Arup Study

Innovative Structural Engineering for Tall Buildings in Fire.

See slideshow of study here.

Contracts on WTC investigation from NIST




R - World Trade Center 7 Structural Models and Collapse Hypothesis, Contract Modifications for Structural and Blast Analyses

Applied Research Associates


Award Information



R - World Trade Center 7 Structural Models and Collapse Hypothesis

Applied Research Associates


Award Information



R --Analysis of Smoke Management Systems in WTC Buildings

Hughes Associates, Inc. (HAI)


Award Information



R --Structural Response to the World Trade Center (WTC) Towers

Simpson Gumpertz & Heger, Inc. (SGH)


Award Information



R-- Development of WTC 7 Structural Models and Collapse Hypotheses

Gilsanz Murray Steficek LLP (GMS)


Award Information



R--Analysis of Active Fire Alarm Systems, WTC 1, 2, and 7

Rolf Jensen & Associates, Inc. (RJA)


Award Information



R -- Analysis of Aircraft Impacts Into the World Trade Center Towers

Applied Research Associates, Inc. (ARA)


Award Information



R--Analysis of Active Fire Protection Systems--Sprinklers, Standpipe, and Pre-Connected Hoses in WTC Buildings 1, 2, and 7

Hughes Associates, Inc. (HAI)


Award Information



R -- Fire Endurance Testing of the World Trade Center Floor System (contract modification)

Underwriters Laboratory, Inc.


Award Information



R -- Analysis of Building and Fire Codes and Practices

Rolf Jensen & Associates, Inc. (RJA)


Award Information



R -- Fire Endurance Testing of the World Trade Center Floor System

Underwriters Laboratory, Inc.


Award Information



R-Outside Experts for Baseline Structural Performance, Impact Analysis, Structural Response to Fire, Collapse ...

Area 4: Teng & Associates, Inc.


Award Information



R-Outside Experts for Baseline Structural Performance, Impact Analysis, Structural Response to Fire, Collapse ...

Area 5: Dr. Daniele Veneziano and Dr. Jozef Van Dyck


Award Information



R-Outside Experts for Baseline Structural Performance, Impact Analysis, Structural Response to Fire, Collapse ...

Area 2: Dr. David M. Parks


Award Information



R-Outside Experts for Baseline Structural Performance, Impact Analysis, Structural Response to Fire, Collapse ...

Area 1: Skidmore, Owings & Merrill LLP


Award Information



R-Outside Experts for Baseline Structural Performance, Impact Analysis, Structural Response to Fire, Collapse ...

Area 3: Dr. Kaspar Willam


Award Information



Document and Evaluate the Steel Recovered from the WTC Towers

Wiss, Janney, Elstner Associates, Inc.


Award Information



World Trade Center (WTC) Investigation Survey Administration and Report Delivery: Questionnaires, Interviews and Focus Group Synopsis



Award Information



(Pre-solicitation Notice/Sole Source) R -- World Trade Center (WTC) Investigation First Person Accounts of Egress

National Fire Protection Association (NFPA)


Award Information



Development of Structural Databases and Baseline Models for the WTC Towers

Leslie E. Robertson Associates (LERA)


Award Information



Fire Safety Engineering Expertise

Mr. Harold Nelson


Award Information



Outside Expert Team Member

Dr. Norman Groner


Award Information



Outside Expert Team Member

Dr. Guylene Proulx


Award Information



Outside Expert Team Member

Dr. Dennis Mileti


Award Information

ASCE Establishes Teams to Study NY and DC Disasters

ASCE Establishes Teams to Study NY and DC Disasters

ASCE, using its Disaster Response Procedure, has authorized the formation of two teams to study the collapse of the World Trade Center in New York and the damage to the Pentagon in suburban Washington, DC. Each team is composed of ASCE members who are experts in the design of complex structures. Team members will use their contacts in their area of expertise and work with local officials in the disaster area to gain access to the site. When the teams complete their assignment their work will be documented in the form of a report that is made available to the ASCE membership and other interested parties. ASCE's Disaster Response Procedure has been in place for approximately 10 years. This is the fifth time in 2001 the procedure has been used to create study teams. Earlier teams, whose members were experts in earthquakes and lifeline engineering, were dispatched to study and document the devastation from the earthquakes in El Salvador, India, the Seattle area, and Peru. Funding for the teams is made available through the ASCE Voluntary Fund.

