Massive force of blasts triggered towers' collapse

Massive force of blasts triggered towers' collapse

Paul Kelso and Tim Radford
Wednesday September 12, 2001
The Guardian

Until yesterday's devastating impacts reduced them to rubble, the twin towers of the World Trade Centre dominated the New York skyline, iconic symbols of American achievement.

Situated at the southern end of Manhattan island, the 110-storey, 1,350ft silver slabs towered over Wall Street and provided office space to around 40,000 employees. Ninety thousand more visited on a daily basis. The towers were the largest commercial complex in the world, complete with their own subway station.

Commissioned in the 1960s by the Port Authority of New York from architect Minoru Yamasaki - who was famously afraid of heights - they were built a deliberate 100ft taller than the Empire State Building, and when they opened in 1973 added 10m square feet of office space to the city's cramped financial district.

The ground excavated for the foundations was shoved into the Hudson river and used to create Battery Park, a green space facing Staten Island that was yesterday swamped beneath the shattered towers.

The towers were initially derided as white elephants, but the economic boom of the 1980s ensured near total occupancy, and with their distinctive appearance they became an instantly recognisable landmark as well as an economic success.

Graham Masterton, a structural engineer, said towers on this scale are almost impossible to protect from the impact of two passenger planes ploughing into them. The only building capable of remaining standing after such an impact would have to be designed "like a nuclear bunker".

"Initially you have localised damage at the point of impact, although the scale of this would have been beyond what an architect usually takes into account," said Mr Masterton, a senior spokesman for the Institute of Civil Engineers.

"This massive force would have reverberated through the building, above and below ground, and this would have a significant weakening effect.

"The twin towers didn't collapse immediately, but the structural strength was affected by the subsequent explosion and the progressive effects of the fire seem to have triggered the final collapse.

"Of course it is up to investigators to identify the exact cause of the collapse but it seems to have been a combination of catastrophic events beyond any reasonable expectations."

Duncan Steel, a physicist at the University of Salford, said the impact of a laden passenger airliner, even at several hundred miles an hour, is less than the force imposed on a tall building by normal high winds. However, he calculated that the subsequent explosion of fuel would have dwarfed the initial impact.

"I would imagine that the major damage that caused the building to collapse is the release of that fuel," he said. "It was a delayed explosion in each case."

"It's a very large explosion," said Scott Steedman, a civil engineer and an expert in natural disasters. "You have the energy in the plane itself, its weight and speed, which is absorbed by the building on a monumental scale.

"Typically that deflection would be likely to be less than the wind deflection. The impact of an aeroplane on the side of the building will also cause it to deflect, and then it will bounce back again, and the energy will be absorbed into the structure.

"But of course if it is full of aviation fuel then that impact is absolutely monumental. In this case, it is obviously clear that the impact was so enormous it has knocked the building sideways."

Tall buildings are designed to absorb horizontal shock. The World Trade Centre towers, built on rubber bases, normally swayed by a metre or more in the wind.

The architect and the engineers developed a rigid "hollow tube" design of closely spaced steel columns with floor trusses extending across to a central core.

The foundations of the centre extend some 70ft underground in huge concrete piles that connect with the unusually hard bedrock of Manhattan island. This rock is ideally suited to take the load of tall buildings. Without it the island would not be able to support even its more modest buildings, and the World Trade Centre towers could never have been contemplated.

The towers became a potent symbol of America's global dominance, as well as a tourist attraction.

In 1993 Ramzi Ahmed Yousef and Eyad Ismoil, motivated by hatred of Israel and America, planted a bomb in the underground car park of Tower 1, the second to collapse yesterday. The subsequent explosion killed six people but failed to fulfil Ismoil's ambition of toppling one tower into the other and killing 250,000 people. Yesterday's attacks succeeded where he failed.

Twin towers' collapses due to fire, expert says

Twin towers' collapses due to fire, expert says

Engineers talks of Trade Center myths, argues fire-proofing doomed building

Tania Ganguli

An expert on the Sept. 11 collapse of the World Trade Center explained Monday how researchers discovered that the fires from the explosion caused the twin towers to collapse.

W. Gene Corley, who headed the committee of postmortem studies for the American Society of Civil Engineers, detailed for Northwestern engineering professors and students both past misinterpretations and truths about why the towers collapsed.

Corley maintained that the buildings did not collapse because the fires were hotter or lasted longer than the building could stand. He illustrated his point by showing a photograph, taken of the hole in one of the towers shortly after the crash, in which a person standing inside the building did not appear affected by the heat.

"There is a person standing there who is not bothered by the heat and from that we determined that this is a normal office fire," Corley said.

He also presented the audience with a photograph of a large fireball enveloping a portion of each of the towers, arguing the initial fire burned out within three to nine minutes and exhausted all the fuel from the plane.

According to Corley, the initial impact weakened the buildings so much that any extra disturbance, including the fires that followed, would have caused a collapse.

"A strong wind would've done the same thing," Corley said. "It is our position that if there had been no fire, the buildings would have stood indefinitely until some other major occurrence."

Although the fires did not burn long, they burned thoroughly, he said. One of the major reasons the fires burned the buildings as completely as they did was the fact that the sprinklers failed to work, he said.

When the planes hit the towers, they cut through the water supply that would otherwise have helped calm the fires, Corley said.

Corley went on to discuss the fire-proofing of the internal columns done during the construction of the buildings in the 1960s, which also contributed to the building's collapse. Much of the World Trade Center's fire-proofing was not secured strongly enough to withstand the impact of the planes hitting the buildings.

Civil engineering Prof. Ray Krizek pointed out the preparation that was done on the part of the World Trade Center's structural engineers.

"They did design for an (airplane) hitting it," Krizek said.

Structurally, the buildings should have been able to withstand the impact, Corley said. The fire-proofing, an aspect that the architect designs, was the ultimate problem.

"You've never heard anyone say the architect screwed up," Corley said. "They say, 'What did the structural engineers do wrong?'"

Ahmad Hadavi, a civil engineering professor and associate director of the masters of project management program, attributed this misunderstanding to the public's image of an architect as just a designer.

"People think of an architect as the artist who thinks of what to do with the space, not (as a person) who thinks of structural considerations," Hadavi said.

A Berkeley Engineer Searches for the Truth About the Twin Towers' Collapse

A Berkeley Engineer Searches for the Truth About the Twin Towers' Collapse

Abolhassan Astaneh-Asl's computer simulations suggest more-conventional skyscrapers might have withstood the attacks


When the World Trade Center's burning south tower crumbled to the ground five years ago, just 56 minutes after terrorists crashed a Boeing 767 passenger jet into its upper floors, Abolhassan Astaneh-Asl's horror was mixed with professional surprise.

As a professor of structural engineering at the University of California at Berkeley and an expert on steel structures, he thought that the buildings should have stood longer, even after such a catastrophic impact, and that the collapse should not have been so nearly vertical.

