Clearly, prevention is the best defense against this kind of assault. 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. How the Towers Kept Standing As with all large buildings, the main structural engineering design criteria for the facility's 1,foot-tall south tower and 1,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.
He calculated that the approximate maximum wind shear force that a single face needed to withstand to be somewhere around 11,, pounds. The gravity loads weight produced by the towers at their bases were on the order of , tons, Fowler said. To handle these immense forces, the engineers "designed the World Trade Center essentially as a large beam section," explained another panel member, Robert McNamara, president of the engineering firm McNamara and Salvia.
Called structural tubes in the business, each twin tower was strongly framed in structural steel. The frame comprised inner and outer rectangular box tubes consisting of closely spaced steel box columns connected by steel spandrel members or truss beams that supported 40,square-foot cross-braced floors, each nearly an acre in area, the empaneled engineers said. This configuration created a complete exterior tube around the building and a center tube down the middle.
The foot-long central core, formed of massive vertical steel columns that held most of the building's weight, contained elevator shafts, stairways and utility spaces, they said. The core's columns were thicker toward the base to support huge accumulated gravity loads. The outer perimeter tube, a tight prefabricated latticework with 61 inch steel box columns spaced 39 inches on center on each building face, provided all the bracing resistance against lateral and twisting forces from wind and seismic action.
This exterior grid served as a moment frame, providing a large moment arm of torque against overturning and deflection forces. The outer tube bore part of the gravity-induced downward load as well as, they noted.
The huge inner and outer rectangular tubes "needed to be protected to maintain their structural integrity, so the floors acted as reinforcing diaphragms or bulkheads [the term used in shipbuilding]," said panel member Jerome Connor, professor of civil and environmental engineering at M. The office floors, which each comprised a to foot clear span from the core to the exterior grid, were panelized structural members supported by open web joists with steel decks above them, he said.
The horizontal truss struts, bolted and welded to the exterior grid and the core column structures, included viscoelastic stringers that provided increased damping to help make the structure less lively in the wind, according to Connor.
Each steel floor deck was covered with four inches of concrete. After the deletion of key structural members from about the 90th to 96th floors on the north face of the north tower, One WTC, and from about the 75th to the 84th floors of the south, east and north faces of the south tower, Two WTC, the buildings' skeletons found alternative paths to take the loads.
Each impact and following explosion imparted first a large local lateral force and then an omnidirectional force to the structures, together causing massive initial damage to the columns and floor systems at the elevation of the crash. Despite shocks and explosions estimated to be equivalent to that of the truck bombing of the Alfred P. It's a testament to the system that they lasted so long. The events of September 11th show that this was indeed the case.
The towers collapsed only after the kerosene fuel fire compromised the integrity of their structural tubes: One WTC lasted for minutes, whereas Two WTC remained standing for 47 minutes.
The aviation fuel fires that broke out burned at a much hotter temperature than the typical contents of an office. Some have raised questions about the degree of fire protection available to guard the structural steel. According to press reports, the original asbestos cementitious fireproofing applied to the steel framework of the north tower and the lower 30 stories of the south were removed after the terrorist truck bombing.
Others have pointed out the possibility that the aviation fuel fires burned sufficiently hot to melt and ignite the airliners' aluminum airframe structures. Aluminum, a pyrophoric metal, could have added to the conflagrations. Hot molten aluminum, suggests one well-informed correspondent, could have seeped down into the floor systems, doing significant damage. The goblets are white hot, burning at an estimated degrees Celsius. At this temperature, the water of hydration in the concrete is vaporized and consumed by the aluminum.
It was the deadliest terrorist attack in the history of the United States. Just over an hour and forty minutes later, both towers had collapsed. In total, 2, died that day. Many more have died in the years since. Out of the ashes of this atrocity arose several conspiracy theories. Some of them were so fanciful that they were easy to debunk. Others have proven much harder for the authorities to shake off. Here we present some of the more persistent conspiracy theories surrounding the events of that fateful day, as well as the most plausible explanations put forward by experts, official reports and eyewitnesses.
One of the most persistent claims made by sceptics is that the two planes that hit the North and South towers could not have possibly caused so much damage to the floors below impact. The first jet hit the North Tower between the 98th and 94th floors; the second jet hit the South Tower between the 84th and 78th floors.
Despite the height of both impacts, eyewitnesses reported seeing damage to both lobbies of the Twin Towers. How can that be possible?
Surely a more plausible explanation is that secondary explosives were set off further down the towers, thus causing damage to the lower floors and both lobbies. The temperature at which jet fuel burns is between This allowed jet fuel to pour down elevator shafts. The fuel ignited, leading to raging fires gushing down the shafts. Many elevator cables were cut and braking systems disabled on impact, causing elevators to crash to the ground, smashing open lobby doors on impact and sending jets of burning fuel into both lobbies.
These explosive jets caused considerable damage and set many people on fire - something witnessed by Jules Naudet, a cameraman who was one of the first on the scene in the lobby of the North Tower shortly after the impact of Flight So how could the towers collapse due to structural failure when the fires were not hot enough to melt steel?
While it is indeed true that burning jet fuel alone cannot melt steel, a fire that was also fueled by burning furniture, curtains, blinds, chairs, desks, computer equipment and an enormous amount of paper would have burned hot enough to warp steel. The point at which steel weakens is As the steel warped and buckled, the columns and steel beams holding the towers up were no longer able to support the weight of the buildings, leading to the pancake collapse of both towers.
Many sceptics point to film of mysterious puffs of smoke emanating from each of the floors of the towers as they collapsed. There are two problems with this theory. I agree with Mr. The computer models they used rely on many assumptions chosen to make the collapse possible, but that again and again violate what was observed and the evidence that was left over.
The mystery is that three skyscrapers on the same day collapsed virtually into their own footprints due to fire. Nothing like that has ever happened before. Building 7 is especially important because unlike WTC 1 and 2, the excuse that jet fuel and the impact of a plane somehow made those happen is unavailable.
Yet the building collapses in almost pure symmetry. But three times in one day? Delivering affordable housing is an urgent issue across Canada, yet there are ex Actions speak louder than words. Yet, what we hear from world leaders concerning Pre-Bid Projects.
Click here for free access to Conceptual and Planning stage projects from across Canada. Ian Harvey November 24, Korol and other likeminded colleagues, stubbornly disagree. Of the 2, deaths terror attack related deaths on Sept. Recent Comments 2 comments comments for this post are closed Phillip Michaels December 17,
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