Team Members confirmed as of 11/14/01 are:

World Trade Center
Data Collection Team

W. Gene Corley, Ph.D., P.E., Lead
Senior Vice President,
Construction Technologies Laboratories
Skokie, Illinois
Expert in building collapse investigations; principal investigator,
Murrah Federal Office Building Study
Jonathan Barnett, Ph.D.
Professor, Center for Fire Safety Studies
Worcester Polytechnic Institute
Worcester, Massachusetts
Expert in building fire safety design and fire computer modeling
David T. Biggs, P.E.
Ryan-Biggs Associates
Troy, New York
Expert in facades
Bill Coulbourne, P.E., S.E.
Principal, URS Corporation
Gaithersburg, Maryland
Edward M. DePaola, P.E.
Partner, Severud Associates
Consulting Engineers
New York City
Expert in structural engineering
Robert F. Duval
Senior Fire Investigator
National Fire Protection AssociationExpert in fire investigations
John T. Fisher, P.E.
Joseph T. Stuart Professor of
Civil and Environmental Engineering
Lehigh University
Bethlehem, Pennsylvania
Expert in metallurgy and connections
Richard G. Gewain
Senior Engineer, Hughes Associates, Inc.
Expert in fire engineering
Ramon Gilsanz
Managing Partner, Gilsanz Murray Steficek
New York City
Expert in structural engineering
John L. Gross, Ph.D., P.E.
Leader, Structural Systems and Design Group
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, Maryland
Expert in steel design and fire-structure interaction
Ronald Hamburger, P.E., S.E.
Senior Vice President, EQE Structural
Engineers Division
ABS Consulting
Belmont, California
Expert in structural analysis and design
Nestor Iwankiw
Vice President, Engineering and Research
American Institute for Steel Construction
Expert in steel design
Venkatesh Kodur, Ph.D., P.E.
Institute for Research in Construction
National Research Council of Canada
Ottawa, Ontario
Expert in fire effects on materials
Eric Letvin
Department Head, Hazards Engineering Group
Greenhorne & O'Mara
Greenbelt, Maryland
Jon Magnusson, P.E.
Chairman of the Board, Chief Executive Officer
Skilling Ward Magnusson Barkshire, Inc.
Expert in structural analysis and high-rise design
Christopher E. Marrion, P.E.
Fire Strategist, Arup Fire
New York City
Expert in fire engineering
Therese P. McAllister, Ph.D., P.E.
Senior Structural Engineer
Greenhorne & O'Mara
Greenbelt, Maryland
James Milke, Ph. D., P.E.
Professor, Department of Fire Protection Engineering
University of Maryland
Expert in fire resistance analysis
James A. Rossberg, P.E.
Director, Structural Engineering Institute
Reston, Virginia
Saw-Teen See, P.E.
Managing Partner
Leslie E. Robertson Associates
New York City
Expert in structural analysis and high-rise design
Robert Smilowitz
Principal, Weidlinger Associates
New York City
Expert in blast effects
Bruce Swiren
Hurricane Program Manager, Region II
Federal Emergency Management Agency
New York City
Paul Tertell, P.E.Program Manager, Building Performance Assessment Team
Federal Emergency Management Agency
Washington, D.C.

Dr. Karl Frank

Engineers receive $817,000 to create university facility
to better understand response of building materials to fire

September 9, 2005

AUSTIN, Texas—Civil and mechanical engineers at The University of Texas at Austin have received an $817,000 grant to create the first university center to study the impact of fire on the integrity of building components.

Dr. Karl Frank, director of the Phil M. Ferguson Structural Engineering Lab that received the National Science Foundation grant, said the Sept. 11, 2001, collapse of buildings at the World Trade Center (WTC) was a wake-up call for American structural engineers to focus more attention on high-temperature scenarios.

“The World Trade Center catastrophe stunned the world, and it shocked structural engineers,” said Frank, holder of the Warren S. Bellows Centennial Professorship in Civil Engineering. “We didn’t expect those buildings to fall.”

The impact of the hijacked planes that hit the WTC started a chain of events that ultimately led steel columns in the core of the two buildings and joists supporting their floors to lose their strength as the temperature of the members increased from raging fires. The columns and joists carried the weight of each floor. Once these elements gave way, the weight of a falling floor overloaded those beneath it, causing a domino-like failure.