"From the day that I stood there and watched it collapse" on television, he says, "I was thinking that this is impossible. That there's something strange here."

Mr. Astaneh-Asl says he knew immediately that he wanted to be a part of the scientific response to the tragedy. He felt that his unique expertise could help in understanding how the two towers collapsed. He was well versed in the effects of terrorist bombings on buildings, having conducted research on blast effects after a car bomber brought down the Alfred P. Murrah Federal Building in Oklahoma City. And he had long studied how buildings responded to earthquakes and other natural disasters, so he knew that researchers must act fast to get the clues needed to understand what had happened.

The day after the attacks of September 11, 2001, Mr. Astaneh-Asl submitted an emergency grant proposal to the National Science Foundation asking for money to examine the steel at ground zero firsthand. Only days later, the request granted, Mr. Astaneh-Asl flew to New York and spent weeks at a recycling center where the towers' remains were being scrapped. There he inspected and collected samples of joints and other scraps of steel from what were once two of the tallest buildings in the world.

Though his NSF grant soon ended, questions about the collapse remained. Mr. Astaneh-Asl decided to create a computer simulation of the plane attacks, with as much detail as possible, in the hope that the unprecedented tragedy might yield lessons that could be used in the design of future skyscrapers.

He did not expect the discoveries he would make, the political obstacles he would face — or that, five years later, he would be involved in a struggle for skyscraper safety.

This week Mr. Astaneh-Asl is scheduled to present findings from his latest simulations in a lecture at Berkeley, making an argument that is sure to raise eyebrows in the engineering community and beyond.

If the World Trade Center towers had been built in a more conventional way and in strict accordance with New York City building codes — from which they were exempt because they were built under the auspices of the Port Authority of New York and New Jersey — the buildings probably would not have collapsed, he argues, and thousands of lives might have been saved.

Two extensive government investigations found no fault with the towers' structural design, and many engineers say that no engineer could have anticipated or shielded against kamikaze attacks by fuel-laden jetliners. And some say that what-if scenarios are fruitless lines of research.

Leslie E. Robertson, who helped design the twin towers, could not be reached for comment. But William Faschan, a partner at Leslie E. Robertson Associates, defended the building's performance. "It's extraordinary that any building could withstand that event and remain standing," he says. Mr. Faschan says he is not familiar with Mr. Astaneh-Asl's research and would not comment on its findings.

Even Mr. Astaneh-Asl is careful when discussing his findings, stressing that the people who perished in the buildings' collapse "were murdered by terrorists." But he insists that it is his obligation as an engineer to seek "the truth" about the buildings' history and structure. He speaks excitedly about the importance of his findings, and talks eagerly for hours about the topic.

"We can avoid this happening to someone's loved ones in the future," he says.

'Virtual Replica'

His work examining steel near ground zero gave Mr. Astaneh-Asl a few short minutes of fame. He was interviewed on The NewsHour With Jim Lehrer, CNN, and National Public Radio, as well as by several newspapers, including The Chronicle. At the time, he stressed the positive aspects of the twin towers' performance on the day of the attacks, noting that the length of time the buildings stood allowed most occupants to escape.

Mr. Astaneh-Asl says that from his inspection of the steel, he decided the collapse was not due to faulty welding or poor workmanship. That meant he was still not sure exactly how the collapse happened.

Then he got a call from an analyst from MSC Software Corporation, which makes high-end computer-modeling software used by carmakers and other businesses. Company officials had seen the Berkeley professor quoted in the news media, and offered to donate the company's software for his efforts. He quickly took them up on their offer and began the next phase of his research.

Mr. Astaneh-Asl wanted the computer simulation to be as true to life as possible. That would require the blueprints and construction specifications for the twin towers.

"Basically you build the towers inside your computer with all the dimensions — you represent each element of your structure," he says. "It's really a virtual replica of your physical structure."

But the building plans for the towers turned out to be hard to come by, as the developers kept them sealed from public view. Experts say that most developers keep such documents private, viewing them as proprietary information, but Mr. Astaneh-Asl says he had hoped that, considering the circumstances, the plans would be made available to researchers.

He nearly got access by joining an investigation team led by the Federal Emergency Management Agency and the American Society of Civil Engineers, which brought together some two dozen researchers and engineers in late 2001.

Mr. Astaneh-Asl was initially asked to participate, but he says he was troubled that team members were all required to sign a nondisclosure form promising to keep certain details of the investigation, including the buildings' architectural plans, to themselves. Mr. Astaneh-Asl's says he felt the agreement violated his academic freedom, and so he resigned from the team before its investigation got under way.

(The leader of that investigation, W. Gene Corley, says he believes the wording of the nondisclosure agreement would not have stopped any participant in the investigation from publishing academic papers about the structures. "It essentially said that we would not use information we obtained there to be used in a lawsuit against the owners and designers of the building," says Mr. Corley, who is senior vice president of the CTL Group, in Skokie, Ill.)

After the Berkeley professor had nearly lost hope of obtaining the blueprints, he was invited to testify before the U.S. House of Representatives' Committee on Science, in March of 2002, at a hearing titled "Learning From 9/11: Understanding the Collapse of the World Trade Center."

There, he was asked what "impediments" he had encountered in his research, and he replied that the largest one was his inability to get the design and engineering documents. Soon afterward, he was sent a copy of the plans by an official from FEMA. (He jokes that his wife wishes he had also asked for money to support his research.)

As Mr. Astaneh-Asl examined the construction documents, however, he was horrified by aspects of the design. He says the structure essentially threw out the rule book on skyscraper construction. "This building was so strange, and so many violations of practice and code were introduced," he says.

The design contains at least 10 unusual elements, he says. For example, rather than using a traditional skeletal framework of vertical and horizontal columns, the twin towers relied partly on a "bearing wall" system in which the floors and walls worked together to support each other, says Mr. Astaneh-Asl. That system allowed designers to use thinner steel in the buildings' columns and exterior than would be used in a traditional design, he says, adding that in some places the steel in columns was only one-quarter of an inch thick. And he says the designers used stronger steel (measured in what is known as "yield strength") in some columns than is allowed by any U.S. building codes, and that such steel is less flexible — and therefore more brittle — than the type traditionally used in such buildings.

As a result of such design elements, he argues, when the two airliners smashed into the upper floors of the towers, both planes plunged all the way in, wings and all. Airliners carry much of their fuel in their wings. His model clearly shows that in the initial fight between the plane and the building's exterior, the plane won, easily breaching the structure.

"It's like a soda can hit with a pencil," says Mr. Astaneh-Asl. "It was so easy that the plane went in without any damage and took the thousands of gallons of jet fuel in."

The structural innovations meant the developers saved money because they could use less steel, says Mr. Astaneh-Asl.

Efforts were made at the time of construction to verify the buildings could withstand anticipated forces, including high winds. The towers were among the first buildings ever to be modeled and tested in a wind tunnel before they were built. The buildings were widely praised for the efficiency of their construction.

