Barnett, R. Biederman, and R. Sisson, Jr. The collapse of the World Trade Center WTC towers on September 11, , was as sudden as it was dramatic; the complete destruction of such massive buildings shocked nearly everyone. Immediately afterward and even today, there is widespread speculation that the buildings were structurally deficient, that the steel columns melted, or that the fire suppression equipment failed to operate.
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. In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame.
A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber. Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet.
This is what occurs in a jet engine, and this is the flame type that generates the most intense heat. In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions.
It does not attain the flame velocities of a jet burner. Besides the twin towers, a storey third tower, known as Tower Seven, was also collapsed after seven hours on the same day.
The then published reports had claimed that building fires were growing and spreading throughout the multiple floors, which became the ultimate reason for the collapse of the buildings.
Before the official investigative report was published, some conspiracy theorists argued that the third tower was "brought down" in a controlled demolition as it was not hit by a plane. Some theorists continue to argue even after the government refuted the claims constantly. The building was utterly important due to the presence of the Secret Service, the CIA, the Department of Defense and the Office of Emergency Management, which was supposed to coordinate any response to terrorist attacks.
Even after 20 years since the deadly attacks, some conspiracies continue to contest the government claims but so far, no concrete evidence has been provided to support them and theories remain Later in the day Andrew helped work the phones with other employees who survived and by the end of the day they had an idea of who got out and who didn't.
Derek, his Dundee friend, was one of the 2, who lost their lives in the Twin Towers attack. The pair bonded over their Scottish roots on Andrew's first day in the job. The firm lost 67 employees that day. The surviving staff spent the rest of the week in and out of New York hospitals checking logs to see if any of their colleagues had survived.
Andrew, now 51, is a history and economics teacher in South Carolina. He has attended a few memorial services since that day but prefers to keep a low profile. It is always there and it might change your life but it shouldn't stop you living it. Image source, Getty Images. Andrew was in the South Tower when the first plane hit and was trying to descend the building when the second plane caused another explosion.
It was a beautiful day. Image source, Andrew Cullen. If the lift had gone up, they would have been killed. A wall of dust and smoke races through streets as one of the World Trade Center towers collapses. Andrew was blown behind a desk in a health club as the store fronts along Wall Street were blown in.
0コメント