With the grant, Frank and others at the J.J. Pickle Research Center lab will test how steel and other materials respond to increasingly high temperatures. Specimens will be placed in digitally controlled test frames that can apply loads to the material while the material undergoes heating that can occur in a fire. One test frame purchased with grant funds can apply a maximum of 22,000 pounds of force on a test material; the other, with a 20-foot height, up to 550,000 pounds of force.

Each test frame will be outfitted with digitally controlled furnaces which Frank is purchasing, creating enclosed heating chambers. The furnaces will heat material up to 2,000 degrees Fahrenheit, or twice the temperature that is usually considered failure in standard fire tests.

“We will test a variety of materials at different temperatures so we can determine the loss of their strength as the temperature goes up,” Frank said.

Wooden trusses, concrete connections and new materials will be studied as well as steel. New materials include fiber-reinforced plastics, which are being used to strengthen and repair connections between beams and columns that support the loads in concrete structures.

Traditional building steel, favored for its strength at room temperatures, is easier to weaken than other materials like wood at high temperatures. Frank and colleagues will test the heat response of steel alloys that have chemicals added to enhance the steel’s strength at high temperatures. They will also test connection details such as steel connections with high-strength bolts, wood trusses connected with steel plates, and reinforced concrete connections.

Civil engineering faculty besides Frank who will perform heating studies using the new equipment are: Professor Michael D. Englehardt, Professor Sharon Wood, Associate Professor Dan L. Wheat and Assistant Professor Oguzhan Bayrack. In the Department of Mechanical Engineering, participants are Associate Professors Ofodike Ezekoye and Eric Taleff. The experiments will also be videotaped and shared with students in the College of Engineering.

“The behavior of building materials in fire, and how rapidly some materials deteriorate, is something most engineers don’t have a conceptual understanding of,” Frank said.

Did The Building Do It?

Did The Building Do It?

FEA Study Shows Design May Have Played a Role in the World Trade Center Collapse

Karen Auguston Field, Editor-in-Chief -- Design News, September 25, 2006

Seven days after the World Trade Center attack on September 11, 2001, Professor Abolhassen Astaneh-Asl was in New York, armed with a digital camera and a notebook and documenting what he could of the devastation. A Professor of structural engineering at the University of California Berkeley working under a grant from the National Science Foundation to collect perishable data and conduct a reconnaissance of the collapsed towers from a structural engineering point of view, Astaneh-Asl knew he had a very brief window of opportunity in which to gather evidence that might be useful in any future analysis of the buildings’ collapse.

What he did not realize, however, was that he was embarking on a half-a-decade-long odyssey that would have him scrambling for resources and trying to obtain construction drawings and design documents for the towers from the Port Authority of New York and New Jersey, the owner of the WTC.

The release of these documents—which ultimately required an order from Congress’ Committee on Science --was critical because they contained design specifics that Astaneh-Asl needed in order to develop a detailed structural model to simulate the impact of the airplanes on the twin towers. “Basically we wanted to find out what role the buildings themselves played in the tragic events of that day and to learn lessons that can be used in protection of other structures,” he explains.

Now, as the five-year anniversary of the World Trade Center attack draws near, Astaneh-Asl finally expects to have the results of his analysis published in an academic journal. The project, requiring thousands of hours to complete, was self-funded and conducted by Astaneh-Asl, his students, and analysts from the MSC.Software Corporation, which donated the structural analysis software (MD Nastran and Dytran).

Engineers use this kind of software to simulate the behavior of objects subjected to any number of punishing scenarios, including bending, shaking, and collisions.

A roomful of engineers and analysts—whose own simulation studies revolve around more practical, everyday concerns like how much torque a new wrench design can sustain—got a sneak preview of those upcoming results at MSC. Software’s2006 Virtual Product Development Conference in Huntington Beach, CA, in July.

“The simulation model shows the plane slicing right through the outer walls of the as-built building like it was a thin soda can,” Astaneh-Asl explained to the spellbound crowd.

He described the issue in a nutshell: “Because of their unique design and the use of the so called “steel bearing wall” tube structural system, which as far as we know has never been used before or after its application in the WTC towers, the buildings essentially showed no resistance to the impact of a medium-sized plane flying into them at about 450miles per hour.”