But Mr. Astaneh-Asl argues that in engineering, innovations should — and usually do — emerge slowly, through evolutionary processes that follow time-tested practices. "Structural engineering is something that evolved," he says. "It was not invented."

"Unfortunately and tragically, when [this design] was subjected to this terrorist attack, there's no way this building could stand it."

Ross B. Corotis, a professor of civil engineering at the University of Colorado at Boulder, disagrees, arguing that the innovations did not mean that the towers were poorly designed. "I think our understanding of materials and our ability to analyze structural behavior gives us the ability to innovate more without introducing additional risk," he says.

Would a traditional structure have done better?

To try to answer that question, Mr. Astaneh-Asl and his team made another computer model in which they altered the design of the north tower's structure to make it more consistent with what the researcher calls standard engineering-design practice. Then he ran the same simulated plane into the structure in the same place it hit on September 11, 2001.

In that scenario, the airplane's wings are torn off, and therefore kept out of the building, when they hit the outer wall, while the fuselage still pierces the wall. "When it gets inside, there's not very much fuel," he says. Government reports found that it was not the damage from the planes, but the subsequent fires that weakened the steel and caused the buildings' collapse.

Mr. Astaneh-Asl says he cannot be certain whether a more-traditional building would have survived the smaller fire that would have followed because he is not an expert on fires. Even so, he argues, if the World Trade Center towers had been designed "using the codes and traditional systems, the building most likely would have survived — it most likely would not have collapsed."

The Prevailing View

Many engineers disagree with Mr. Astaneh-Asl's conclusions.

Mr. Corotis argues that "that particular design probably did better than most traditional designs would have done." One feature that helped, he says, was a cap across the tops of the towers that helped the buildings redistribute weight after the planes knocked gaps in them. Though he has not seen the blueprints for the structures, he says he is familiar with the building's innovative design, which has been widely publicized.

"Given the size of the planes and given the fire," he says, "the fact that they did come down is not surprising — but it's still shocking."

The most extensive investigation of the towers' collapse, completed by the National Institute of Standards and Technology, also found no fault with the structure, but it did acknowledge the buildings' unique history and design.

"The buildings were unlike any others previously built, both in their height and in their innovative structural features," a report on the investigation says. "Nevertheless, the actual design and approval process produced two buildings that generally were consistent with nearly all of the provisions of the New York City Building Code and other building codes of that time that were reviewed by NIST. ... The departures from the building codes and standards identified by NIST did not have a significant effect on the outcome of September 11."

And the other government investigation into the collapse, led by FEMA, reached the same conclusion.

"We didn't really cite anything we thought the designers should have known about at that time," says Mr. Corley, who led that investigation. "It would have made no difference to what happened regardless of what building code it was built under."

Mr. Corley says Mr. Astaneh-Asl's simulations do not prove that the design was flawed. "If I know what's going to happen, I can design something that can do better" under those circumstances, he says.

Mr. Astaneh-Asl responds that his modified version of the towers was not designed specifically for an airplane strike. "We designed this building assuming that they were building this building in 1970 following the [New York City] code without any consideration of an airplane," he says.

He sounds exasperated by what has come to be the accepted wisdom among engineers: that there was nothing wrong with the buildings. "I cannot see why the entire profession has agreed to sit in this convenient seat of saying that there is nothing wrong with our work," he says.

Skyscraper Safety

For the most part, Mr. Astaneh-Asl has done little to publicize his findings so far, especially since he still hopes to publish a scientific paper about his latest simulation. He agreed to talk to The Chronicle only after a reporter called him to follow up on its previous coverage of his research.

He did present the findings in July at MSC Software's Virtual Product Development Conference, in Huntington Beach, Calif. An article that ran in Design News says the presentation had audience members "spellbound."

But Mr. Astaneh-Asl has been drawn into the political fight over the new Freedom Tower that is slated to be built at ground zero. Last year he joined an advisory panel of a group started by families of 9/11 victims. The group, the Skyscraper Safety Campaign, is lobbying to, among other things, require the new office tower to adhere to local building codes, rather than to the Port Authority's guidelines.

"They're going to design this building without going to City Hall and getting permits," says Mr. Astaneh-Asl, his voice rising. "Even if you want to change your kitchen, you have to get a permit."

The Berkeley researcher says he initially declined the group's invitation to join because he wanted to remain completely independent. Aside from the free software, Mr. Astaneh-Asl says that his simulations research is not financially supported by anyone, and that he and the graduate students who helped with the project have volunteered their time, even using their personal computers. He says he later agreed to join the Skyscraper Safety Campaign's advisory panel because he supports their argument about building codes, but that the group has not given him any money.

Sally Regenhard is the leader of the Skyscraper Safety Campaign. Her son was a firefighter who perished responding to the attacks. Ms. Regenhard says that the government was slow to investigate the performance of the World Trade Center on September 11, 2001, and of the emergency response that followed. "There was a huge, huge force of don't ask, don't tell — don't ask questions," she says.

Many people outside engineering and government have developed their own theories about how and why the World Trade Center buildings fell. Some, wondering how buildings that easily withstood fierce wind gusts for decades were so quickly brought down by airplanes, argue that explosives planted before the attacks must also have been involved. Even some college professors have advanced such theories, though they have largely been dismissed (The Chronicle, June 23).

Mr. Astaneh-Asl also rejects such alternative theories. "I certainly don't buy into any of the conspiracy stuff," he says.

"Those are lightweight buildings," he adds. "There was no need for explosives to bring them down."

Professor S. Kitipornchai

Are tall buildings in Hong Kong safe from terrorist plane crashes?

by Professor S. Kitipornchai

No-one will ever forget September 11, 2001 – a day when the most powerful country in the world was caught completely off guard. Groups of well co-ordinated terrorists managed with relative ease to hijack a number of commercial planes and slam them into the 110-storey twin towers of the World Trade Centre and a section of the Pentagon.

The two Boeing 767 planes that crashed into the twin towers had just taken off, and both carried full tanks of fuel (91,000 litres). The twin towers were completely destroyed with the loss of over 6000 lives. The north tower, which was hit first, at about the 90th floor, stood for 1 hour and 45 minutes, whilst the south tower, which was hit at about the 60th floor, collapsed after only 45 minutes. Both towers collapsed spectacularly in front of millions of television viewers around the world. This raises some fundamental questions: why and how did the towers collapse, and are tall buildings in Hong Kong safe from similar attacks?

Engineers have anticipated planes crashing into tall buildings

The twin towers were the tallest buildings (416 m tall) in the world when they were completed in the 1970s. The buildings, 63.4m x 63.4m in plan, were framed in structural steel with closely spaced exterior steel columns, each of which were 476 mm wide and 560 mm apart, forming an exterior hollow tube wall. This wall acted as a structural frame, and provided the necessary lateral resistance. The central steel core of the building was designed to carry vertical or gravity loads. Horizontal steel trusses that spanned 18.3 m from exterior tube wall to the core supported the concrete floor, which also acted as a rigid diaphragm at each level.

