1. GENERAL INFORMATION

Height: 1,368 and 1,362 feet (417 and 415 meters)
Owners: Port Authority of New York and New Jersey
Architect: Minoru Yamasaki, Emery Roth and Sons consulting
Engineer: John Skilling and Leslie Robertson of Worthington, Skilling, Helle and Jackson
Ground Breaking: August 5, 1966
Opened: 1970-73; April 4, 1973 ribbon cutting
Destroyed:  Terrorist attack, September 11, 2001

2. THE STRUTURAL SYSTEM

^^ Voltar acima

Yamasaki and engineers John Skilling and Les Robertson worked closely, and the relationship between the towers’ design and structure is clear. Faced with the difficulties of building to unprecedented heights, the engineers employed an innovative structural model: a rigid "hollow tube" of closely spaced steel columns with floor trusses extending across to a central core. The columns, finished with a silver-colored aluminum alloy, were 18 3/4" wide and set only 22" apart, making the towers appear from afar to have no windows at all. 

Also unique to the engineering design were its core and elevator system. The twin towers were the first supertall buildings designed without any masonry. Worried that the intense air pressure created by the buildings’ high speed elevators might buckle conventional shafts, engineers designed a solution using a drywall system fixed to the reinforced steel core. For the elevators, to serve 110 stories with a traditional configuration would have required half the area of the lower stories be used for shaftways. Otis Elevators developed an express and local system, whereby passengers would change at "sky lobbies" on the 44th and 78th floors, halving the number of shaftways.

(Taken from http://www.skyscraper.org/)

Typical Floor Plan of the World Trade Center:

A perimeter of closely spaced columns, with an internal lift core.  The floors were supported by a series of light trusses on rubber pads, which spanned between the outer columns and the lift core.

3. WHY DID IT COLLAPSE?

^^ Voltar acima

Tim Wilkinson, Lecturer in Civil Engineering

(This is an initial suggestion on one possible reason for failure, and should not be regarded as official advice)

The structural integrity of the World Trade Center depends on the closely spaced columns around the perimeter.  Lightweight steel trusses span between the central elevator core and the perimeter columns on each floor.  These trusses support the concrete slab of each floor and tie the perimeter columns to the core, preventing the columns from buckling outwards.

After the initial plane impacts, it appeared to most observers that the structure had been severely damaged, but not necessarily 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 gradually affecting the behaviour of the remaining material.  As the planes had only recently taken off, the fire would have been initially fuelled by large volumes of jet fuel, creating potentially enormously high temperatures. The strength of the steel drops markedly with prolonged exposure to fire, while the elastic modulus of the steel reduces (stiffness drops), increasing deflections.

Modern structures are designed to resist fire for a specific length of time.  Safety features such as fire retarding materials and sprinkler systems help to contain fires, help extinguish flames, or prevent steel from being exposed to excessively high temperatures.  This gives occupants time to escape and allow fire fighters to extinguish blazes, before the building is catastrophically damaged.

It is possible that the blaze, started by jet fuel and then engulfing the contents of the offices, in a highly confined area, generated fire conditions significantly more severe than those anticipated in a typical office fire.  These conditions may have overcome the building's fire defences considerably faster than expected.

Eventually, the loss of strength and stiffness of the materials resulting from 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.  Regardless of which of these possibilities actually occurred, it would have resulted in the complete collapse of at least one complete storey at the level of impact.

Once one storey collapsed all floors above would have begun to fall.  The huge mass of falling structure would gain momentum, crushing the structurally intact floors below, resulting in catastrophic failure of the entire structure.

Sydney Morning Herald graphic

The only evidence so far are photographs and television footage.  Whether failure was initiated at the perimeter columns or the core is unknown.  The extent to which the internal parts were damaged during the collision may be evident in the rubble if any forensic investigation  is conducted.  Since the mass of the combined towers is close to 1000000 tons, finding evidence will be an enormous task. 