Elaborating on the novelty of the design, he said that the notion of a ‘structural framing system’ simply didn’t apply in the case of the twin towers. “Rather than traditional columns and beams, the designers employed a steel bearing wall tube system for the perimeter and steel truss joists in the floors that connected the gravity load-carrying inner core columns to the outside perimeter steel bearing walls. The relatively thin steel bearing wall pre-fabricated units of the perimeter bearing tube were bolted together in a Lego-like fashion to expedite construction” he explained.

He also noted that designers chose to fabricate many of the building columns out of very high strength steel [90 ksi steel as opposed to the more typical 36-65 ksi steel]. “This is not allowed by the structural design codes then and is still notallowed in current codes,” he stressed. “But the World Trade Center did not need to obtain a permit from City Hall. Because of special status as Port Authority of New York and New Jersey, they could make such choices outside the prevailing codes.”

This choice, he argued, allowed builders to use less steel in the columns [two to three times thinner than typical columns] presumably to save cost.

But by using high strength steel and thin cross sections, he pointed out, on impact the plane was able to cut through the outside steel bearing wall and enter the building--delivering thousands of gallons of jet fuel to the interior. During the ensuing fire, he said, the thin outside columns of the steel bearing walls were quite vulnerable to the rapid rise of temperature in them and reduction of their strength as a result of rising temperature of the steel.

“When the fires started, they heated up the steel. In my opinion, the truss joists collapsed first, leaving the exterior columns of probably two floors in the impact area with no bracing but still under gravity load from the floors above. As the columns heated up and reached temperatures of nearly 1,000F, their strength was reduced to less than half the design strength and they started to buckle. When the columns buckled, the top portion of the building, losing its supports, was pulled down by gravity and dropping on the floors below, pancaking the floors one after another and leading to progressive collapse in an almost perfect vertical direction of the pull of gravity force.”

In a 2001 article Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, published in the Journal of Metal, MIT Material Engineering Professor Thomas Eagar and Graduate Student Christopher Musso concluded that the failure of the steel resulted from loss of strength due to the temperature of the fire and the loss of structural integrity due to distortion of the steel from non-uniform temperatures in the fire. They did not comment on the type of steel used in the design.

In that paper, they concluded that the World Trade Center was not defectively designed, and that “no designer of the WTC anticipated, nor should have anticipated, a 90,000 L Molotov cocktail on one of the building floors.”

Astaneh-Asl says that the reason for undertaking his studies is not to implicate the designers, but rather to look into the design and answer the basic question that has bothered him since September 11: “Why did these towers collapse so quickly and so completely while other steel structures, including skyscrapers, under intense fire for hours, have not failed?”

He says that he feels he is closing in on the answer. “These structures were so unique that their collapse does not represent the performance expected of any other existing steel high-rise structure subjected to the same scenario,” he says.

Related links:

Berkeley Researcher Believes Fire Led to Collapse of Towers

Why Did the World Trade Center Collapse? Science, Engineering, and Speculation

Skyscraper Safety Campaign Website

Skyscraper Safety Campaign

9/11 probers say updated fire standards for skyscrapers could save lives
NEWSDAY - Friday, June 24, 2005

Associated Press Writer

NEW YORK -- Updated building standards requiring fire-protected elevators and widely spaced stairwells, among other features, could save lives and would add only marginally to the cost of new skycrapers, investigators who examined the World Trade Center collapse said.

The lead investigator with the National Institute of Standards and Technology said Thursday that reports that the recommendations might add 2 to 5 percent to the cost of a building were not unreasonable.

"It would differ depending on the location, depending on the city ... but certainly for the vast bulk of buildings that are throughout the United States I would say the costs would be modest," said Shyam Sunder.

NIST, an arm of the U.S. Department of Commerce, does not have the authority to institute the changes but hopes to persuade local authorities to change their building codes.

NIST released a draft of its findings at a news conference in lower Manhattan and will host a conference Sept. 13-15 after a period of public comment. The conference at NIST's headquarters in Gaithersburg, Md. is intended to encourage implementation of its recommendations.

Those recommendations include installation of structurally hardened elevators designed to function in a fire and stairwells situated apart from each other so that if one is damaged another might still work.