Engineers have long anticipated the scenario of planes crashing into tall buildings. In 1945, a USAF bomber crashed into the 79th floor of the 102-storey Empire State Building in Manhattan. The crash occurred during a misty night, and the damage was restricted to the impact area. There have also been a number of near misses.

One of the criteria used in the design of the World Trade Centre was that if a Boeing 707 should hit either of the towers, then it would go right through without damaging the other storeys. This would be like punching a hole through the wall of a hollow tube without making the tube collapse. Calculations can easily show that such an impact will have little or no effect on the overall structural integrity of the building because the mass of the plane is very small compared to the mass resistance of the building. In a similar manner, the explosion of a terrorist bomb in the north tower basement during 1993 created a large hole, but did no real damage to the overall structural integrity of the building.

However, the Boeing 767s that hit the towers were much larger than expected, and had much greater fuel capacities. The impact of the fuel explosion and the ensuing fire was not considered in the building design.

Buildings collapsed because of explosions and fire

Video evidence suggests that the initial impact would have caused only local structural damage. The towers would have survived the impacts had it not been for the ensuing explosions and fire. The 250 or so fire fighters and police officers were ordered into the towers in the belief that they would not collapse – sadly none of them survived. This was a tragic error of judgment. An order for immediate evacuation should have been made at the very beginning.

With so much fuel, the temperature inside the buildings would have exceeded 1000°C. It was only a matter of time before the steel columns and the supporting floor trusses softened and lost all strength, precipitating the inevitable collapse of at least one complete storey at the level of impact. Once this happened, the huge mass of all the floors above would have simply crushed the intact floors below, resulting in the collapse of one storey after another like a row of dominoes. The south tower collapsed first because the plane entered the building at the corner, cutting through the exterior structural frame to either side, whereas the plane that hit the north tower entered more favourably near the middle of the building. It was fortunate that both towers collapsed almost vertically, otherwise there would have been much more devastation.

Are tall buildings in Hong Kong safe from terrorist plane crashes?

The attack on the World Trade Centre towers is thought to have been in the planning for some years, and seems to have been the result of perceived American foreign policy bias in the Middle East. There is no reason to suggest that Hong Kong will be targeted in a similar manner. However, we need not be complacent. Tall buildings in Hong Kong or anywhere else in the world would have met the same fate as did the towers in New York. There was nothing structurally wrong with the design of the twin towers. On the contrary, the buildings performed extremely well in the circumstances. Most other buildings hit by a Boeing 767 with a capacity fuel load would have collapsed almost immediately. The high degree of redundancy inherent in the exterior structural frame delayed the collapse of the buildings by about an hour, thus allowing thousands of occupants the precious time to escape.

It would be hard to imagine how any tall building could be designed to withstand attacks of this nature. Such buildings would be massive and the cost prohibitive. Only nuclear power plants are designed to withstand explosions and fire. Even if we can design and construct new buildings to resist such attacks, how do we make safe a multitude of existing buildings all over the world?

The answer may well lie in the design of aircraft. How do we keep planes out of the hands of terrorists? The challenge is to design them in a manner that would prohibit unauthorised entry into the cockpit even if terrorists could get on board. Only in that way could we arrest the tragedy of passenger planes being used as deadly bombs.

AIIB Newsletter, October edition.

Professor S. Kitipornchai is a Chair Professor in the Department of Building and Construction at the City University of Hong Kong

World Trade Center Investigation "Exonerates" Twin Towers' Design

Structural steel of the twin 110-story towers of the World Trade Center was stripped of its fireproofing by debris from the aircraft impact and weakened by the resulting fires, eventually causing the towers to collapse, according to an interim report by the National Institute of Standards & Technology. The report says the region of dislodged fireproofing was determined from the predicted path of the debris.

“Had the fireproofing not been dislodged, the temperature rise of the structural components would likely have been insufficient to cause the global collapse of the towers,” says NIST in the Oct. 19 release of another interim report of its $16-million study of the WTC destruction on Sept. 11, 2001, by terrorists. “Fireproofing dislodged by debris left the components more sensitive to heat than any areas where there was missing or thin fireproofing before the aircraft impacts,” says the report.

Many experts familiar with the twin towers design are not surprised by the findings. But they are worth noting, say sources, because there are others, both structural engineers and fire experts, who have questioned whether the design by Skilling Helle Christiansen Robertson in some way contributed to the collapse.

According to S. Shyam Sunder, NIST’s lead investigator for the study, an ordinary office fire would likely have resulted in burn-out, not collapse.

Sunder says the working hypothesis for a conventional fire, where sprinklers are not working, is that it IS more likely TO have burn-out without collapse. "If the sprinkler system were not operational in a multifloor fire, our current working hypothesis, without having done the calculations," is that the building would not have collapsed, he says. On 9/11, the building collapsed because the fireproofing on the steel was dislodged by direct debris impact. The steel heated up and softened and lost strength.

In addition, NIST has determined that the majority of the steel was stronger than minimum requirements. “The safety of the towers was most likely not affected by the small percentage of steel below the minimum,” says the report. “Building designs routinely allow structures to withstand greater loads than are expected by including significant factors of safety. Moreover, the structural loads on Sept. 11, 2001, were well below this design level. “

In fire tests in August, NIST also determined that the floor systems in the towers met the New York City building code of the time (ENR 9/13 p. 16).

The findings include an explanation for the time delay between the collapses of the two towers. (The south tower, Two WTC, survived for 56 minutes; the north tower, One WTC, for 103 minutes). NIST says the difference was primarily due to five items: the asymmetrical structural damage of the aircraft impact to Two WTC compared to the aircraft damage to One WTC; the time it took for heat to soften, buckle and shorten core columns that had fireproofing dislodged by debris impact; the structure’s ability to redistribute loads as the core columns shortened; the time it took for fires to traverse from their initial location to the face of the towers where perimeter columns were bowing inward (as seen only minutes before the collapse of each tower); and the time it took for heat to soften and buckle those columns.

NIST plans to release its final draft of the twin towers’ report in December or January. A four to six-week public comment period will follow. The final release is expected in May. The draft report on Seven WTC is set to be released in May. The final report is expected out in July.

Experts: Impact, fire too much for twin towers

Experts: Impact, fire too much for twin towers

©Washington Post,
published September 12, 2001

NEW YORK -- Built to withstand earthquakes and hurricane-force winds, and equipped with enhanced security after a 1993 terrorist bombing, the twin towers of New York's World Trade Center were supposed to last.

Their architect boasted that they could withstand the impact of a jumbo jet.

But when two hijacked commercial jetliners crashed into the 110-story structures within 15 minutes of each other early Tuesday morning, experts flinched, for "what we saw today was several orders of magnitude beyond anything we'd seen before," said the National Academy of Sciences' Richard Little, who has overseen several studies on how to protect buildings from terrorist attacks.

