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Firefighter looks up at the remains of the World Trade Center, two days after its collapse]] Following the September 11, 2001 Attacks , that sent one hijacked airliner into each of the main towers of the World Trade Center complex, 1 WTC, 2 WTC and 7 WTC collapsed. Both main towers lost structural integrity and fell that morning, killing 2,595 people within and nearby, as well as 157 people who were aboard the flights. Other nearby buildings, including 7 WTC, were damaged by the debris. 7 WTC was evacuated, and collapsed seven hours later at 5:20pm, without casualties. In the first few months after the attacks, most experts who gave statements to the media lauded the performance of the Twin Towers, suggesting that loss of life could have been far worse if design and construction of the buildings had been of lesser quality. Radical design decisions made by the WTC team were compared to more time-tested , 2006 issued by the Federal Emergency Management Agency (FEMA) in May 2002, pronounced the WTC design fundamentally safe and attributed the collapse wholly to extraordinary factors beyond the control of the builders. The , 2006 documenting emergency response efforts and events leading up to the collapse. NIST concluded "the buildings would likely not have collapsed under the combined effects of aircraft impact and the subsequent jet-fuel ignited multi-floor fires, if the fireproofing had not been dislodged or had been only minimally dislodged by aircraft impact." NIST also found the Towers' stairwell design lacked adequate reinforcement. The FEMA and NIST reports have not resolved all disagreements among engineers. ''New Civil Engineer'' published an article titled ''Row erupts over why twin towers collapsed'', in which one party claims "the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact." Fireproofing was added after a minor electrical fire in 1975Federal Emergency Management Agency, WTC1 and WTC2 .pdf, ''World Trade Center Building Performance Study'', URL accessed , 2006 . CONSTRUCTION OF 1 AND 2 WTC , 2004 ]] Construction of the towers began in 1968 and were completed in 1972 and 1973. During the period, implementation of an innovative elevator system halved the number of elevator shafts. The express elevators took people to "sky lobbies" on the 44th and 78th floors, where they could board local elevators. Also unique was its grouping of columns into the core and perimeter of the building, a structural system called a "tube". To meet the challenges of wind load, gravity load and related architectural stresses, the WTC's structural engineers took a then-unusual approach in its construction: instead of employing a traditional grid-like plan with beams evenly spaced throughout a floor, the WTCs columns were grouped in the building's core and perimeter. The core of each tower was a rectangular area 87 by 133 feet (27 by 41 meter) and consisted of steel box columns running from the bedrock to the tops of the tower. The columns tapered to the top, where they transitioned to lightweight H-beams, but the exact dimensions are unknown as the blueprints are under the jurisdiction of the Port Authority and are not public domain. Each tower had 240 steel perimeter columns (from 2.5 inches thick at the bottom tapering to .25 inch at the top to 0.6 cm ) placed 14 inches (36 cm) around the perimeter. This signature feature of grouping columns in the core and perimeter allowed large tracks of floorspace that were uninterrupted by columns, a significant marketing feature for the towers. Early tests conducted on steel beams from the World Trade Center show they generally met or were stronger than design requirements, ruling them out as a contributing cause of the collapse of the towers, federal investigators from the NIST stated (Boston Globe, August 28, 2003, www.boston.com/news/nation/articles/2003/08/28/steel_type_in_wtc_met_standards_group_says?mode=PF). IMPACTS OF AIRLINERS '']] The towers were each struck by hijacked , 2006 (c. 40,000 L) of jet fuel and immediately spread the fire to several different floors simultaneously while consuming paper, furniture, carpeting, computers, books, walls, framing and other items in all the affected floors. After the airliners hit, it appeared to most ground observers that the buildings had been severely but not fatally damaged. However, intense heat from the burning , 2006 A combination of factors including the impact from aircraft flying at high speed caused internal structural damage.Clifton, G. Charles, Collapse of the World Trade Centre Towers .pdf, URL accessed , 2006 Fires resulting from the aviation fuel spread widely through the impact zone and ultimately led to the collapse of the Twin Towers. As the Towers collapsed, the destabilization, debris, and associated fires damaged many of the buildings in the immediate vicinity. COLLAPSE OF THE TWO TOWERS The north tower, 1 WTC, was struck at 8:46:26 am and