When New York City's giant World Trade Center towers plunged to earth following successive suicide terrorist attacks on September 11th, the world was confronted with one of most shockingand sickeningsights of modern times. The mechanisms by which these huge and seemingly solid edifices suddenly collapsed, snuffing out the lives of thousands, was the subject of a preliminary postmortem conducted last week in Cambridge, Mass.
They also presented recommendations for more detailed engineering studies of the disaster. This investigation was completed in September The NIST investigators did not find anything substandard The twin towers collapse the design of the WTC towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.
They also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns: The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors.
The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half. It has been suggested, that the fires might not have been as centrally positioned, nor as intense, had traditionally heavy high-rise construction been standing in the way of the aircraft.
Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores, which would then not have become unique failure points.
In this scenario, the towers might have stood far longer, perhaps indefinitely. The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above.
As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions to dispatchers. The New York City Police Department aviation unit also relayed information about the deteriorating condition of the buildings to police commanders.
Only 14 people escaped from above the impact zone of the South Tower after it was hit including Stanley Praimnath, who saw the plane coming at himand only four from the floors above it.
They escaped via Stairwell A, the only stairwell which had been left intact after the impact.
Numerous police hotline operators who received calls from individuals inside the South Tower were not well informed of the situation as it rapidly unfolded. Many operators told callers not to descend the tower on their own, even though it is now believed that Stairwell A was most likely passable at and above the point of impact.
With the warnings, the NYPD issued orders for its officers to evacuate. The North Tower collapsed at Due to radio communications problems, firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed.
No one was able to escape the North Tower from the impact zone or above, as all stairwells and elevator shafts on those floors were destroyed or blocked. After the collapse, light dust reached as far as the Empire State Building, located 2.
Both buildings collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides.
In both cases, the section of the building that had been damaged by the airplanes failed, which allowed the floors above the impact zone to fall onto the undamaged structure below. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air from the upper levels.
The first fragments of the outer walls of the collapsed North Tower struck the ground 11 seconds after the collapse started, and parts of the South Tower after 9 seconds. While the buildings were designed to support enormous static loads, they provided little resistance to the moving mass of the sections above the floors where the collapses initiated.
Structural systems respond very differently to static and dynamic loads, and since the motion of the falling portion began as a free fall through the height of at least one story roughly three meters or 10 feetthe structure beneath them was unable to stop the collapses once they began.
Indeed, a fall of only half a meter about 20 inches would have been enough to release the necessary energy to begin an unstoppable collapse.
After the planes struck the buildings, but before the buildings collapsed, the cores of both towers consisted of three distinct sections. Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating.
The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel.Sep 10, · The NYPD (New York Police Department) logs the first tower's (South) collapse at a.m., and the second tower's (North) at a.m.
The Twin Towers collapsed . The towers collapsed only after the kerosene fuel fire compromised the integrity of their structural tubes: One WTC lasted for minutes, whereas Two WTC remained standing for 47 minutes.
Minutes: The Unforgettable Story of the Fight to Survive Inside the Twin Towers - Kindle edition by Jim Dwyer, Kevin Flynn. Download it once and read it on your Kindle device, PC, phones or tablets.
Use features like bookmarks, note taking and highlighting while reading Minutes: The Unforgettable Story of the Fight to Survive Inside the Twin Towers. Many of the peculiar anomalies violating the laws of physics in the collapse of Building 7 are also present and problematic in the collapse of the Twin Towers.
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