Re: Crollo Torri Gemelle: perché si parla di effetto pancake?
Inviato da Henry62 il 8/8/2006 20:30:55
Ciao Seatnineb,
ti allego qualcosa che ho trovato in rete, pubblicato da Scientific American nell'Ottobre 2001:
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.
"Though the WTC towers stood over 1,360 feet above the street level, the structures' bases were actually set 70 feet into the ground, and one had a 100-foot-tall antenna atop it, so with 205-foot widths, they had a lot of [exterior] area facing the wind," the engineer stated. He calculated that the approximate maximum wind shear force that a single face needed to withstand to be somewhere around 11,000,000 pounds. The gravity loads (weight) produced by the towers at their bases were on the order of 500,000 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 40,000-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 90-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 14-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.I.T. The office floors, which each comprised a 35- to 60-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. "With almost an acre of area for each floor and figuring about 100 pounds per square foot of area," he estimated that "each floor system weighed about 3,200,000 pounds."
l'indirizzo é il seguente:
http://www.public-action.com/911/jmcm/sciam/
Ciao
ti allego qualcosa che ho trovato in rete, pubblicato da Scientific American nell'Ottobre 2001:
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.
"Though the WTC towers stood over 1,360 feet above the street level, the structures' bases were actually set 70 feet into the ground, and one had a 100-foot-tall antenna atop it, so with 205-foot widths, they had a lot of [exterior] area facing the wind," the engineer stated. He calculated that the approximate maximum wind shear force that a single face needed to withstand to be somewhere around 11,000,000 pounds. The gravity loads (weight) produced by the towers at their bases were on the order of 500,000 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 40,000-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 90-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 14-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.I.T. The office floors, which each comprised a 35- to 60-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. "With almost an acre of area for each floor and figuring about 100 pounds per square foot of area," he estimated that "each floor system weighed about 3,200,000 pounds."
l'indirizzo é il seguente:
http://www.public-action.com/911/jmcm/sciam/
Ciao
Messaggio orinale: https://old.luogocomune.net/site/newbb/viewtopic.php?forum=4&topic_id=1908&post_id=41444