Perimeter columns, several storeys high, and still linked together, lie amongst all the debris on the ground.

The bottom left photograph shows the south tower just as it is collapsing.  It is evident that the building is falling over to the left.  The North Tower collapsed directly downwards, on top of itself.  The same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers.  However, it is possible that a storey on only one side of the South Tower initially collapsed, resulting in the "skewed" failure of the entire tower.

The gigantic impact forces caused by the huge mass of the falling structure landing on the floors below travelled down the columns like a shockwave faster than the entire structure fell.  The clouds of debris coming from the tower, several storeys below the huge falling mass, probably result from the sudden and almost explosive failure of each floor, caused by the "shockwave".

(Pictures taken from various news sources on the Internet)

4. WHAT OTHER ENGINEERS SAY

^^ Voltar acima

Taken from Engineering News Record

In the aftermath of the Sept. 11 terrorist attacks in New York City and Washington, D.C., which brought down the twin 110-story towers of the World Trade Center and damaged the Pentagon, designers and contractors say they are skeptical that signature structures can be hardened against extreme acts of barbarism. 

"Only the containment building at a nuclear powerplant" is designed to withstand such an impact and explosion, says Robert S. Vecchio, principal of metallurgical engineer Lucius Pitkin Inc., referring to the hijacked Boeing 767 airplanes, heavy with fuel, that slammed into each WTC tower. 

The attacks appeared to be coordinated and in parallel. In Manhattan, at least two hijacked Boeing 767 airplanes, one with 92 people on board and the other with 65, crashed into the World Trade Center's twin towers, disappearing within and triggering fire and explosions. The north tower, called One WTC, was hit at 8:45 a.m.; the south tower, Two WTC, at 9:03 a.m. Another hijacked airliner, a Boeing 757 with 64 people on board, crashed into a section of the Pentagon at 9:40 a.m. 

Some 50,000 people work at the World Trade Center and some 23,000 at the Pentagon. Specifics on death tolls and damage were not available at press time. WTC rescue efforts were put off until Sept. 12 because the area was still a "hot zone" late in the day on Sept. 11, with falling debris from the stumps that were once among the world's tallest buildings, and a thick blanket of soot for blocks around. In addition, the 47-story Seven WTC collapsed in the evening, causing more chaos. "It'll take about a year to clean up" the remains, says Vecchio. 

Sources close to the carnage indicate that those below the 89th floor in the north tower and the 60th floor in the south tower were most likely to have survived. Reports indicated that at least 200 of the 400 firefighters at the scene are presumed dead and 78 police officers were missing. 

Confirmation of collateral damage to the numerous subway lines that converge under the WTC was not available. The owner, the Port Authority of New York and New Jersey, which just leased the WTC complex to Silverstein Properties, New York City, set up a command center across the Hudson River in Jersey City, N.J., trying to locate its employees, who were on floors 63 to 85 in the north tower. Additionally, in the hours after the attack, the port authority located temporary quarters at sites at John F. Kennedy International Airport in Queens; at the Teleport on Staten Island; and in Newark, N.J. 

The port authority issued the following press release: "Our hearts and our prayers go out to the families of the countless people, including many members of the port authority family, who were killed today in this brutal and cowardly attack. We at the port authority are doing everything in our power to rescue and care for the injured, and to comfort and assist the families of the victims." 

Meanwhile, Jim Rossberg, director of the Structural Engineering Institute of the American Society of Civil Engineers, is organizing two forensic teams, one for the WTC and the other for the Pentagon. 

The twin towers, framed in structural steel, had exterior moment frames with 14-in. steel box columns spaced 39 in. on center. The configuration created a complete tube around the building. The central steel core carried gravity loads only. The exterior tube provided all the lateral resistance. Horizontal steel trusses spanned 60 ft from the exterior wall to the core. Concrete on metal deck completed the floor diaphragm. 