"In general it's good practice to have them remote, not clustered," Sunder said.

NIST has determined that the death toll of 2,749 at the World Trade Center would have been much higher _ perhaps as much as 14,000 _ if the twin towers had been struck later in the day at full occupancy.

But if the buildings' elevators had been better protected, many of them would have remained functional after the attacks, Sunder said. Those elevators could have helped more people escape the building before the collapse, or deliver firefighters quickly to the inferno and perhaps rescue those trapped above.

The three-year probe has gathered data on everything from fire tests on steel to office worker behavior in evacuating, to create an exhaustive sequence of exactly how the towers fell.

While many of the recommendations would apply to new construction, Sunder urged managers of older high-rises to consider whether the recommendations for new codes and practices would make their buildings safer too.

"Building owners and public officials should look at these recommendations in light of the inventory of existing buildings and take steps to mitigate any unwarranted risks," he said.

But Cincinnati-based architect David Collins, who acted as an adviser to the NIST investigation, said building owners will resist the recommendations for changes to older buildings.

"I don't think it's likely to happen in the vast majority of them," Collins said. "I think the vast majority of owners are hard-pressed to even consider some retrofit activities that have already been suggested many times over."

Brian Meacham, a principal with Arup, an engineering consulting firm, said the report contains useful recommendations that should be weighed against other design considerations.

"Don't take an extreme terrorist event and raise the bar so high for buildings that it restricts the uses that we want," he said.

Patricia Lancaster, buildings commissioner for New York City, said the report would help planners improve the safety of high-rise buildings.

"My staff and I look forward to thoroughly examining the NIST findings and considering how to integrate the best practices into the new building code for New York," she said.

Sally Regenhard, chairwoman of the Skyscraper Safety Campaign and the mother of a firefighter who died at the trade center, said the report was not specific enough about the communications failures that plagued the Fire Department on Sept. 11, 2001.

"It's indisputable that the majority of the 343 firefighters perished because their radios did not work in the buildings," Regenhard said. "They could not escape. They did not know what they were getting into."

How the World Trade Center fell

By BBC News Online's Sheila Barter

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

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

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

The steel and concrete structures 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 cores 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.

Nothing is designed or will be designed to withstand that fire

World Trade Center construction manager
"It was the fire that killed the buildings. There's nothing on earth that could survive those temperatures with that amount of fuel burning," said structural engineer Chris Wise.

"The columns would have melted, the floors would have melted and eventually they would have collapsed one on top of each other."

The buildings' construction manager, Hyman Brown, agreed that nothing could have saved them from the inferno.

"The buildings would have stood had a plane or a force caused by a plane smashed into it," he said.

I would have given the order to get out - you would have thought someone with technical expertise would have been advising them

Professor John Knapton, Newcastle University
"But steel melts, and 24,000 gallons (91,000 litres) of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."

Once the steel frame on one floor had melted, it collapsed downwards, inflicting massive forces on the already-weakened floor below.

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.

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

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 was like a horror film and I think people's rationale had gone

Professor John Knapton
"It sounds harsh - this had never happened in the world, so you can hardly criticise them.

"But I would have given the order to get out. You would have thought someone with technical expertise would have been advising them."

But he acknowledged that the sheer scale of the tragedy probably overwhelmed the operation commanders.

"I think everyone was not thinking. It was like a horror film and I think people's rationale had gone," he said.

Steel-core design

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.

Masoud Sanayei,professor of civil and environmental engineering

A 'bland' skyscraper became the symbol of a city

On a clear, beautiful day a few years ago, Masoud Sanayei, professor of civil and environmental engineering, decided to visit the World Trade Center and see for himself the two towers that dominated the New York City skyline. He took the nearly one-minute elevator ride up 110 stories, enjoyed the breathtaking view and had nothing but admiration for the architects, civil engineers and construction workers who had made the buildings possible.

"As a structural engineer, I wanted to see these buildings," he remembers. "I was so impressed with the people who designed, financed and constructed the towers."

The lower Manhattan skyline © Donovan Reese/PhotoDisc

Sanayei took a stroll on the rooftop. He felt so relaxed that he sat down on a bench in the viewing area and took a nap.

On September 11, another clear and beautiful day, Sanayei, along with the rest of the world, watched in horror as the towers collapsed.