"We were hopeful at first," added Pennsylvania State University architectural engineer Kevin Parfitt, who teaches a course in building failures. "But the longer the fire burned, the more we feared the outcome."

With justification. In just under an hour, a raging fire from burning aviation gasoline softened or perhaps even melted the steel strength members supporting 50 floors of undamaged skyscraper above the point of impact in South Tower.

The top floors slumped to the damaged area and the impact of the dead weight caused the entire building to pancake to the ground. A half-hour later, the North Tower collapsed in the same way.

By late Tuesday afternoon, the 47-story Building 7, another of the center's seven buildings, had also fallen after burning all day. Building 6, the U.S. Customs House, was a smoldering, soot-blackened, hulk.

Experts agreed that collapse of the two towers was almost inevitable, for while their "tube structure" design was their greatest source of strength, it was also an Achilles' heel. For someone who wanted to bring them down, a gasoline-filled guided missile was perhaps the only way.

The towers were built like "rectangular doughnuts," Parfitt said. Strength came from a central steel core and from steel columns spaced closely around the perimeter of each building. There was no structural support between the core and the outer walls.

"When the planes come through, they cut through a number of those (perimeter) columns," Parfitt said. "At the same time, the planes are starting transcontinental flights, and they have full tanks of aviation fuel. You get a massive explosion and a fire."

The initial gasoline explosions most likely blew the insulation off the towers' girders, Parfitt suggested, incinerated easy combustibles and gave the ensuing fires free access to the unguarded steel.

For the people trying to escape, what followed was a macabre race against time, and the odds weren't good. Each of the Trade Center towers had 250 elevators, but only three stairwells. Between 20,000 and 25,000 people had to get out of each building as rapidly as possible.

Hyman Brown, a University of Colorado civil engineering professor and the Trade Center's construction manager, said it appeared the attack was meticulously planned.

"If they did it lower in the building the fire department could have gotten to it sooner. In its simplicity, it was brilliant."

He said that the two towers have staircases in all four corners and were designed to be evacuated in an hour, but it appeared that since the planes crashed into the corners, escape was cut off for those on the floors above.

The end came when the fire had softened the girders to the point that the weight above the crash sites became unsupportable. The South Tower, hit lower down, fell first beneath the greater weight. The North Tower, with less weight above the explosion, held out for a bit longer.

Angus Kress Gillespie, author of the 1999 book Twin Towers: The Life of New York City's World Trade Center, said architect Minoru Yamasaki designed the towers to withstand the impact of a jumbo jet, "but planes have become bigger" since the center was built in 1972.

Minoru Yamasaki Associates issued a statement Tuesday saying the firm was in contact with authorities and had offered assistance.

Cesar Pelli, designer of the Petronas Towers in Kuala Lumpur, Malaysia, the world's tallest buildings, suggested that while "it will take structural engineers a long time to figure out exactly how" the towers collapsed, he agreed that "no building is prepared for this kind of stress."

Fireproofing Blown Off Twin Towers

Fireproofing Blown Off Twin Towers

Report Details 9/11 Collapse in N.Y.

By Michelle Garcia
Special to The Washington Post
Wednesday, April 6, 2005; Page A03

NEW YORK, April 5 -- The hijacked airplanes that struck the World Trade Center hit with such force that the resulting explosions blew the fireproofing off the steel columns, accelerating heat buildup and weakening the structural core -- contributing to the towers' eventual collapse, according to a report issued Tuesday.

The process was hastened by fires outside that consumed the buildings' face and caused the exterior columns to bow in, according to the report.

Still, the study by the National Institute of Standards and Technology concluded that no amount of fireproofing could have saved the buildings.

Poor evacuation procedures, lack of communication and weak staircases cost the lives of civilians and emergency workers at the towers, as workers waited for directions and were slow to leave after the Sept. 11, 2001, attack, the report said.

Only two of the 198 elevators in the towers survived the initial explosions -- forcing most survivors to escape down emergency stairwells, which had suffered extensive damage. The report found that building codes lacked requirements sufficient to protect the structure of emergency stairwells.

Had such codes been in place, said S. Shyam Sunder, the lead investigator of the institute, "there would have been greater opportunity for people to evacuate."

Another federal report issued Tuesday found that the economic impact of the attacks was less than New York officials had originally estimated. After the attacks, state and city officials said the loss of tax revenue could approach $5.8 billion.

But the Government Accountability Office said the loss attributable to the attacks was closer to $2.9 billion and cited the city's recession, which had begun to take a toll before Sept. 11, for the rest of the loss.

The institute's report on the building collapse was long awaited by city officials. The institute based its analysis on extensive interviews with about 1,000 survivors, computer modeling, recovered steel and communications records.

The institute will use the findings in the 3,000-page report to formulate recommendations -- expected for release in September -- for changes in national building codes for office towers. A spokesman at the Port Authority of New York & New Jersey, which owned the World Trade Center, said local and state officials will review the recommendations and use them to guide reconstruction at Ground Zero.

"Whatever recommendations are adopted we will follow," said authority spokesman Steve Coleman. "Our engineering department has oversight over the buildings [and] will ensure the codes are followed."

In the past, city safety codes for office buildings often were a sort of informal compromise between safety and commercial imperatives. In 1968, New York City officials drastically reduced the number of required stairwells in skyscrapers, at the request of the real estate industry, to increase the amount of available rental space.

New York was, in fact, fortunate that the attacks took place in the morning, when most people had not yet reached their offices. If the building had been fully occupied, the report found, a full evacuation would have taken four hours and cost 14,000 lives.

The agency interviewed survivors and found that, although most had participated in a fire drill, nearly one-half had never used the stairwells in the buildings before the attacks. In fact, New York City prevents the use of stairwells during fire drills.

"I've never heard of another jurisdiction having such a prohibition," Sunder said.

Engineering solutions

Engineering solutions

In late 2001, engineers were working on designs for BP1 - a new tower for Canary Wharf, London. But following 9/11, they changed their plans. Together with steelwork specialists, they devised a pioneering system to strengthen beam and column connections.

Usually, each end of a floor beam is bolted directly to the face of a supporting column. But by pushing a short steel plate through the column, engineers found they could tie neighbouring beams together. Even with two adjacent columns removed, floor beams could hang from the remaining structure.

When the Twin Towers Fell

In Focus
October 09, 2001

When the Twin Towers Fell
One month after the attack on the World Trade Center, M.I.T. structural engineers offer their take on how and why the towers came down
By Steven Ashley

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.


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."