Each tower contained about 100,000 tons of steel and 4 in. of concrete topping on the 40,000-sq-ft floors, according to Henry H. Deutch, assistant to the chief structural engineer for construction manager Tishman Realty & Construction Co. Inc., New York City, during the construction of the WTC and currently head of HHD Consultants Inc., Osceola County, Fla. 

Deutch says that originally, the north tower contained asbestos in its cementitious fireproofing as did the first 30 stories of the south tower. He believes the asbestos, which had been encapsulated, was removed after the 1993 bombing. In a press conference, Mayor Rudolph W. Giuliani said the city's health department had tested the air in the area and found no undue amount of chemical agents. 

The attacks were witnessed by hundreds of people in each of the locales. Vecchio, who was part of the investigation of the 1993 bombing of the WTC, was an eyewitness to the Manhattan debacle from Pitkin's office about 1¼2-mile north of the trade center. "The explosions were so tremendous," he says. "You could smell the jet fuel in the air." 

Millions across the nation also "saw" the towers collapse, through live television news coverage. The south tower fell at 10 a.m. and the north tower at 10:29 a.m. 

Reports indicate that the impact of each plane compromised the structural integrity of each tower, knocking out perimeter columns and the interior structure. The explosions then caused further damage, sweeping through several floors. "These were airliners scheduled for long flights, full of fuel, causing massive explosions," says Richard M. Kielar, a Tishman senior vice president. "No structure could have sustained this kind of assault," says Kielar. 

As the fires burned, the structural steel on the breached floors and above would have softened and warped because of the intense heat, say sources. Fireproofed steel is only rated to resist 1,500 to 1,600° F. As the structure warped and weakened at the top of each tower, the frame, along with concrete slabs, furniture, file cabinets, and other materials, became an enormous, consolidated weight that eventually crushed the lower portions of the frame below. 

Jon D. Magnusson, chairman-CEO of Skilling Ward Magnusson Barkshire Inc., Seattle, one of the successor firms of Skilling Ward Christiansen Robertson, structural engineer for the original World Trade Center, agrees: "From what I observed on TV, it appeared that the floor diaphragm, necessary to brace the exterior columns, had lost connection to the exterior wall." 

When the stability was lost, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down, he says. 

A big question for implosion expert Mark Loizeaux, president of Controlled Demolition Inc., the Phoenix, Md., is why the twin towers appeared to have collapsed in such different ways. 

Observing the collapses on television news, Loizeaux says the 1,362-ft-tall south tower, which was hit at about the 60th floor, failed much as one would like fell a tree. That is what was expected, says Loizeaux. But the 1,368-ft-tall north tower, similarly hit but at about the 90th floor, "telescoped," says Loizeaux. It failed vertically, he adds, rather than falling over. "I don't have a clue," says Loizeaux, regarding the cause of the telescoping. 

The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covers eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, is under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 (ENR 7/9/64 p. 36). The 1.8-million-sq-ft Seven World Trade Center, constructed in the mid-80s, also had a steel moment frame from the seventh story up (ENR 11/28/85 p. 30). 

Security measures were tightened at the 12-million-sq-ft WTC complex after a terrorist bomb on Feb. 26, 1993. That bomb blew out one section of a north tower basement X-brace between two of the perimeter columns. The blast ripped out sections of three structural slabs in the basement levels between the north tower and the hotel, threatening the structural integrity of the foundation box. It did little damage to the north tower's structural tube, other than the affected X-brace. Damage was extensive to the other building systems, however, because the bomb compromised major utility lines in the basement, and the brace compromised the central core wall, allowing soot and smoke to shoot up the building core (ENR 3/15/93 p. 12). 

In Washington, Pentagon officials were still assessing the damage and the fire was still burning nearly seven hours after the building was hit. At press time there were no details about injuries and fatalities. 

The impact was between the newly renovated Wedge I and the about-to-be-renovated Wedge II, according to an aide in the Pentagon's Renovation Office. Wedge I "did hold up," the aide says. 

