How the World Trade Center fell
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. But the towers' ultimate collapse was inevitable, as the steel cores inside them reached temperatures of 800C - raising questions as to 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."
Temperatures at 800C
But as fires raged in the towers, driven by aviation fuel, the steel core in each building would have eventually reached 800C - hot enough to start buckling and collapsing. The protective concrete cladding on the cores would have been no permanent defence in these extraordinary circumstances - keeping the intense heat at bay for only a limited timespan.
"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. "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.
Science of collapse
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.
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. Other buildings - including the 47-storey Salomon Brothers building - 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 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.
By The Associated Press,
Facts about the World Trade Center:
- SIZE: Seven buildings, including the twin 110-floor towers. The others: a 47-story office building, two nine-story office buildings, and eight story U.S. Customs office and 22-story hotel.
- HEIGHT: The twin towers were each 1,350 feet tall.
- COST: Roughly US$1.25 billion.
- RENTABLE SPACE: 12 million square feet of rentable office space; some 1,200 companies and organizations leased space.
- WORKERS: Roughly 50,000 people were employed in the twin towers.
- TOURISTS: On average, 90,000 tourists and business visitors each day.
- HISTORY: Development of a downtown Manhattan World Trade Center was recommended in 1960; the complex was dedicated in 1973; final completion of twin towers in 1976.
- CHIEF ARCHITECT: Minoru Yamasaki.
Great Buildings Online: World Trade Center - photos and architectural details.
2 Planes Hit Twin Towers at the Weakest Spot
The terrorists who piloted two planes into the World Trade Center apparently managed--either by careful calculation or evil luck--to have hit the buildings at their weakest spot to cause their disastrous collapse, structural engineers said Tuesday.
"It's like hitting someone at the back of the knee," said Nabih Youssef, a structural engineer who heads the Tall Building Council in Los Angeles and is an expert on the design and strength of skyscrapers. "With enough weight above you, you take the entire building down."
Government officials believe the terrorists wrested control of the passenger jets, then skillfully steered the planes toward the doomed towers. "Whoever took over the plane knew what they were doing," said Greg Fenves, a professor of civil engineering at UC Berkeley.
Had the buildings been hit higher up the towers, they would have sustained damage but probably would not have collapsed because the weight on the damaged portion of the building would not have been enough to overwhelm a tower's structural supports, engineers said.
Planes Had to Clear Nearby Buildings
The planes might have done more damage if they had hit the buildings lower, but they had to fly at a height of about 60 stories to clear nearby buildings. The first tower was hit at about the 80th story. The second tower was hit at about the 60th story.
"They showed some knowledge of physics in the attempt to make the hits as low as possible," said Ron Hamburger, chief structural engineer for ABS Consulting in Oakland and a past president of the Structural Engineers Assn. of California.
To many who saw the buildings fall on television, the collapse resembled a planned demolition, especially in the way that the twin towers imploded--tumbling in on themselves. But engineering experts discounted the notion that additional explosives had been planted around the base of the buildings to ensure that they came down.
Demolition of a building the size of the ones in the World Trade Center would require "literally hundreds of charges around the building," Hamburger said. "It's inconceivable to me anyone would be able to place that many charges--even with years of planning."
Instead, the impact of the planes themselves, and the tremendous heat generated by tons of burning jet fuel--upward of 1,000 degrees Fahrenheit--would suffice to destroy the buildings, said Scott Gustafson, owner of Demtech Inc. of Blue Springs, Mo., one of the world's leading demolition experts.
A Boeing 767 has a fuel capacity of 20,000 gallons. A Boeing 757 has an 11,000-gallon fuel capacity. Because the planes were scheduled for transcontinental flights, they would have been fully loaded with fuel.
Plane Was 'a Highly Explosive Bomb'
"The plane probably made its way halfway to the core of the structure," Gustafson said. "The fuel went through a couple of floors, loaded them with fuel, and the impact opened a corridor to the outside for air. Some fuel probably got into the elevators and spread the fire. One thing led to another, and it just kept snowballing."
"It was very well thought out," said Hank Koffman, who directs the construction engineering department at USC. "These guys were evil geniuses. "The plane was really a highly explosive bomb," he said. Terrorism experts were calling the attack "low-tech and high-concept."
Even though structural steel used in buildings is coated with a fireproof material, extreme amounts of heat cause the steel to soften and lose its strength. The weight of the floors above then causes them to crash. "The technical term is progressive collapse--the slang term is pancaking," said Ron Klemencic, president of Skilling Ward Magnusson Barkshire, the Seattle firm that engineered the World Trade Center. "What basically happens is that one floor falls on top of the floor below it, and with one floor falling on top of another there's no way to stop it."