Why the Towers Fell


With all of its structural redundancies, "the World Trade Center was probably one of the more resistant tall building structures," McNamara said, adding that "nowadays, they just don't build them as tough as the World Trade Center." His statement is bolstered by the fact that the support structures of both twin towers withstood the initial hits of the two kamikaze airliners despite the breaching of many levels of framing. 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 1995 truck bombing of the Alfred P. Murrah Federal Building in Oklahoma City (about 400 tons of TNT), the towers remained upright. "The buildings displayed a tremendous capacity to stand there despite the damage to a major portion of the gravity system, and for an hour or so they did stand there," McNamara said. "The lateral truss systems redistributed the load when other critical members were lost. It's a testament to the system that they lasted so long."

Newspapers and TV newscasts reported that the twin towers had been designed to withstand a collision with a Boeing 707. The events of September 11th show that this was indeed the case. "However, the World Trade Center was never designed for the massive explosions nor the intense jet fuel fires that came next—a key design omission," stated Eduardo Kausel, another M.I.T. professor of civil and environmental engineering and panel member. The towers collapsed only after the kerosene fuel fire compromised the integrity of their structural tubes: One WTC lasted for 105 minutes, whereas Two WTC remained standing for 47 minutes. "It was designed for the type of fire you'd expect in an office building—paper, desks, drapes," McNamara said. The aviation fuel fires that broke out burned at a much hotter temperature than the typical contents of an office. "At about 800 degrees Fahrenheit structural steel starts to lose its strength; at 1,500 degrees F, all bets are off as steel members become significantly weakened," he explained.

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 1993 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. "Aluminum melts into burning 'goblet puddles' that would pool around depressions, [such as] beam joints, service openings in the floor, stair wells and so forth...The goblets are white hot, burning at an estimated 1800 degrees Celsius. At this temperature, the water of hydration in the concrete is vaporized and consumed by the aluminum. This evolves hydrogen gas that burns. Aluminum burning in concrete produces a calcium oxide/silicate slag covered by a white aluminum oxide ash, all of which serve to insulate and contain the aluminum puddle. This keeps the metal hot and burning. If you look at pictures of Iraqi aircraft destroyed in their concrete shelters [during the Persian Gulf war], you will notice a deep imprint of the burned aircraft on the concrete floor.

Though the Boeing 767s airliners that hit the towers were somewhat larger than the Boeing 707 (maximum takeoff weights: 395,000 pounds versus 336,000 pounds) the structures were designed to resist, the planes carried a similarly sized fuel load as the older model—about 24,000 gallons versus 23,000 gallons, according to Kausel. "Most certainly," he continued, "no building has or will resist this kind of fire." The sprinkler system, which was probably compromised, would have been are useless against this kind of fire, he said, adding, "The World Trade Center towers performed admirably; they stood long enough for the majority of the people to be successfully evacuated."

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," he noted.

The M.I.T. professor added that about 30 percent of the collapse energy was expended rupturing the materials of the building, while the rest was converted into the kinetic energy of the falling mass. The huge gray dust clouds that covered lower Manhattan after the collapse were probably formed when the concrete floors were pulverized in the fall and then jetted into the surrounding neighborhood. "Of the kinetic energy impacting the ground, only 0.1 percent was converted to seismic energy," he stated. "Each event created a (modest-sized) magnitude 2 earthquake, as monitored at Columbia University's Lamont-Doherty Observatory, which is located about 30 kilometers away from New York City." Kausel concluded that the "the largest share of the kinetic energy was converted to heat, material rupture and deformation of the ground below."

Despite the expert panel's preliminary musings on the failure mechanisms responsible for the twin towers' fall, the definitive cause has yet to be determined. Reportedly, the National Science Foundation has funded eight research projects to probe the WTC catastrophe. The American Society of Civil Engineers is sponsoring several studies of the site. Meanwhile the Structural Engineering Institute of the American Society of Structural Engineers has established an investigative team to analyze the disaster and learn from the failure. W. Gene Corley, senior vice president of the Construction Technology Laboratory in Skokie, Ill., is said to be heading the ASSE study team through its initial phase of data gathering, and then William Baker, a structural engineer at the Chicago-based firm of Skidmore Owings & Merrill in Chicago, will lead the following analysis phase. The Structural Engineering Institute is to partner with the American Institute of Steel Construction, the National Fire Protection Association and the Society of Fire Protection Engineers. The Federal Emergency Management Agency has been invited to join as well.

How the Towers Fell

Given the lack of firm conclusions regarding how the collapses occurred, the M.I.T. panel participants asked their audience to consider various theories they put forth. In general, it was agreed that as the structure warped and weakened at the top of each tower, the frame, along with the concrete slabs, furniture, file cabinets and other materials, became an enormous consolidated weight that eventually crushed the lower portions of the structure below. The details of how the frame members failed remain under contention.


Professor Connor's theory focused on weaknesses in how the vertical and horizontal structural members were tied together. During construction, he explained, each prefabricated floor system was lifted into place by a crane and "supported at the ends like a hinge, where they were bolted and welded to the inner and outer framing tubes" so that part of the gravity load went through the core and the other part through the exterior structure. "The floor trusses sat on beams and were tied down so the core was locked to the exterior," he said. "It was an unusual system and very lightweight. If you lose the connection between them, however, you lose the ability to carry the floor loads and allow the floors to slide back and forth under stress. If a damaged floor system were to fall, it would break the end connections in the lower floors and down and down the floors would go."

"In my theory, the hot fire weakened the supporting joint connection," Connor continued. "When it broke, one end of a floor fell, damaging the floor system underneath, while simultaneously tugging (pulling) the vertical members to which it was still attached toward the center of the building and down." This phenomenon started a parasitic process that accelerated until total failure and the structure fell in on itself, he said.

Eduardo Kausel proposed an alternative failure explanation that he acknowledged was independently developed by Zdenek Bazant, a professor at Northwestern University. "I believe that the intense heat softened or melted the structural elements—floor trusses and columns—so that they became like chewing gum, and that was enough to trigger the collapse," he said. "The floor trusses are likely to have been the first to sag and fail. As soon as the upper floors became unsupported, debris from the failed floor systems rained down onto the floors below, which eventually gave way, starting an unstoppable sequence. The dynamic forces are so large that the downward motion becomes unstoppable."

Via two simple models, Kausel was able to determine that the fall of the upper building portion down onto a single floor must have caused dynamic forces exceeding the buildings’ design loads by at least an order of magnitude. He also performed some computer simulations that indicate the building material fell almost unrestricted at nearly the speed of free-falling objects. "The towers' resistive systems played no role. Otherwise the elapsed time of the fall would have been extended," he noted. As it was, the debris took about nine seconds to reach the ground from the top.

"It's difficult to judge which of these failure mechanisms occurred first; probably all occurred and interacted," said panel member Oral Buyukozturk, professor of civil and environmental engineering at M.I.T.. "The prolonged effect of high heat is likely to have led to the buckling of the columns, collapse of the floors, as well as to the shearing of the floors upon the failure the joints." He noted that videotapes of the catastrophe showed some tilting of the top portion of the south tower before it collapsed. "This indicates the buckling of one building face while the adjacent face was bending [placed into tension]." After that, the upper portions of the tower are shown disintegrating, with "a dynamic effect and amplification process" following that led to a progressive collapse—"a kind of pancaking or deck of cards effect"—down to ground zero, Buyukozturk stated.