Reports of damage also remain sketchy and Pentagon officials decline to discuss specifics. However, the Pentagon aide reports that the plane slammed into the southwest side of the Pentagon adjacent to the heliport. The airliner reportedly hit at the first and second floors, but later the upper three floors collapsed. 

The U.S. Dept. of Defense's 6.5-million-sq-ft headquarters, built as a temporary structure 58 years ago, was not constructed with fire-resistant or bomb-resistant materials. The overdue, multiyear renovation includes technologically advanced materials designed to ward off severe damage from such attacks (ENR 9/4/00 p. 58). 

Fire was leaping out the windows, primarily on the upper floors of the unrenovated section. The adjacent renovated section appeared to have less damage, likely because of the reinforced glass windows and firewalls used in the renovation that was completed less than a year ago. 

In the aftermath of the 1993 bombing, WTC structural consultant Leslie E. Robertson, Skilling's project engineer for the original job, was convinced that the terrorists had meant to take down the twin towers. After the events of Sept. 11, there's little doubt that Robertson was correct. 

^^ Voltar acima

National Council of Structural Engineering Associations

http://dwp.bigplanet.com/engineers/

According to one of the designers of the World Trade Center (WTC), the towers were originally designed to take the impact of a Boeing 707; and the impact of the aircraft this morning did not take the buildings down. In fact, WTC One stood for 1 hour and WTC Two stood for 1 3/4 hours after impact. Engineers familiar with the chain of events suspect that heat from the massive and extraordinary fires weakened the structures and initiated the progressive collapses.

John Hooper, a structural engineer from Skilling, Ward, Magnusson, Barkshire, the structural engineering firm that evolved from Skilling, Helle, Christianson, Robertson, which was the structural engineering firm of record for the WTC, provided the following facts to NCSEA: WTC One was 1368' tall, and WTC Two was 1362' tall. Each 110-story tower had a floor plate that was 208' by 208'. The central core of each was 86' square. Around the perimeter of the buildings, columns were spaced at 3'-3" on center, with 48"- deep plate girders at each floor. At the third level, the columns transitioned in an arch-like formation to a 10'-0" spacing for the lower story. Floors were supported by steel trusses spanning 60', from the core to the perimeter wall, on each side of the building. The buildings are also thought to have been the first buildings to use non-asbestos fireproofing. The fibers of the spray-on fireproofing product were reportedly ceramic rather than asbestos.

NCSEA has contacted FEMA and will coordinate and make available structural engineers in the New York and Washington D.C. areas. NCSEA will also coordinate and provide the services of Member Structural Engineering Associations throughout the U.S., as needed. 

The National Council of Structural Engineers Associations is extremely saddened with the day's news, including the collapse of the World Trade Center in New York City. Our hearts and prayers go out to all the victims and their families.

^^ Voltar acima

AISC Task Force to Investigate World Trade Center Collapse 

The American Institute of Steel Construction, Inc. (AISC) has contacted FEMA and the leading structural engineering associations and is forming a special task force to investigate the structural collapses of the World Trade Center buildings resulting from the terrorist attacks on September 11, 2001. 

AISC is the technical institute responsible for developing and maintaining the standards for design and construction of steel buildings in the United States. Information developed by this task force will enable AISC to determine if modifications are needed in existing standards.

“AISC strives to create a steel building specification that makes use of the latest available design data and construction technology,” said Nestor R. Iwankiw, AISC’s Vice President of Engineering and Research. “The special task force of nationally recognized experts will investigate and determine the various factors that contributed to the collapses and make recommendations to AISC’s Specification, Blast and Fire Committees.”

