World Trade Center Collapse - Essay Example

WTC Collapse (17 April 2007) WTC Collapse Introduction The world saw one of the most tragic terrorist attacks on September 11, 2001. The two main towers of the World Trade Center complex that was a unique feature in the New York city were each hit by aircraft as part of the terrirost attacks. Both the towers collapsed resulting in massive damage to the rest of the complex and also the nearby buildings. It was estimated that the fatalities in all were 2,595 people inside and near the towers, together with the 157 people who on the two airplanes (Hirschkorn n. pag, 2003). If we take a look at the constructiuon of the two towers, it can be noted that the towers were designed as framed tube structures to maximize office space, with columns grouped around the boundary and within the core. The boundary columns supported nearly all lateral loads, like the wind loads, and also shared the gravity loads with the core columns. In total, above the 7th floor there were 59 boundary columns along each face of the building and there were 47 heavier columns in the core. Besides, all of the elevators and stairwells were located in the core, leaving a large column-free space between the boundary that was bridged by pre-fabricated floor trusses. The tower also consisted of truss system that allowed some load redistribution between the perimeter and core columns and supported the transmission tower The towers also consisted of a "hat truss" or "outrigger truss" located between the 107th and 110th floors, which consisted of six trusses along the long axis of core and four along the short axis. It was found to play a key role in the collapse sequence (NIST 6-17, 2005). WTC towers like all modern day skyscrapers were designed in a perfet way to survive major fires. After the fire incidence in 1975 that spread to six floors before being extinguished, fireproofing was also added to the two towers (Hamburger 2:1-40). It is said that while designing these towers, the possibility of any accedent by aircraft was also taken into consederation. John Skilling, who had been in charge of the structural design of the buildings said that an aircraft impact would cause a great deal of damage and loss of life, mainly because of the cunsequent fires, but he claiment that the structure would not collapse (Nalder n. pag). Leslie Robertson, who had also participated in the structural design of the towers, after the 9/11 attack said that the towers had in fact been designed to withstand the impact of the largest airliner such as the Boeing 707-320. According to Robertson, the modeled aircraft weighed 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), as in approach and landing. However, this would have been much slower than the actual impacts of 9/11. He also said that they lacked a good understanding and did not antiocipate the effects of such large fires on the structures (Robertson n. pag). Unfortunately, the designers Skilling and Robertson and many others had not anticipated sush an event while designing the buildings. Impact of the Fire Though both the towers were cinstructed in such a mannar with fireproofing, none of us anticipated such a attack. Once the aircrafts hit the towers the light construction and hollow nature of the structures allowed the jet fuel to penetrate deep inside the towers simultanously igniting many large fires over a wide area of the impacted floors. Once the fuel from the planes burned out the contents of the buildings cought fire and burned over the next hour or hour and a half (Field 2004). Many people has suggested that the fires may not have penetrated deep into the core of the building rathere the debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores. But in this scenario, the towers might have stood far longer, perhaps indefinitely (Gross 2004). The fires were hot enough and were responsible for weakening the columns and cause floors to sag down resulting in pulling perimeter columns inward, and significantly reducing their ability support the weight of the building above (NIST, 2005). In the later studies engineering suggest that the impact points and heights had a significant bearing on time to collapse. They also agreed to the fact that the load-bearing vertical structural components performed extremely well and, possibly, would have continued to stand some more time beyond the actual collapse times if it was not been for the fires. The subsequent fires had greater impact and attacked the lightweight trusses to the point that their bolted connections to the columns failed. This resulted in the progressive, catastrophic collapses (Onderzoek 911). From the initial stages of investigation, the Bush administration claimed that burning jet fuel caused the collapse of the towers. However there were many who disagreed with these facts that fire was the only factor the caused the collapses. Some facts representing the flaws in the government jet fuel theory include: There were photos showing people walking around in the hole in the North Tower where 10,000 gallons of jet fuel supposed to be burning. When the South Tower was hit by the second aircraft, most of the North Tower's flames