Sanayei and Daniel Abramson, associate professor of art history at Tufts, co-teach a course called "Skyscrapers" that is offered to both engineering and liberal arts students. They recently assessed the World Trade Center, both aesthetically and in terms of its engineering and design.

In his new book, Skyscraper Rivals (Princeton Architectural Press), Abramson explains that the Singer Tower, built in 1908, was demolished to make way for the World Trade Center, which was completed in 1974. The Singer Tower was 47 stories high and remains the tallest building ever deliberately brought down.

ÔFairly blank'
"Aesthetically, the World Trade Center was never thought of highly," said Abramson. "It was considered fairly bland, fairly blank and was not particularly interestingly articulated. The columns were very close together, and the windows about 30 inches wide. Because the windows were so narrow, we saw a blank faade."

Even the plaza at the base of the towers was dull, he said. Although architecturally uninteresting, the towers' fame rested on their height.

Briefly the tallest buildings in the world until the construction of the Sears Tower in Chicago and the Petronas Towers in Kuala Lumpur, the World Trade Center dwarfed everything else in lower Manhattan. Since their completion, nothing else built in New York City has come close to their height.

"New York is a great skyscraper city, and skyscrapers are the archetypal building type of the 20th century," said Abramson. "I'm dumbfounded to think that something so big could come down so quickly. One presumed they would never be torn down and that they would always be there. This would be as if the Pyramids were destroyed."

Heat brought them down
Sanayei said the buildings were strong enough to survive the impact of the airplanes but collapsed as a result of the ensuing fire, which may have burned with temperatures greater than 1000 degrees Fahrenheit.

The buildings were designed, he said, using a tube-in-tube structural arrangement, "creating a system that is very stiff and strong and able to resist lateral loads such as wind or an earthquake. The towers survived the initial impact. Each building rocked back and forth, and people would have felt the buildings move."

But, Sanayei said, the large amount of jet fuel delivered to the towers followed by explosions and subsequent fire weakened the floor systems and the columns. "I suspect the fire created temperatures higher than what is normally experienced in office building fires caused by burning furniture or rugs or paper," he said.

Structural steel melts at around 2000 degrees Fahrenheit but starts to soften at around 800 degrees. Somewhere between 1200 and 1500 degrees, it loses two-thirds of its strength, Sanayei said.

"The building still had to carry the massive loads of higher floors above the plane crash location. In the first tower, one third of the building was above the plane, and in the second, about half the floors were above the impact. The structural columns could no longer carry the gravity loads of the floors above the fire.

"In addition," Sanayei said, "the heat could have weakened the floor systems above, as well as the floor-to-column connections. This combination caused the top-down collapse, producing a domino effect. The exterior columns that formed the outside tube of the World Trade Center buildings guided the self-contained collapse within these buildings. It looked like a designed and time-delayed implosion that took only a few seconds to bring each tower down. It should have felt like an earthquake to the surrounding buildings. In such catastrophic events, it is hard for anyone to survive."

If there had been no fire, he said, potentially the buildings could have survived, although at a later date, they might have had to be brought down. Without the collapse, he noted, not as many lives would have been lost.

Lessons for the future
"There are lessons to be learned from this tragedy," Sanayei said. "Our structural and geotechnical engineers and architects who design these buildings are doing a fine job of designing safe, high-rise buildings, as is shown by the fact that both towers survived the initial impact.

But the challenge ahead, Sanayei said, is to make these buildings more fire-resistant, to develop better systems to extinguish fires, even one like this. "It is going to happen again someday, if not by aircraft crash than by something else," he said.

"We need to develop more reliable methods of evacuating tall buildings. We need redundant systems such that if one evacuation system fails, there are other effective ways of getting out. The airplane crash, the subsequent explosion and fire and the catastrophic collapse of both towers will drastically change the way investors, architects and engineers will construct our future skyscrapers. We need to rethink our cities."

Leslie E. Robertson

Reflections on the World Trade Center
Leslie E. Robertson
Volume 32, Number 1 - Spring 2002

The lead structural engineer reflects on the rise and fall of the World Trade Center towers.

