 Image: NOAA BEFORE SEPTEMBER 11, 2001, the twin towers of the World Trade Center seemed a permanent part of the Manhattan skyline. |
When New York City's giant World Trade Center towers plunged to earth following successive suicide terrorist attacks on September 11th, the world was confronted with one of most shocking¿and sickening¿sights of modern times. The mechanisms by which these huge and seemingly solid edifices suddenly collapsed, snuffing out the lives of thousands, was the subject of a preliminary postmortem conducted last week in Cambridge, Mass. A panel of Boston area-based civil and structural engineers convened to discuss the fate of the superskyscrapers, struck by hijacked passenger planes, in front of an overflow audience on the campus of the Massachusetts Institute of Technology. Their starkly sobering analyses highlighted the vulnerabilities of ultra-tall buildings to fire and pointed out steps that could be taken to lessen them.
After first describing the highly redundant structural system that kept the 110-story twin towers standing for decades despite hurricane-force winds and a terrorist truck bomb, the engineers then delineated how that system was breached and finally overcome on that fateful day when America was attacked. The main culprits in bringing the famously lofty buildings down, they concluded, were the two intensely hot infernos that erupted when tens of thousands of gallons of aviation fuel spilled from the doomed airliners. Once high temperatures weakened the towers' supporting steel structures, it was only a matter of time until the mass of the stories above initiated a rapid-sequence "pancaking" phenomena in which floor after floor was instantly crushed and then sent into near free fall to the ground below. Significantly, the panel stated that any mitigating reinforcements and redundancies added to these buildings could have only delayed the inevitable failure, though they would have bought more time for the evacuation of the occupants. No existing or foreseeable economically viable skyscraper structure, they agreed, could have withstood this kind of cruel onslaught. Clearly, prevention is the best defense against this kind of assault.
"Though the twin towers were not much taller than their famous uptown predecessor, the Empire State Building, the World Trade Center rose during the late 1960s, a new era of construction characterized by rapidly erected, lightweight steel structures rather than heavy masonry walls," explained Robert Fowler, senior engineer at the structural engineering firm of McNamara and Salvia. Fowler was then a junior member of the WTC's engineering firm of record, Worthington, Skilling, Helle & Jackson, later renamed Skilling Helle Christiansen Robertson. "As the Trade Center was so much lighter in comparison to earlier designs, it was a watershed building in the history of skyscrapers," he added. Leslie E. Robertson, then the project manager, was the engineer most responsible for the superskyscraper's design, Fowler noted. He is currently principal partner at Leslie E. Robertson Associates, the current structural consultants to the WTC. The late Seattle-based architect Minoru Yamasaki designed the World Trade Center.
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How the Towers Kept Standing
As with all large buildings, the main structural engineering design criteria for the facility's 1,362-foot-tall south tower and 1,368-foot-tall north tower centered on two things: ensuring resistance to the gigantic gravity loads of the buildings themselves as well as to sideways or lateral forces caused by high winds and earthquakes, which can generate huge overturning forces at the bases. The former condition, Fowler explained, depends on specifying strong vertical columns that can efficiently transmit the mass of the building to the ground. The latter consideration concerns not only structural integrity but also "requires developing an acceptable comfort level for the occupants" by avoiding too much swaying. Opposition to lateral motion is controlled by "the design's structural mass [weight], the stiffness of its lateral members and the degree of structural damping employed," Fowler said.
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Add CommentI did not notice any reference to lamellar tearing.An eminent welding engineer/metallurgist friend of mine who visited the twin towers during construction told me that the large welds holding the floors up were riddled with lamellar tearing - in fact there were 'miles' of this type of cracking in the building ie the welds could not sustain the loads for which they were designed.Records of these defects should be around somewhere.I wonder whether this could explain the 'pack of cards' collapse of the towers?
Reply | Report Abuse | Link to thisi know who planned it i think it was bin laden cuz he did london boming and 2 of his followers flew a plane in to world trade center but the good news is on 2012 on september 11th is cuz on that day twin towers will be made again but this time twin towers is going to be taller its ganna be tallest buildings in the world. i no how it collapes when the plane went in the fire burnt the floor witch the floor will melt and the steel at the outside of wtc bended and it collapsed that is what realy happend people thank u very much if u read my comment :)
Reply | Report Abuse | Link to thisProfessor Kausel made a very basic error in his calculations when he said: Kausel also reported that he had made estimates of the amount of energy generated during the collapse of each tower. "The gravitational energy of a building is like water backed up behind a dam," he explained. When released, the accumulated potential energy is converted to kinetic energy. With a mass of about 500,000 tons (5 x 108 kilograms), a height of about 1,350 ft. (411 meters), and the acceleration of gravity at 9.8 meters per second 2, he came up with a potential energy total of 1019 ergs (1012 Joules or 278 Megawatt-hours). "That's about 1 percent of the energy released by a small atomic bomb,"
Reply | Report Abuse | Link to thisHe assumed the whole 500,000 tons was at 1,350 ft. (The total weight of the building was consentrated at the very top of the building? The lower floors had no weight? The lower floors fell from 1,350 ft?)
If he made this sort of grade school error, what others did he make?
I just noticed that when I cut and pasted his quote, the exponents did not copy correctly, the rest of my comment still stands.
Reply | Report Abuse | Link to thisjchere, I noticed that right off the bat. A serious math error for the Professor to make. Definitely had difficulty with work problems in Calculus II.
Reply | Report Abuse | Link to thisomg i cant believe that they would do that it puts me to tears i goin to go into the war and the kill the people who did that to my country
Reply | Report Abuse | Link to thiswow i hate the people who did that i mean i was young when it happened but still come on guys lets get along you know
Reply | Report Abuse | Link to thisSo much history I wish I could learn more about it . I love listening to this stuff it's so interesting and so real.
Reply | Report Abuse | Link to thisSo much history I wish I could learn more about it . I love listening to this stuff it's so interesting and so real.
Reply | Report Abuse | Link to thisso much so sad to and so interesting love learnig about it
Reply | Report Abuse | Link to thisJ Chere.
Reply | Report Abuse | Link to thisYou are incorrect (and rude). You need to bother to actually do the calculation: 411 x 9.8 x 5E8 = 2E12 Joules. Kausel assumed all the mass of the building was half way up, giving an energy estimate of 1E12 J, as stated in the article. Although not completely rigorous in this case, locating all the mass at the centre of gravity of the structure is reasonable.
So now we have had TEN YEARS of UNscientific American. How does the steel and concrete have to be distributed in a skyscraper so it can hold itself up and withstand the wind. This should have been resolved in 2002. How could the top 15 stories of the north tower destroy the intact structure below when the building had to get stronger and therefore heavier all of the way down?
Reply | Report Abuse | Link to thishttp://psikeyhackr.livejournal.com/1276.html