would have resulted in significant structural damage. It is not known whether the windows that were broken shortly after impact were broken by these external overpressures, overpressures internal to the building, the heat of the fire, or flying debris.
The first arriving firefighters observed that the windows of WTC 1 were broken out at the Concourse level. This breakage was most likely caused by overpressure in the elevator shafts. Damage to the walls of the elevator shafts was also observed as low as the 23rd floor, presumably as a result of the overpressures developed by the burning of the vapor cloud on the impact floors.
If one assumes that approximately 3,000 gallons of fuel were consumed in the initial fireballs, then the remainder either escaped the impact floors in the manners described above or was consumed by the fire on the impact floors. If half flowed away, then approximately 4,000 gallons remained on the impact floors to be consumed in the fires that followed. The jet fuel in the aerosol would have burned out as fast as the flame could spread through it, igniting almost every combustible on the floors involved. Fuel that fell to the floor and did not flow out of the building would have burned as a pool or spill fire at the point where it came to rest.
The time to consume the jet fuel can be reasonably computed. At the upper bound, if one assumes that all 10,000 gallons of fuel were evenly spread across a single building floor, it would form a pool that would be consumed by fire in less than 5 minutes (SFPE 1995) provided sufficient air for combustion was available. In reality, the jet fuel would have been distributed over multiple floors, and some would have been transported to other locations. Some would have been absorbed by carpeting or other furnishings, consumed in the flash fire in the aerosol, expelled and consumed externally in the fireballs, or flowed away from the fire floors. Accounting for these factors, it is believed that almost all of the jet fuel that remained on the impact floors was consumed in the first few minutes of the fire.
As the jet fuel burned, the resulting heat ignited office contents throughout a major portion of several of the impact floors, as well as combustible material within the aircraft itself.
A limited amount of physical evidence about the fires is available in the form of videos and still photographs of the buildings and the smoke plume generated soon after the initial attack. Estimates of the buoyant energy in the plume were obtained by plotting the rise of the smoke plume, which is governed by buoyancy in the vertical direction and by the wind in the horizontal direction. Using the Computational Fluid Dynamics (CFD) fire model, Fire Dynamics Simulator Ver. 1 (FDS1), fire scientists at the National Institute of Standards and Technology (NIST) (Rehm, et al. 2002) were able to mathematically approximate the size of fires required to produce such a smoke plume. As input to this model, an estimate of the openings available to provide ventilation for the fires was obtained from an examination of photographs taken of the damaged tower. Meteorological data on wind velocity and atmospheric temperatures were provided by the National Oceanic and Atmospheric Administration (NOAA) based on reports from the Aircraft Communications Addressing and Reporting System (ACARS). The information used weather monitoring instruments onboard three aircraft that departed from LaGuardia and Newark airports between 7:15 a.m. and 9:00 a.m. on September 11, 2001. The wind speed at heights equal to the upper stories of the towers was in the range of 10-20 mph. The outside temperatures over the height of the building were 20-21 degrees Centigrade (68-70 degrees Fahrenheit).
The modeling suggests a peak total rate of fire energy output on the order of 3-5 trillion Btu/hr, around 1-1.5 gigawatts (GW), for each of the two towers. From one third to one half of this energy flowed out of the structures. This vented energy was the force that drove the external smoke plume. The vented energy and accompanying smoke from both towers combined into a single plume. The energy output from each of the two buildings is similar to the power output of a commercial power generating station (this is the same type of misleading statement that the «Scientific» American article made, in its description of the aircraft strikes and fires in the WTC as equivalent to small nuclear weapons going off). The modeling also suggests ceiling gas temperatures of 1,000 degrees Centigrade (1,800 degrees Fahrenheit), for all of 5 minutes, until the jet-fuel burnt off, with an estimated confidence of plus or minus 100 degrees Centigrade (200 degrees Fahrenheit) or about 900-1,100 degrees Centigrade (1,600-2,000 degrees Fahrenheit).
This is impossible, as it is well known that the maximum temperature that can be reached by a non-stoichiometric hydrocarbon burn (that is, hydrocarbons like jet-fuel, burning in air) is 825 degrees Centigrade (1520 degrees Fahrenheit). Even worse, the WTC fires were fuel rich (as evidenced by the thick black smoke) and thus did not reach anywhere near this upper limit of 825 degrees. In fact, the WTC fires would have burnt at, or below, temperatures typical in office fires.
If the temperatures inside large regions of the building were above 700 degrees Centigrade, then these regions would have glowing red hot and there would have been visible signs of this from the outside. Even pictures taken from the air looking horizontally into the impact region show little or no sign of severe burning (above 700 degrees Centigrade).
When temperatures above 700 degrees Centigrade are reached within a region, this results in the breaking of the windows within that region. However, once the blast and fireball effects of the impacts had subsided, there appeared to be no ongoing window breakage from either tower, either as evidenced from pictures or video footage or as reported from the ground. In fact, significant areas of window even remained intact within the impact region. This is further evidence that fully developed fire conditions did not spread much through and beyond the initial devastated region, following the impacts.
In contrast, the First Interstate Bank fire in Los Angeles showed greater heating effects over larger regions than those observed in either tower. The temperature attained by the First Interstate Bank fire was clearly greater than that of either of the twin towers as the fire was hot enough to break the window glass (which rained down on the streets below presenting a considerable hazard to those on the ground).
The First Interstate Bank did not collapse.
A major portion of the uncertainty in these estimates is due to the scarcity of data regarding the initial conditions within the building and how the aircraft impact changed the geometry and fuel loading. Temperatures may have been as high as 900-1,100 degrees Centigrade (1,700-2,000 degrees Fahrenheit) in some areas and 400-800 degrees Centigrade (800-1,500 degrees Fahrenheit) in others.
All this talk of such high temperatures is to convince you that the steel beams and columns must have got really hot, but this is not so. For example, a ceiling gas temperature of 1,800 degrees Fahrenheit, for 5 minutes, would not heat the steel beams and columns significantly and the typical office fire that followed would not heat them to the point of collapse (trusses however, may have been significantly affected (this is the reason why the «truss theory» became popular)). It should be noted that the twin towers were designed to survive much more serious fires than those that occurred on September 11. That is the law.
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