The journey toward the design and construction of the World Trade Center began prior to 1960 when Minoru Yamasaki Associates was selected to design the Federal Science Pavilion, a key element of the Seattle World’s Fair; NBBJ was selected as the local architect. Having accomplished many structural designs for NBBJ, it was only natural that we would obtain the commission for the structural design of the Pavilion. That structural design, reflecting the very highest attainments of our profession, was creatively conceived and executed by John V. Christiansen (NAE). Indeed, the Pavilion stands today as an example of the importance of fine structural engineering as it influences the overall architectural process. The entrepreneurship and skills of another of our partners, John B. Skilling (NAE), were instrumental in the development of the close relationship between our firm and that of Minoru Yamasaki and Associates; many wonderful projects were to follow.

When Yamasaki was commissioned to design the World Trade Center in New York, he proposed that we be retained as structural engineers. Although his recommendation was influential, we were in competition with many New York firms that had more experience in high-rise design than we had. Although we worked hard preparing for our interview with the Port Authority of New York and New Jersey, we wouldn’t have obtained the commission without the presence and the skills of John Skilling.

Once we had been awarded the commission, I moved from Seattle to New York with a team of expert engineers—Wayne A. Brewer (drawing production and coordination), Paul S.A. Foster (towers), Ernest T. Liu (plaza buildings and below-grade structures), Jostein Ness (detailing), Richard E. Taylor (computers), and E. James White (construction technology). Professor Alan G. Davenport (NAE), on sabbatical from the University of Western Ontario, joined us to head the wind-engineering research group. Although I was the titular leader, the energies and talents of the entire team led to our successes.

A list of the innovations incorporated into the World Trade Center would be very long. In the following pages, I describe just a few of the ideas and innovations conceived and developed by our team. Most, if not all, of this technology is now a part of the standard vocabulary of structural engineers.

The tubular framing system for the perimeter walls resisted all of the lateral forces imposed by wind and earthquake, as well as the impact loads imposed on September 11. Although we had used closely spaced columns in an earlier building, it was Minoru Yamasaki who proposed that we use narrow windows in the WTC towers to give people a sense of security as they looked down from on high. Our contribution was to make the closely spaced columns the fundamental lateral-force-resisting system for the two towers. The tubular framing system also precluded the need for the customary 30-foot column spacing in interior areas, making column-free, rentable space structurally desirable.

In support of Yamasaki’s design, during the construction, before the windows were installed, I noticed that people felt comfortable walking up to the outside wall, placing their hands on the columns to either side, and enjoying the wonderful view. If the wind was blowing toward them, they would walk right up to the outside wall; however, if they felt even a trace of pressure from a breeze from behind, they would at least hesitate before walking to within five feet of the wall . . . and many would not approach the wall at all.

Another structural innovation was the outrigger space frame, which structurally linked the outside wall to the services core. This system performed several functions. First, gravity-induced vertical deformations between the columns of the services core and the columns of the outside wall were made equal at the top of the building; at other levels, the differential deformations were ameliorated. Second, wind-induced overturning moments were resisted in part by the columns of the services core, thus providing additional lateral stiffness. Finally, the weight of, and the wind-induced overturning moment from the rooftop antenna (440 feet tall) was distributed to all columns in the building . . . adding additional redundancy and toughness to the design.

Prefabricated structural steel was used to an unprecedented degree. Two examples will give you an idea. Exterior wall panels three stories high and three columns wide were fabricated in Washington state. Floor panels 60 feet long and 20 feet wide, complete with profiled metal deck and electrical distribution cells, were assembled in New Jersey from components fabricated in Missouri and elsewhere.

We mounted a comprehensive program to determine the design-level gradient wind speed for New York City. Data were collected from all available sources and incorporated into an appropriate mathematical model. For the first time, we were able to obtain full-scale measurements of the turbulent structure of the wind and compare them with the turbulent structure obtained in a boundary-layer wind tunnel. This was done by mounting anemometers atop three high points in lower Manhattan and by making similar measurements on our wind tunnel model (Figure 1). The boundary-layer wind tunnel was further developed and used to predict the steady-state and dynamic forces on the structure and the glazing, as well as to develop the dynamic component of wind-induced motion of the structure. Jensen and Frank, two brilliant Danish engineers, had discovered that surface roughness in the wind tunnel allowed them to accurately predict wind pressures on farm structures. We expanded this technology upward to 110 stories by using a wind tunnel, constructed under the guidance of Dr. Jack E. Cermak, (NAE, Colorado State University), designed to study the dispersion of gases emitted from tall stacks. Thus, for the first time, we were able to analyze the steady-state and dynamic components of wind-induced structure deflections.

