момент, как раз когда огонь стал гаснуть.
Где потеряли 32.000 тонн стали?
The World Trade Center demolition
On the 11th September, 2001, three steel framed skyscrapers, World Trade Center One, World Trade Center Two and World Trade Center Seven, collapsed entirely. Other than structures bought down in controlled demolitions, these three buildings are the only steel framed skyscrapers, in the entire history of high rise buildings, to have suffered total collapse. World Trade Centers 3, 4, 5 and 6 also suffered significant damage, but none of these suffered the total collapse seen in World Trade Centers 1, 2 and 7 (in fact, these other buildings showed amazing survivability given that they were repeatedly hit by hundreds of tons of pieces of World Trade Centers 1 and 2, which on impact were traveling at well over 100 miles per hour). On the 23rd July, 2001, just seven weeks previous, the Port Authority of New York and New Jersey signed a deal with a consortium led by Larry Silverstein for a 99 year lease of the World Trade Center complex. The leased buildings included WTCs One, Two, Four, Five and 400,000 square feet of retail space. The Marriott Hotel (WTC 3), U.S. Customs building (WTC 6) and Silverstein's own 47-story office building (WTC 7) were already under lease. Silverstein is seeking $7.2 billion from insurers for the destruction of the center. One would estimate that the chances of the insurers paying out anything at all, are close to zero. It should be emphasized that World Trade Center Seven suffered total collapse. World Trade Center Seven was neither hit by an aircraft nor by falling debris from the twin towers. If the claim that it was destroyed by fire were true (it is not) then it would be the only steel framed skyscraper ever to have collapsed exclusively due to fire. Although the WTC Seven collapse warrants the writing of a book, we will deal only with the collapses of WTCs One and Two.
The WTC was designed to survive the impact of a Boeing 767
Fact. The twin towers were designed to withstand a collision with a Boeing 707.
The maximum takeoff weight for a Boeing 707-320B is 336,000 pounds.
The maximum takeoff weight for a Boeing 767-200ER is 395,000 pounds.
The wingspan of a Boeing 707 is 146 feet.
The wingspan of a Boeing 767 is 156 feet.
The length of a Boeing 707 is 153 feet.
The length of a Boeing 767 is 159 feet.
The Boeing 707 could carry 23,000 gallons of fuel.
The Boeing 767 could carry 23,980 gallons of fuel.
The cruise speed of a Boeing 707 is 607 mph = 890 ft/s,
The cruise speed of a Boeing 767 is 530 mph = 777 ft/s.
So, the Boeing 707 and 767 are very similar aircraft, with the main differences being that the 767 is slightly heavier and the 707 is faster.
In designing the towers to withstand the impact of a Boeing 707, the designers would have assumed that the aircraft was operated normally. So they would have assumed that the aircraft was traveling at its cruise speed and not at the break neck speed of some kamikaze. With this in mind, we can calculate the energy that the plane would impart to the towers in any accidental collision.
The kinetic energy released by the impact of a Boeing 707 at cruise speed is
= 0.5 x 336,000 x (890)^2/32.174 = 4.136 billion ft lbs force (5,607,720 Kilojoules).
The kinetic energy released by the impact of a Boeing 767 at cruise speed is
= 0.5 x 395,000 x (777)^2/32.174 = 3.706 billion ft lbs force (5,024,650 Kilojoules).
From this, we see that under normal flying conditions, a Boeing 707 would smash into the WTC with about 10 percent more energy than would the slightly heavier Boeing 767. That is, under normal flying conditions, a Boeing 707 would do more damage than a Boeing 767.
In conclusion we can say that if the towers were designed to survive the impact of a Boeing 707, then they were necessarily designed to survive the impact of a Boeing 767.
So what can be said about the actual impacts?
The speed of impact of AA Flight 11 was 470 mph = 689 ft/s.
The speed of impact of UA Flight 175 was 590 mph = 865 ft/s.
The kinetic energy released by the impact of AA Flight 11 was
= 0.5 x 395,000 x (689)^2/32.174 = 2.914 billion ft lbs force (3,950,950 Kilojoules).
This is well within limits that the towers were built to survive. So why did the North tower fall?
The kinetic energy released by the impact of UA Flight 175 was
= 0.5 x 395,000 x (865)^2/32.174 = 4.593 billion ft lbs force (6,227,270 Kilojoules).
This is within 10 percent of the energy released by the impact of a Boeing 707 at cruise speed. So, it is also a surprise that the 767 impact caused the South tower to fall.