Kausel addressed the oft-asked question of why the towers did not tip over like a falling tree. "A tree is solid, whereas building is mostly air or empty space; only about 10 percent is solid material. Since there is no solid stump underneath to force it to the side, the building cannot tip over. It could only collapse upon itself." Robert McNamara said his failure mechanism theory "focuses on the connections that hold the structure together," but he cautioned that "we really need to wait for a detailed investigation, before we decide if we have to up the code ratings for these connections in signature structures."

Protecting Skyscraper Occupants

The expert panel then turned its attention to changes for future tall structures in the wake of what has been learned. Though the recent "disaster couldn't be envisioned as a design scenario in the 1970s, it means we have to change the way we design and construct tall buildings in the future," Buyukozturk said.


Existing skyscrapers should probably be retrofitted with some additional safety measures, but the professors say that it doesn't make sense economically—and aesthetically—to protect them all physically from similar catastrophes. "Retrofitting is very expensive and is therefore usually done only for monumental buildings," Connor said.

"There will never be a building that won't fall," Kausel noted. "The best we can do is to ensure that it will stand long enough for all the people to escape." Back when the WTC was built, no one seems to have anticipated the need to evacuate an entire large building at once. To do so successfully means boosting a building's structural redundancy—the provision of additional means to assist system function. Panel members discussed providing improved fire protection for the structural elements, alternative load paths to stand in for damaged structures and fixing diaphragm floor beams more strongly to vertical members. Also mentioned was the idea of installing blast-resistant, energy-absorbing materials such as concrete-encased steel exterior columns and/or cavities (reinforced concrete cores) in future large structures that could help them survive or at least promote failure in certain slower, less deleterious sequences.

One audience attendee, a West Coast-based structural engineer who did not give his name, created a provocative moment when he claimed that it would cost about 10 percent more than the original building cost to install floor joint reinforcements for greater redundancy. "According to our analysis, it could add several more hours to the evacuation period," he stated. "If each tower cost about $1 billion to build, then an extra hundred million dollars could have saved most of the occupants. Though it's horrible to contemplate," he continued, "a human life is valued for insurance purposes at about a million dollars apiece, so this helps put the extra investment into perspective. After all, the World Trade Center was retrofitted with 10,000 viscoelastic dampers to reduce its swaying, so safety improvements can't be ignored. Building clients have to become more demanding, even if the probabilities of a repeat disaster are very slim...."

The panel also considered the need to improve the effectiveness of building safety systems. Kausel pointed out problems with the twin towers' emergency communications systems ("just when coordination was most critical, the people didn't know what to do"), the emergency illumination system and protection against smoke ("the great killer in building fires is smoke inhalation"). He also suggested that more effort should be expended to create "alternative escape routes, so evacuees aren't faced with a wall of smoke. If two stairwells are close together," he noted, "one explosion could block them both." Other ideas floated included installing better fire-suppression systems, using the aqueous film-forming foams employed in aviation fires, and creating protected access ways for firefighters. There was also mention of the need to harden stairwells and egress pathways, and perhaps develop "deployable evacuation systems" for building occupants. Beyond robotic stairway evacuation devices, deployable systems might include escape tubes deployed out windows, exterior people-lowering machines, flying platforms or even parachutes."

One audience member asked the assembled experts whether a reinforced concrete skyscraper such as the current height record-holder, the 452-meter Petronas Towers in Kuala Lumpur, Malaysia, would have better resisted a collision with a fuel-filled airliner. Their response indicated that a concrete structure would have probably lasted for a couple of more hours than did the steel World Trade Center towers. Robert McNamara stated that many of the more recent superskyscrapers were constructed of reinforced concrete mainly because of the high cost of steel in Asia. He also mentioned that the Petronas Towers contain "safe refuge floors" to allow building occupants to reach fresh air during fires. McNamara said that this concept was now somewhat discredited, as similar refuges in the WTC would not have ultimately saved anyone.

A lively discussion then ensued about whether the terrorist pilots knew where to hit the buildings for maximum effect. McNamara opined that the position of impact seems significant. "They hit them at just the right place—about two thirds to three quarters of the way up. The earlier [truck bomb] attack showed that the explosion at bottom had little effect and that it's much easier to collapse a building from the top than the bottom. If they had hit the very top of the building, the fire damage wouldn't have had such a catastrophic effect. At the bottom, the columns are much heavier and stronger and so they would have taken a much larger load." Connor offered that one would "need graduate-level engineering training to choose the prime target location."

In the aftermath of the World Trade Center disaster, questions arose whether superskyscrapers should be built in the future. Clearly, these top engineers would reply in the definite affirmative. Inevitably, they said, new tall towers will rise.

Why the Twin Towers Fell

Why the Twin Towers Fell

Engineers who studied the World Trade Center after the September 11 attacks tell why the Twin Towers stood as long as they did, and why they eventually collapsed. For the most current information on New York's World Trade Center, be sure to visit our WTC Index.

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Join the Discussion

"Can we truly build an indestructible building? Let alone having it survive a fire being fed by jet fuel?" ~VORRATUS

New York's World Trade Center Twin Towers were designed to withstand fire and hurricane-force winds. Some engineers believed the Twin Towers could even survive impact from a Boeing 707. But no engineer or architect could have anticipated the terrorist attack that turned the Twin Towers to rubble, and experts often don't agree what steps might have been taken to make the buildings stronger. For the most recent findings and detailed technical information, be sure to visit the World Trade Center Coverage page at icivilengineer.

How did the Twin Towers fall?

1. Impact from the Terrorist Planes
When Boeing jets piloted by terrorists struck the Twin Towers, some 10,000 gallons (38 kiloliters) of jet fuel fed an enormous fireball. But, the impact of the planes and the burst of flames did not make the Towers collapse right away. Like most buildings, the Twin Towers had redundant design. The term redundant design means that when one system fails, another carries the load. Each of the Twin Towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. When some columns were damaged, others could still support the building.

2. Heat from the Fires
The sprinkler system was damaged by the impact of the planes. But even if the sprinklers had been working, they could not have maintained enough pressure to stop the fire. Fed by the remaining jet fuel, the heat became intense. Most fires don't get hotter than 900 to 1,100 degrees F. The World Trade Center fire may have reached 1,300 or 1,400 degrees F. Structural steel does not easily melt, but it will lose about half its strength at 1,200 degrees F. The steel structure of the Twin Towers was weakened by the extreme heat. The steel also became distorted because the heat was not a uniform temperature.

3. Collapsing Floors
Most fires start in one area and then spread. The fire from the terrorist planes covered the area of an entire floor almost instantly. As the weakened floors began to collapse, they crashed into the floors below. With the weight of the plunging floors accelerating, the exterior walls buckled.

