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April 2016 On the physics of high-rise building collapses features. In August 2002, the U.S. National Institute of Standards and Technology (NIST) launched what would become a six-year investigation of the three building failures that occurred on September 11, 2001 (9/11): the well-known collapses of the World Trade Center (WTC) Twin Towers that morning and the lesser-known collapse late that afternoon of the 47-story World Trade Center Building 7, which was not struck by an airplane. NIST conducted its investigation based on the stated premise that the “WTC Towers and WTC 7 [were] the only known cases of total structural collapse in high-rise buildings where fires played a significant role.” Indeed, neither before nor since 9/11 have fires caused the total collapse of a steel-framed high-rise - nor has any other natural event, with the exception of the 1985 Mexico City earthquake, which toppled a 21-story office building. Otherwise, the only phenomenon capable of collapsing such buildings completely has been by way of a procedure known as controlled demolition, where by explosives or other devices are used to bring down a structure intentionally. Although NIST finally concluded after several years of investigation that all three collapses on 9/11 were due primarily to fires, fifteen years after the event a growing number of architects, engineers, and scientists are unconvinced by that explanation. Steel-framed high-rises have endured large fires without suffering total collapse. Throughout history, three steel-framed high-rises are known to have suffered partial collapses due to fires; none of those led to a total collapse. Countless other steel-framed high-rises have experienced large, long-lasting fires without suffering either partial or total collapse. WTC 5 is an example of how steel-framed high-rises typically perform in large fires. It burned for over eight hours on September 11, 2001, and did not suffer a total collapse. Because the only loads present on 9/11 after the impact of the airplanes were gravity and fire (there were no high winds that day), many engineers were surprised that the Towers completely collapsed. The towers, in fact, had been designed specifically to withstand the impact of a jetliner. The only mechanism that could bring down the Twin Towers was controlled demolition. In 1935, the two 191-m-tall Sky Ride towers of the 1933 World’s Fair in Chicago were demolished with 680 kg of thermite and 58 kg of dynamite. Thermite is an incendiary containing a metal powder fuel (most commonly aluminum) and a metal oxide (most commonly iron oxide or “rust”). Eventually, when there were enough large steel-framed buildings that needed to be brought down more efficiently and inexpensively, the use of shaped cutter charges became the norm. Because shaped charges have the ability to focus explosive energy, they can be placed so as to diagonally cut through steel columns quickly and reliably. In general, the technique used to demolish large buildings involves cutting the columns in a large enough area of the building to cause the intact portion above that area to fall and crush itself as well as crush whatever remains below it. This technique can be done in an even more sophisticated way, by timing the charges to go off in a sequence so that the columns closest to the center are destroyed first. WTC 7 fell symmetrically and at free-fall acceleration for a period of 2.25 seconds of its collapse. The calculations show that - in the absence of explosives - the upper section of WTC 1 would have arrested after falling for two stories. The total collapse of WTC 7 at 5:20 PM on 9/11 is remarkable because it exemplified all the signature features of an implosion: the building dropped in absolute free fall for the first 2.25 seconds of its descent over a distance of 32 meters or eight stories. Its transition from stasis to free fall was sudden, occurring in approximately one-half second. It fell symmetrically straight down. Its steel frame was almost entirely dismembered and deposited mostly inside the building’s footprint, while most of its concrete was pulverized into tiny particles. Finally, the collapse was rapid, occurring in less than seven seconds. Instead, NIST (as well as the Federal Emergency Management Agency (FEMA), which conducted a preliminary study prior to the NIST investigation) began with the predetermined conclusion that the collapse was caused by fires. NIST’s final report provides an elaborate scenario involving an unprecedented failure mechanism: the thermal expansion of floor beams pushing an adjoining girder off its seat. The final frame of NIST’s WTC 7 computer model shows large deformations to the exterior not observed in the videos. Unfortunately, NIST’s computer modelling cannot be independently verified because NIST has refused to release a large portion of its modelling data on the basis that doing so “might jeopardize public safety.” In the case of the Twin Towers: high-velocity bursts of debris, or “squibs,” were ejected from point-like sources in WTC 1 and WTC 2, as many as 20 to 30 stories below the collapse front. NIST sidesteps the well-documented presence of molten metal throughout the debris field and asserts that the orange molten metal seen pouring out of WTC 2 for the seven minutes before its collapse was aluminum from the aircraft combined with organic materials. Yet experiments have shown that molten aluminum, even when mixed with organic materials, has a silvery appearance — thus suggesting that the orange molten metal was instead emanating from a thermite reaction being used to weaken the structure. Meanwhile, unreacted nano-thermitic material has since been discovered in multiple independent WTC dust samples. As for eyewitness accounts, some 156 witnesses, including 135 first responders, have been documented as saying that they saw, heard, and/or felt explosions prior to and/or during the collapses. The evidence points overwhelmingly to the conclusion that all three buildings were destroyed by controlled demolition. Given the far-reaching implications, it is morally imperative that this hypothesis be the subject of a truly scientific and impartial investigation by responsible authorities. (Source: EuroPhysicsNews): https://tinyurl.com/y84zvk6x
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