Much speculation is currently underway about the cause of the collapses. Most engineers currently believe the collapses occurred as a result of a combination of extraordinary events, including the initial aircraft impacts and explosions, which destroyed part of the structure, and the subsequent extreme fire, which progressively weakened the remaining structure. The ensuing collapses may have occurred when the weight of the buildings above the points of impact exceeded the reduced load carrying capacity of the remaining structure. It is believed that the collapse of the 47-story building adjacent to the twin towers was caused by a combination of its foundations and structure being weakened by the collapsing twin towers and fire. 

“We’re saddened about this terrible disaster and the loss of life,” said AISC President H. Louis Gurthet, P.E. “Our hearts and prayers go out to the victims and their families.”

^^ Voltar acima

British Engineers Give Their Theory

Taken from BBC

http://news.bbc.co.uk/hi/english/world/americas/newsid_1540000/1540044.stm

The design of the World Trade Center saved thousands of lives by standing for well over an hour after the planes crashed into its twin towers, say structural engineers. 

"It was the fire that killed the buildings - nothing on Earth could survive those temperatures with that amount of fuel burning" Structural engineer Chris Wise 

But the towers' ultimate collapse was inevitable, as the steel cores inside them reached temperatures of 800C - raising questions why hundreds of rescue workers were sent into the doomed buildings to their deaths. 

The steel and concrete structure performed amazingly well, said John Knapton, professor in structural engineering at Newcastle University, UK. "I believe tens of thousands of lives have been saved by the structural integrity of the buildings," he told BBC News Online. "They had a lot of their structure taken out, yet they remained intact for more than an hour, allowing thousands to escape." 

But as fires raged in the towers, driven by aviation fuel, the steel core in each building would have eventually reached melting point - 800C. The protective concrete cladding on the cores would certainly have been no defence in these extraordinary circumstances. 

"Nothing is designed or will be designed to withstand that fire" World Trade Center construction manager 

"It was the fire that killed the buildings. There's nothing on Earth that could survive those temperatures with that amount of fuel burning," said structural engineer Chris Wise. "The columns would have melted, the floors would have melted and eventually they would have collapsed one on top of each other." 

The building's construction manager, Hyman Brown, agreed that nothing could have saved it from the inferno. "This building would have stood had a plane or a force caused by a plane smashed into it," he said. 

"I would have given the order to get out - you would have thought someone with technical expertise would have been advising them" Professor John Knapton, Newcastle University "But steel melts, and 24,000 gallons (91,000 litres) of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."

Once the steel frame on one floor had melted, it collapsed downwards, inflicting massive forces on the already-weakened floor below. From then on, the collapse became inevitable, as each new falling floor added to the downward forces. Further down the building, even steel at normal temperatures gave way under the enormous weight - an estimated 100,000 tonnes from the upper floors alone. 

"It was as if the top of the building was acting like a huge pile-driver, crashing down on to the floors underneath," said Chris Wise. 

Early in the unfolding horror, some office workers were told to stay where they were - dreadful advice, said Professor Knapton. 

The towers withstood impact but not inferno. People's only hope was to run and keep running - reaching open ground. The building could have fallen over sideways, he points out, potentially bringing even greater devastation. 

Another 47-storey building belonging to Salomon Brothers caved in later, weakened by the earlier collapses, and more nearby buildings may still fall, say engineers. 

But the eventual collapse of the twin towers was so predictable that the order should have been given to withdraw emergency services within an hour, said Professor Knapton. He watched in horror, knowing the building would fall within two hours. The hundreds of dead firemen and police officers should simply not have been there, he said. 

"I think they should not have gone in at all," he said. "If they did decide to take the risk, they should have been pulled out after an hour." 

But in the panic and horror, the order was never given for rescue workers to abandon the building. "Mistakes were made," said Professor Knapton. 

"It was like a horror film and I think people's rationale had gone" Professor John Knapton 

"It sounds harsh - this had never happened in the world, so you can hardly criticise them. But I would have given the order to get out. You would have thought someone with technical expertise would have been advising them." But he acknowledged that the sheer scale of the tragedy probably overwhelmed the operation commanders. "I think everyone was not thinking. It was like a horror film and I think people's rationale had gone," he said. 

