The coronavirus pandemic has stalled its completion, with a performing arts center under construction and a fourth and final skyscraper planned. Ground Zero became both a solemn memorial and a leisure destination.
It is the great public space. At its heart are two reflecting pools designed by Michael Arad, marking the footprints of where the Twin Towers once stood, with a pair of four-sided waterfalls draining into an abyss. The names of the victims are etched into its bronze borders. The fact the collapse of 7 World Trade Center was announced in a live report by BBC News correspondent Jane Stanley - while it was still visibly standing behind her - has been cited by conspiracy theorists as evidence major media organisations were part of the inside-job plot.
The Reuters news agency had mistakenly reported the collapse of the building, which was also picked up by CNN, just before the live report. Some online conspiracy theories suggest US missiles were fired at the Pentagon, as part of a government plot, and the hole left in the building was too small to have been caused by a passenger plane.
But a member of the American Society of Civil Engineers told Popular Mechanics magazine the size and shape of the hole was due to one wing of the Boeing hitting the ground and the other being severed on impact with the building.
Meanwhile, United Airlines Flight 93 crashed near Shanksville, Pennsylvania, after passengers tried to take control of the plane from the hijackers.
Online theories claim it was shot down by a white business jet flying into a nearby airport. But aviation officials had requested the jet inspect the area, which it did, reporting back evidence of a big hole in the ground with smoke coming out of it. Vice-President Dick Cheney later revealed in his autobiography that following the attack on the Twin Towers, he had ordered the shooting down of any commercial airliner believed to have been hijacked.
Another theory falsely claims no Jewish people were killed in the attacks because 4, Jewish employees at the World Trade Center had received advance notice not to turn up for work. 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.
Evacuees included hundreds of people with physical disabilities, some in wheelchairs, being carried down by their co-workers through the narrow stairs. Survivors said even as the descent moved at a snail's pace, people did not push. Below the points of impact, the stairs saved thousands of people, with about 14, occupants of the lower floors making it out alive.
The death toll would likely have been much higher that day, but for one crucial factor: The time of attack. Investigators estimate the buildings were only half full when the first plane struck at am. Had the attack taken place later, the overcrowding they saw in the stairwells during the evacuation would have been catastrophic, with thousands more trapped as the towers collapsed.
There were no video cameras in the stairwells and radio communications had broken down, leaving them unable to coordinate rescue efforts or receive warnings to evacuate. Accounts from survivors paint a grim picture of how the vacuum of information inside the towers compounded the death toll.
Even with the sole stairwell in the south tower remaining passable, the occupants above the point of impact didn't get the information they needed to make the right choice. When emergency operator recordings were released of distress calls that day, large numbers of victims had called for help from their mobile phones, only to be told to stay put and "defend-in-place".
Professor Corbett believes that if information from survivors who had made it down the stairs had been relayed to those still in the building with a quick call to their phones, many more may have made it out alive.
The US approved 23 building and fire code modifications in , following investigations into the World Trade Centre disaster. They included measures to improve fire resistance in building materials, to reinforce structures against collapse, and add blast-resistant walls to elevator and stairwell shafts — all designed to help buildings stay intact long enough to get people out.
High-rise buildings were required to improve radio coverage systems to ensure emergency crews can communicate with each other inside, and with personnel outside. A requirement for an extra stairwell did get through, but only in buildings above metres, more than 40 storeys high. The width of stairways would be increased by 50 per cent, but only in a building code that does not cover most of the new high rise buildings being built across the United States today, including New York City.
0コメント