The plane then flipped over and sped toward the ground at upwards of miles per hour, crashing in a rural field near Shanksville in western Pennsylvania at a. All 44 people aboard were killed.
Its intended target is not known, but theories include the White House , the U. Capitol, the Camp David presidential retreat in Maryland or one of several nuclear power plants along the eastern seaboard. Citizens of 78 countries died in New York, Washington, D. At the World Trade Center, 2, died after the two planes slammed into the twin towers. That figure includes firefighters and paramedics, 23 New York City police officers and 37 Port Authority police officers who were struggling to complete an evacuation of the buildings and save the office workers trapped on higher floors.
At the Pentagon, people were killed, including 64 on American Airlines Flight 77, the airliner that struck the building. On Flight 93 , 44 people died when the plane crash-landed in Pennsylvania. Bush , who was in Florida at the time of the attacks and had spent the day being shuttled around the country because of security concerns, returned to the White House.
These acts shatter steel, but they cannot dent the steel of American resolve. In a reference to the eventual U. Within two months, U. Osama bin Laden, the mastermind behind the September 11th attacks, remained at large until May 2, , when he was finally tracked down and killed by U. In June , then-President Barack Obama announced the beginning of large-scale troop withdrawals from Afghanistan; it took until August for all U. It was signed into law by President George W.
Bush on November 25, Today, the Department of Homeland Security is a cabinet responsible for preventing terror attacks, border security, immigrations and customs and disaster relief and prevention.
Mohammed led propaganda operations for al Qaeda from In August , a U. More than professionals and technical subject matter experts, including 85 NIST staff members, answered the call and participated in the investigation. As part of the investigation, the NIST team gathered every bit of evidence they could find. Activities included:. Throughout the process, NIST held 23 public meetings and provided multiple opportunities for the public to review and comment on drafts of the reports. Read first-hand accounts of what it was like to work on the WTC investigation in our blog series.
Start with Shyam Sunder's reflections on the investigation and its legacy, and then keep reading for insights on everything from what it was like to catalogue the images and video to how changed one researcher The WTC investigation has had a significant legacy. In the reports, NIST made 31 recommendations for improvements to building and fire codes, standards, and practices based on the WTC investigation. While the federal government has no regulatory authority for building and fire codes, many U.
In addition, NIST scientists conducted tests of steel from the WTC buildings to measure their mechanical properties at normal and elevated temperatures. These tests led to the development and validation of performance criteria for fire resistive steel. This type of steel, which was not available at the time the WTC was built, would not necessarily prevent a building from collapsing during a fire, but it would give occupants more time to escape.
Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a , t structure has too much inertia to fall in any direction other than nearly straight down. The World Trade Center was not defectively designed. No designer of the WTC anticipated, nor should have anticipated, a 90, L Molotov cocktail on one of the building floors.
Skyscrapers are designed to support themselves for three hours in a fire even if the sprinkler system fails to operate. This time should be long enough to evacuate the occupants. No normal office fires would fill 4, square meters of floor space in the seconds in which the WTC fire developed.
Usually, the fire would take up to an hour to spread so uniformly across the width and breadth of the building. This was a very large and rapidly progressing fire very high heat but not unusually high temperature. After all, 1,, t of rubble will require 20, to 30, truckloads to haul away the material. The asbestos fire insulation makes the task hazardous for those working nearby.
Separation of the stone and concrete is a common matter for modern steel shredders. The land-filling of , t of concrete and stone rubble is more problematic. There will undoubtedly be a number of changes in the building codes as a result of the WTC catastrophe.
For example, emergency communication systems need to be upgraded to speed up the notice for evacuation and the safest paths of egress. Emergency illumination systems, separate from the normal building lighting, are already on the drawing boards as a result of lessons learned from the WTC bombing in There will certainly be better fire protection of structural members.
Protection from smoke inhalation, energy-absorbing materials, and redundant means of egress will all be considered. A basic engineering assessment of the design of the World Trade Center dispels many of the myths about its collapse.
First, the perimeter tube design of the towers protected them from failing upon impact. The outer columns were engineered to stiffen the towers in heavy wind, and they protected the inner core, which held the gravity load. Removal of some of the outer columns alone could not bring the building down. Furthermore, because of the stiffness of the perimeter design, it was impossible for the aircraft impact to topple the building.
However, the building was not able to withstand the intense heat of the jet fuel fire. While it was impossible for the fuel-rich, diffuse-flame fire to burn at a temperature high enough to melt the steel, its quick ignition and intense heat caused the steel to lose at least half its strength and to deform, causing buckling or crippling.
This weakening and deformation caused a few floors to fall, while the weight of the stories above them crushed the floors below, initiating a domino collapse. It would be impractical to design buildings to withstand the fuel load induced by a burning commercial airliner.
Instead of saving the building, engineers and officials should focus on saving the lives of those inside by designing better safety and evacuation systems. As scientists and engineers, we must not succumb to speculative thinking when a tragedy such as this occurs. Quantitative reasoning can help sort fact from fiction, and can help us learn from this unfortunate disaster. As Lord Kelvin said,. In , the National Institute of Standards and Technology also published a best practice guide recommending solutions to reduce risk of progressive collapse.
There have since been changes to building codes and standards on improving the structural performance of buildings on fire, as well as opportunities to escape such as added stairwell requirements. At the same time, the collapse of the twin towers demonstrated the very real dangers of fire in high-rise buildings. In the decades since the World Trade Center was designed, buildings have become taller and more complex, as societies demand sustainable and cost-effective housing in large cities.
This can be partly attributed to the wide use of flammable cladding. It is marketed as an innovative, cost-effective and sustainable material, yet it has shown significant shortcomings in terms of fire safety — as witnessed in the Grenfell Disaster. The Grenfell fire and similar cladding fires are proof that fire safety in tall buildings is still a problem.
And as structures get taller and more complex, with new and innovative designs and materials, questions around fire safety will only become more difficult to answer. Read more: Cladding fire risks have been known for years.
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