The building's design was standard in the 1960s, when construction began on what was then the world's tallest building. At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells.   Steel beams radiate outwards and connect with steel uprights, forming the building's outer wall. All the steel was covered in concrete to guarantee firefighters a minimum period of one or two hours in which they could operate - although aviation fuel would have driven the fire to higher-than-normal temperatures. The floors were also concrete. The building had to be tough enough to withstand not just the impact of a plane - and the previous bomb attack in 1993 - but also of the enormous structural pressures created by strong winds. 

Newer skyscrapers are constructed using cheaper methods. But this building was magnificent, say experts, in the face of utterly unpredictable disaster.

^^ Voltar acima

Trade Center architect discusses buildings

Taken from CNN

http://www.cnn.com/2001/US/09/12/wtc.architect.cnna/index.html

(CNN) -- When they were completed in the early 1970s, the Twin Towers of New York's World Trade Center were the tallest buildings in the world. That designation didn't last long -- Chicago's Sears Tower took the title in 1974, a year after Two World Trade Center was finished -- but the buildings' standing as a New Yoork City landmark, anchors amid the office-tower canyons of Manhattan's financial district, remained unchallenged. 

Tuesday, the buildings -- daytime home of more than 50,000 workers -- were destroyed when two hijacked passenger jets were flown into the structures. 

CNN's Leon Harris spoke with Aaron Swirsky, part of the architectural team led by World Trade Center chief architect Minoru Yamasaki, on the way the building was designed. 

LEON HARRIS, CNN ANCHOR: So many of us had thought for so long that the Twin Towers were invincible. We had heard for so many times over the years that the buildings have been built to withstand an impact from the crash of a plane. 

Let's talk right now on the telephone with Aaron Swirsky. He's in Jerusalem. He was one of the original architects of the complex, as I understand it. 

Is that the case, Mr. Swirsky? 

AARON SWIRSKY, ARCHITECT: I was working with Minoru Yamasaki, who is the architect of the building. But I was one of the workers with him. We were a team of 14 architects, and I was one of the members of the team. 

HARRIS: As a member of the team, and having such insight to how this building was constructed, could you believe that a plane could bring these buildings down? 

SWIRSKY: No, as a matter of fact, one of the rationales of the structure of the building was that it would be built as a pipe. And that proved itself to work during the explosion of 1993, when a hole was brought into the building, and it survived. But somehow, nobody could foresee anything like (Tuesday's incident). 

Also, at that time, the planes were not like these types of planes that we have now. I think the biggest plane was a 100-passenger plane, something like that, and the fuel capacity of those planes was not like they are today. 

The criterion was that if a plane hits, it would go right through it. And nobody could foresee something like that. The tower was protected in such a way that the damage would be limited to one story, but it wouldn't travel to the other stories. 

HARRIS: The planes that crashed yesterday were much bigger than that. They were 757s. 

SWIRSKY: And also the fuel capacity is much more tremendous. 

HARRIS: Exactly. That's what I want to ask you about. Which was it that made the biggest difference? Was it the impact felt from the larger plane, or was it the heat generated by the burning and that much fuel. 

SWIRSKY: I imagine, when I saw the pictures of the implosion of the building, it looks like the fuel must have leaked right to the core of the building, and from there it was the massive explosion that caused the building to collapse. So it was something completely unforeseen, so far as the design criteria was (concerned). 

HARRIS: Let me ask one final question, if I may. Considering what you know about the building -- you say it was constructed like a pipe, these two buildings -- and the manner in which we saw them collapse, does that give you any hope at all that the way it collapsed, there will be more packets inside, at the bottom, where survivors could be found? 

SWIRSKY: Well, I sure hope so. We pray that there will be survivors and that this won't happen again. It's a terrible, terrible, incredible tragedy

^^ Voltar acima