had vanished, burning for only 16 about minutes, making it comparatively easy to contain and control without a total collapse. The fire did not grow over time, most likely because it quickly ran out of fuel and was suffocating, demonstrating without added explosive devices the fires could have been easily controlled. FDNY firefighters still remain under a tight government gag order to not talk about the explosions they heard, felt and saw. FAA personnel are also under a similar 9-11 gag order. Even the defective 9-11 commission report acknowledges that "none of the [fire] chiefs present thought that a total collapse of either tower was possible." Fire had never before caused steel-frame buildings to collapse, nor has fire collapsed any steel high rise since 9-11. The fires, especially in the South Tower and WTC-7, were relatively small. WTC-7 was undamaged by any airplane and had only minor fires on the seventh and 12th floors of this 47-story steel building, yet it collapsed in less than 10 seconds. WTC-5 and WTC-6 had powerful fires but did not collapse despite much thinner steel beams. It's difficult if not impossible for hydrocarbon fires like those fed by jet fuel to raise the temperature of steel close to melting (Szymanski 2005). The impact of the plane crashes directly caused noteworthy structural damages to both WTC towers. The towers resisted this level of damage and did not collapse instantaneously showing that the redundancy of the tube-frame structures enabled the redistribution of the loads from the damaged zones to the remaining structures. However, many of them commonly believed that the impact also largely striped off the fire protection materials from structural steel at least in the place where the aircraft primarily hit and caused the damage. The multiple floors fires were ignited by the jet fuel that finally weakened the remaining structures and the towers collapsed. The composite floor truss system provided the lateral resistance or stability of the perimeter columns in the tube-frame system of the WTC towers. This lateral control is reduced as the floor trusses deteriorate and sag in the heat. In a multiple floors fire, it was anticipated that the effective length of laterally unrestrained perimeter columns would increase at least twice or triple. Besides the direct thermal effects, the compressive resistance of these columns ultimately reduced until a point that they could not sustain the applied load and buckling occurred. In later stages, after the initial plane impacts, it appeared that the structures had been severely damaged, however it did not look like it was fatally. It appears likely that the impact of the plane crash destroyed a significant number of perimeter columns on several floors of the building, severely weakening the entire system. Initially this was not enough to cause collapse. However, as fire raged in the upper floors, the heat would have been increasingly affecting the behavior of the remaining material. While the fire would not have been hot enough to melt any of the steel, it is believed that the strength of the steel drops noticeably with prolonged exposure to fire, while the elastic modulus of the steel reduces (stiffness drops), increasing deflections (Wilkinson 2007). Finally, it can be said that a combination of various factors resulted in the loss of strength and stiffness of the materials. The fire, combined with the initial impact damage, would have caused a failure of the truss system supporting a floor, or the remaining perimeter columns, or even the internal core, or some combination. Failure of the flooring system would have subsequently allowed the perimeter columns to buckle outwards. Once one storey collapsed all floors above would have begun to fall. The enormous mass of falling structure would gain momentum, devastating the structurally intact floors below, resulting in disastrous failure of the entire structure. Work cited Field, A. A Look Inside a Radical New Theory of the WTC Collapse. Fire/Rescue News. (2004). 16 April 2007 Gross, J. L., Therese P. McAllister Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers (pdf). (2004). Federal Building and Fire Safety Investigation of the World Trade Center Disaster NIST NCSTAR 1-6. National Institute of Standards and Technology. Hamburger, R., et al. World Trade Center Building Performance Study. Federal Emergency Management Agency. 2:1-40. 16 April 2007 Hirschkorn, P. New York reduces 9/11 death toll by 40. (October 29, 2003) CNN. 16 April 2007 < http://www.cnn.com/2003/US/Northeast/10/29/wtc.deaths/> NIST Final Report on the Collapse of the World Trade Center (chapter 1). (2005) National Institute of Standards and Technology (NIST) (September 2005). 6-17. Nalder, E. Twin Towers Engineered to Withstand Jet Collision. The Seattle Times. Saturday, February 27, 1993. Onderzoek 911 Fire Engineering - World Trade Center Respons 16 April 2007 Robertson, Leslie E. Reflections on the World Trade Center. The Bridge (2002).Vol 32, No1. National Academy of Engineering. Szymanski, G. Bush Insider Claims WTC Collapse Bogus (2005) 16 April 2007 Wilkinson, T. World Trade Center - Some Engineering Aspects (2007) 16 April 2007 Read More