The steel is protected to certain temperatures and for certain periods of time, but "an explosion of this magnitude would have exceeded all those limits," said James C. Anderson, a professor of civil engineering at USC. "Buildings are not designed for this. Not in their wildest dreams."
"Buildings are designed thinking of internally generated heat," added Jon Magnusson, the chairman and chief executive of the Skilling firm. "Nobody anticipates putting jet fuel in a building. If you had to build buildings to withstand this sort of event, you wouldn't be able to build any buildings."
In 1945, a B-25 bomber smashed into the 79th floor of the Empire State Building. It caused an explosion and fire and killed 14 people but did not destroy the building. That plane was not loaded with the huge amount of fuel that Tuesday's jetliners carried.
Engineers suggested that the south tower of the World Trade Center collapsed first, even though it was hit by the second plane, because the fireball caused by the crash was larger and because the plane hit the corner of the building, rather than the center, where there is more structural support.
The World Trade Center towers have always been a familiar icon to those visiting New York. They were New York's most frequently purchased postcard image.
To structural engineers, however, they are famous for something else: their strength.
"I was personally very surprised to see the entire building collapse," Hamburger said.
'Tubular Skyscraper' Construction Hailed
The towers were so tall--1,362 and 1,368 feet--that they swayed by up to 11 inches in a strong wind. Building towers of such height posed challenges for engineers, requiring development of a new system of construction that placed major supporting elements in the outer portions of the building to increase stability. Traditionally, such elements had been placed in the building's core around elevators and restrooms.
At the time of the towers' construction, this "tubular skyscraper" scheme was hailed as the key that would push the world's buildings to elevations undreamed-of by previous generations.
In most buildings, structural steel supports are 20 to 25 feet apart. In the World Trade Center, the supports are only 39 inches apart, said John Hooper, a structural engineer with the Skilling Ward Magnusson firm.
"Watching something come down without our say-so is just a nightmare," Hooper said.
Because the tubular steel supports are so close to each other, they act as a rigid box, encircling the building and giving it strength.
"It's quite a famous structural system," said Fenves. "It's very well-designed."
Steel buildings in general are known for their strength. Even less well-designed steel buildings survived the 1906 earthquake in San Francisco and the 1933 Long Beach quake, Youssef said. When they were built in 1970, the World Trade Center towers were the world's tallest buildings. But the strength designed to withstand wind was no match for the fireball of an exploding plane. "We didn't have any terrorism in mind when the buildings originally went up," Hooper said.
While engineers are exploring ways to "bomb-proof" buildings to protect occupants from flying glass and crumbling walls, it is considered too costly and not socially desirable to attempt to protect buildings from the type of attack that occurred Tuesday.
"We'd be living in bunkers," Youssef said. "We cannot turn the country into bunkers."
The towers were the focus of one previous terrorist attack--a deadly 1993 car bomb blast in the 16-acre subbasement that tore a 60-by-100-foot hole. That explosion, however, did not compromise the buildings' structural integrity, and experts said there was no indication that the damage then, which was repaired years ago, had any connection with Tuesday's collapse. Six men ultimately were convicted for the 1993 bombing, which killed six and injured more than 1,000.
The buildings housed about 55,000 workers employed by more than 700 firms, including the executive offices of the New York Stock Exchange and leading investment, law and accounting firms. Counting those passing through the buildings to transact business, their population during working hours routinely exceeded 100,000 people.
The towers were the centerpiece of an urban renewal project intended to revitalize lower Manhattan. Construction began in 1966, and the first tower formally opened four years later. The architect was the late Minoru Yamasaki, who also designed the Century Plaza Towers in Los Angeles. The twin 110-story towers cost their owners, the Port Authority of New York and New Jersey, $700 million. Last July, they were leased to a New York real estate firm for $3.25 billion, one of the largest such deals in history.
At the time of their opening, the towers were praised as technologically marvelous but aesthetically soulless.
Architect Said Towers Would 'Soothe' Spirit
Stung by that reaction, Yamasaki insisted to one architecture critic that his buildings would "soothe" the human spirit. "Above all, with political turmoil, traffic problems and vast increases in populations and the tremendous impact of the machine, we must have serenity. Man needs a serene architecture to save his sanity in today's world."
Tuesday, those humane sentiments were reduced to rubble and ash.
Times staff writers Tim Rutten, Thomas H. Maugh II and Kenneth Reich contributed to this story.