collapsed at 10:28:31 am, standing for 102 minutes and 5 seconds after impact. The south tower, 2 WTC, was struck at 9:02:54 am and collapsed 56 minutes and 10 seconds later, at 9:59:04 am. A combination of three factors allowed the north tower to remain standing longer: the region of impact was higher (so the gravity load on the most damaged area was lighter); the speed of the airplane was lower (so there was less impact damage); and the affected floors had had their fire proofing partially upgraded. One area of fire concentration in the south tower was near a corner of the buildingUnattributed video, video and led to a sudden bursting of , 2006 The two towers collapsed in markedly different ways which may indicate that there were two modes of failure. The north tower collapsed directly downwards, "pancaking" in on itself, while the south tower fell at an angle during which the top 20 or so stories of the building remained intact for the first few seconds of the collapse, then pulverized into dust in mid-air, and the tower then continued straight down. In spite of these differences, an Australian structural engineer believes that the "same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers"Wilkinson, Tim, Why did it collapse? , World trade Center - Some Engineering Aspects, (April 3, 2006), URL accessed May 1 , 2006 Subsequent modeling suggests that in the north tower the internal trusses supporting the building's concrete floors failed as a result of heat-induced warpingEagar, Thomas, The Collapse: An Engineer's Perspective , ''NOVA'', (May 2002), URL accessed , 2006 In the south tower, it is assumed to be fire that warpedNOVA, Bend , ''The Structure of Metal'', (May 2002), URL accessed , 2006 DESIGN CRITICISMS The collapse of the towers set off intense debates within the structural engineering and architectural professions. The largest camp appears to be those who feel the towers did well under the circumstances by standing long enough for the majority of occupants to escape. A minority takes exception to that view. Their criticisms of the WTC design feature six main points: # Longspan floors supported by external columns are inherently weaker than the traditional box frame column/girder arrangement with internal walls. # The bunching of all internal columns in a relatively narrow center shaft in a building is an "all your eggs in one basket" configuration-- if that region on any floor below the top floor is catastrophically damaged, the entire building is doomed. This stands in stark contrast to earlier generations of skyscrapers which utilize full skeletons of stepped columns, usually one row approximately every 25 feet (7.6 m) from the center to the perimeter. # The World Trade Center exclusively used lightweight materialsEagar, Thomas W., Christopher Musso, The Design , ''Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, JOM, vol. 53/12 pp. 8-11, (2001), URL accessed May 1 , 2006 especially in the facade. Had the WTC facade contained even minimal masonry elements and/or traditional heavy steel outermost column rows, it is less likely the aircraft would have cleanly penetrated to the core of each tower— a significant portion of debris and jet fuel would have remained outside, a much different scenario. # Single-bolt connections binding the longspan floorplates with the load-bearing external columns were extremely lightweight for their assigned task. One study group from the , 2006 Double-bolts should have been used. # The use of Gypsum cladding instead of reinforced concrete to shield stairwells. Almost all skyscrapers, including those built since the WTC, shield stairwells in reinforced concrete. On September 11th, it was the collapse of all stairways above the impact level that consigned all people above the impact zone in Tower One to death. Tower Two had two of its three stairwells taken out above the impact area by the plane. Some people above the impact zone survived, as they used the third stairwell. Computer models have shown that most of the stairwells in both towers would likely have remained usable until the general collapse had they been shielded in concrete. Some see the WTC as an irresponsible experiment in lightweight, rent-space-maximized construction and place particular opprobrium on Leslie E. Robertson , its Chief Structural Engineer. Others see it as a landmark in structural engineering simply in need of refinement due to unforeseen, and probably unforeseeable, variables. One of those variables was the size and kinetic energy of aircraft that might accidentally strike the WTC. Mr. Robertson and others involved in design and construction of the WTC have stated that back in the 1960s they could not have planned for the jetliners of 2001. Specifically, they modeled the effects of a hit by the largest aircraft of the day, the Boeing 707-320, and presumably calibrated their design to withstand it. According to the Federal Emergency Management Agency, WTC towers 1 