Why did the collapsed towers look so flat?

Before the terrorist attack, the Twin Towers were 110 stories tall. Constructed of lightweight steel around a central core, they were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.

Could the World Trade Center have been made stronger?

In a report produced by the Federal Emergency Management Agency (FEMA), the American Society of Civil Engineers (ASCE), and other organizations, experts concluded that no skyscraper could have withstood the impact of the terrorist airplanes. Further, the experts warned that it would not be "technically feasible" to design a building that could survive this type of terrorist attack. Instead, engineers and architects are suggesting that we focus our efforts on designing better warning and evacuation systems so that we can save more people inside the buildings.

Is it possible to design a truly indestructible building? Tell us what you think.

See photos and read more about the construction of the Twin Towers >

William F. Baker

First public hearing of the National Commission on Terrorist Attacks Upon the United States

Statement of William F. Baker to the
National Commission on Terrorist Attacks Upon the United States
April 1, 2003

I wish to extend my sincere appreciation for being asked to address the Commission on behalf of the ASCE/FEMA World Trade Center Building Performance Assessment Team.

My comments relate to my role on the ASCE/FEMA team as well as my experiences as a structural engineer assisting the fire fighters and contractors in the early days after the attack on the World Trade Center. In addition, I will offer key recommendations that may be valuable to New York and other urban areas in dealing with the aftermath of future terrorist attacks. These recommendations include:

  • Immediate coordination of on-site structural engineers;
  • An archived depository for construction drawings of all citywide buildings and infrastructure; and
  • Federal "good Samaritan" legislation

When I arrived at the WTC site, I was awestruck at the extent of the devastation. While the media focused on the destruction of the twin towers, the damage went far beyond anything that was conveyed to the general public. The damage included the total collapse of four major buildings, the partial collapse and burnout of three major buildings and extensive damage to seven additional major buildings. Many other buildings suffered minor damage. The district's infrastructure including utilities and the subway system area was extensively damaged and parts of it were destroyed.

The New York City Department of Design and Construction (DDC) performed a brilliant job in organizing the efforts of the engineering and construction industries to support the search and rescue and, later, clean up the site. DDC divided the district into four sectors. Each sector was assigned to a team of contractors. This was a very successful approach.

Because the contractors were top-notch and adept at managing very large private and public sector projects, they knew how to organize the teams, deal with heavy equipment and marshal resources. However, the contractors needed the professional assistance of structural engineers.

The Structural Engineers Association of New York (SEAoNY) stepped forward to organize the services of structural engineers from across the city, state and country. SEAoNY assembled teams of structural engineers to assist each of the (4) contractor teams. I urge the Commission to use this approach developed by the DDC and SEAoNY as a model for dealing with possible large urban disasters. In many ways, New York City was fortunate to have the DDC and major players from the engineering/construction industries available to assist at the WTC site.

In the first, critical hours and days following the attack, what was not available or well organized were the drawings of the buildings, plazas, subway tunnels, freight tunnels, etc. As engineers and contractors were investigating the extent and severity of damage, drawings of the original structure were sorely needed. When it was necessary to bring very large cranes and other equipment across tunnels, vaults and plazas, drawings were an absolute necessity. I strongly urge all major cities to develop archive depositories for construction drawings and other critical information to be available to the authorities on short notice. A duplicate copy should be housed in a redundant location.

The structural engineers who assisted at the WTC site were often in uncharted territory with respect to professional liability. There should be appropriate national "Good Samaritan" legislation to promote the assistance of structural engineers in such situations. While such legislation exists in high seismic states such as California, all states are vulnerable to terrorist attacks and natural disasters and should have appropriate legislation. Federal "good Samaritan" legislation may be appropriate.

At this point, I would like to focus my comments on the efforts of the ASCE/FEMA Building Assessment. The Structural Engineering Institute (SEI), a division of the American Society of Civil Engineers (ASCE), was responsible for organizing this effort and bringing together the relevant professional societies with support from FEMA, for an assessment of the WTC site. Under the leadership of ASCE, a team of structural engineering and fire engineering experts from around the country were brought together. Because of my expertise in tall buildings, I was asked to join the effort and be on the five-member core group that directed the assessment.

This type of effort is important because advancement in the construction of buildings has often come from the analysis of failures. These tragic events of 9-11 provided an opportunity to see how building emergency systems and structural systems behave in extraordinary events. Although the media has focused on the twin towers, there is more to be learned from the behavior of the more "ordinary" buildings that were damaged by the events. We saw and documented the performance of structures that resisted extraordinary forces and maintained their overall integrity. We also saw and documented collapses that, based on previous experiences, were unexpected. It is through the study of these behaviors that the art of building design is advanced.

Unfortunately, the ASCE/FEMA team faced many obstacles while studying the WTC. The team was not able to assemble on the site until October 6th. We could only request and cajole to get drawings and other information. And, in fact, we did not receive access to the twin tower drawings until January. Nonetheless, the team was able to perform an invaluable service in our initial, overall evaluation of the buildings in order to focus and prioritize future investigations and research.

Fortunately, the National Construction Safety Act that was signed into law on October 1, 2002 addresses the difficulties faced by the ASCE/FEMA team at the WTC site. This act authorizes the National Institute of Standards and Technology to investigate building failures. This is similar to the National Transportation Safety Board investigations of airline and other transportation accidents.

This act allows NIST teams to access building failure sites; provides the power to subpoena evidence; provides access drawings, records and other documents; and allows for the removal and storage of evidence.

This legislation is a significant step forward in creating a vehicle by which the design and construction industry can learn from failures. This will help to advance building technology and improve the safety and reliability of future construction.

As a structural engineer, the WTC collapses represent the largest structural failure in the history of mankind. From this tragedy, I am confident that we can learn how to approach catastrophic building failures in the future and through the National Construction Safety Act we will continue to learn how to improve building construction.

Thank you for your attention.

Mr. Baker is the partner in charge of Structural and Civil Engineering for Skidmore, Owings & Merrill LLP (SOM). Mr. Baker has been involved in a variety of innovations in building systems, including low-rise and high-rise buildings, long span roofs and special structures. Current projects include Tower Palace III, a 69-story, 2-million sf residential tower for the Samsung Group, which is nearing completion in Seoul, Korea, as well as Trump Tower Chicago, a 2.6 million sf tower that will be completed in Chicago, Illinois in 2005.

In the weeks just after the WTC disaster, Mr. Baker provided structural engineering assistance to search and rescue teams in addition to evaluating damaged buildings and advising contractors on debris removal. He has recently been appointed as one of five members of the Core Group of the ASCE/SEI Building Assessment Team that is evaluating the WTC disaster.

Mr. Baker was educated at the University of Illinois, where he received a Master of Science in Civil Engineering in 1980. He received his Bachelor of Science in Civil Engineering at the University of Missouri, in 1975.

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