and 2 were designed to withstand the impact of a 707 lost in fog while looking to land. The modeled aircraft was a 707 weighing 263,000 lb (119,000 kg) with a flight speed of 180 mph (290 km/h), as would be used in approach and landing situations.Federal Emergency Management Agency, WTC1 and WTC2 .pdf, ''World Trade Center Building Performance Study'', Pg. 17, URL accessed May 1 , 2006 The 767s that actually hit the towers had a kinetic energy more than seven times greater than the specifically modeled 707 impact. (The Boeing 747 , with an empty weight more than twice that of the 767, was in the final design phase when WTC drafting began and the first 747s were constructed simultaneously with the WTC towers; however the known attributes of the 747 were apparently not modeled in designing the towers). In its final report, the , 2006 had not believed that the buildings would collapse completely, but rather would collapse only above the levels where the planes respectively struck: Although a small group question its authenticity, a , 2006 A federal technical building and fire safety investigation of the collapses of the Twin Towers and 7 WTC was conducted by the , 2006 will serve as the basis for:
The long-anticipated report was partially released in draft for public comment on April 6 , 2005 . In its over 10,000 pages the conclusion reached was that the fireproofing on the steel infrastructure was blown off by the initial impact of the planes into the towers. If this had not occurred the WTC would have likely remained standing. A further finding of the report was that the staircases were not adequately reinforced to provide emergency escape for people above the impact zone. According to the Executive Summary of NIST's final report on the Collapses of the World Trade Center Towers, one of its goals was to "Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft and why and how WTC 7 collapsed" (p xli). However, the report elsewhere says that its "probable collapse sequence" "does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable." (p xliii) SEVEN WORLD TRADE CENTER The World Trade Center complex had a total of 7 buildings. The third building to collapse in the September 11, 2001 Attacks was 7 World Trade Center which fell at 5:20pm, as seen live on television. WTC 7 was a 47 story steel-frame skyscraper that stood across the street from the rest of the WTC complex. Firefighters appeared to have abandoned the building and let the fires burn. It remains unknown what criteria they used to determine that it should be abandoned or who decided to abandon it. Since little could be seen from the outside, and no one was able to observe what happened within the building, the cause of the collapse is disputed. As part of the electrical backup system, there may have been up to a total of 160,000 litres (42,000 gallons) of , 2006 This is report for WTC1 & WTC2 & does not say this - nor does report for WTC7--> Another speculation is that the building's unusual architecture may have contributed to its collapse. Theoretically, cantilevers and structural members, required to transfer building weight off of the pre-existing Con Ed electrical substation that the 7 WTC building was built over, may have failed in the fire leading to the internal mechanism of collapse. , 2006
The final report from the , 2006 In "WTC part IIC - WTC7 Collapse Final", released in April 2005, NIST concludes about the fuel "This finding allows for the possibility, though not conclusively, that the fuel may have contributed to a fire on Floor 5."Sunder, S. Shyam, World Trade Center Investigation Status .pdf, ''NIST Response to the World Trade Center Disaster'', (October 19, 2004), URL accessed May 1 , 2006 CONTROLLED DEMOLITION SPECULATIONS Many people have challenged the findings of U.S. Government engineers and independent researchers, arguing that controlled demolition better explains the full range of observations and available data. For more information, please see: 9/11 Conspiracy Theories . OTHER BUILDINGS Numerous other buildings in the World Trade Center and surrounding it were damaged or destroyed as the Towers fell. 5 World Trade Center suffered a large fire and a partial collapse of its steel structure. Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott Hotel (3 WTC), South Plaza (4 WTC), U.S. Customs (6 WTC), and the Winter Garden at the World Financial Center . World Financial Center buildings 4 , 5 , 6 , and 7 , 90 West Street , and 130 Cedar Street suffered fires. The Bankers Trust Building , Verizon, and World Financial Center 3 suffered impact damage from the Towers collapse, as did 90 West Street. 30 West Broadway was damaged by the collapse of 7 WTC. The Deutsche Bank building, though left standing, is currently being demolished because of water and mold damage, and severe damage caused by the neighboring towers' collapse. REFERENCES Cited references References
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