We designed motion simulators to determine acceptable levels of wind-induced structure motion. The simulators measured the response of human subjects to lateral motions similar to those anticipated for the two towers. The accumulated data were used to establish the criteria for an acceptable level of the swaying motion of the two towers.

A viscoelastic damping system was invented and patented to ameliorate the wind-induced dynamic component of building motion by dissipating much of the energy of that motion . . . acting more or less like shock absorbers in an automobile. With these dampers, we could control the swaying motion without having to use large quantities of structural steel. This was the first time engineered dampers were used to resist the wind-induced swaying motion of a building.

A theory was developed for integrating the statistical strength of glass with the dynamic forces of the wind to predict the breakage rate of the glass of the exterior wall. Coupled with a testing program of actual glass samples, we were able to determine rationally the necessary thickness and grade of the glass. Another theory was developed to predict stack action and temperature-induced and wind-induced airflow within a high-rise building; an understanding of these airflows is crucial to controlling fire-generated smoke and reducing the energy consumption of the building. A theory to predict appropriate “parking floors” for elevators was developed to minimize the oscillation of elevator cables, which oscillation is stimulated by the wind-induced, swaying motion of a building. Figure 2 is a comparison of the wind-induced dynamic components of the structure response of the two towers and of the Empire State Building.

The two towers were the first structures outside of the military and nuclear industries designed to resist the impact of a jet airliner, the Boeing 707. It was assumed that the jetliner would be lost in the fog, seeking to land at JFK or at Newark. To the best of our knowledge, little was known about the effects of a fire from such an aircraft, and no designs were prepared for that circumstance. Indeed, at that time, no fireproofing systems were available to control the effects of such fires.

We developed the concept of and made use of the fire-rated shaft-wall partition system, which is now widely used in place of masonry and plaster walls. At that time, masonry was the standard enclosure for elevators, stairs, duct shafts, and other internal structures. The partition system eliminates the need for within-the-shaft scaffolding, which was the common practice, provides more smoke-proof stairs and shafts, and improves safety on the job site. The shaft-wall completely changed the nature of the structural system for the two towers, making them the first of a new kind of high-rise building.

A computerized system was conceived and developed for ordering structural steel and producing shop drawings for structural steel, as well as the operation of digitally directed tools, all directly from digital information developed as a part of our design.

When the two towers were finished, the World Trade Center stood proud, strong, and tall. Indeed, with little effort, the towers shrugged off the efforts of terrorist bombers in 1993 to bring them down. The events of September 11, however, are not well understood by me . . . and perhaps cannot really be understood by anyone. So I will simply state matters of fact:

The events of September 11 ended the lives of almost 2,900 people, many of them snuffed out by the collapse of structures designed by me. The damage created by the impact of the aircraft was followed by raging fires, which were enormously enhanced by the fuel aboard the aircraft. The temperatures above the impact zones must have been unimaginable; none of us will ever forget the sight of those who took destiny into their own hands by leaping into space.

It appears that about 25,000 people safely exited the buildings, almost all of them from below the impact floors; almost everyone above the impact floors perished, either from the impact and fire or from the subsequent collapse. The structures of the buildings were heroic in some ways but less so in others. The buildings survived the impact of the Boeing 767 aircraft, an impact very much greater than had been contemplated in our design (a slow-flying Boeing 707 lost in the fog and seeking a landing field). Therefore, the robustness of the towers was exemplary. At the same time, the fires raging in the inner reaches of the buildings undermined their strength. In time, the unimaginable happened . . . wounded by the impact of the aircraft and bleeding from the fires, both of the towers of the World Trade Center collapsed.

Figure 3 shows the comparative energy of impact for the Mitchell bomber that hit the Empire State Building during World War II, a 707, and a 767. The energy contained in the fuel is shown in Figure 4. Considerations of larger aircraft are shown in Figures 5 and 6. The physical sizes of these aircraft are compared with the size of the floor plate of one of the towers in Figure 7. These charts demonstrate conclusively that we should not and cannot design buildings and structures to resist the impact of these aircraft. Instead, we must concentrate our efforts on keeping aircraft away from our tall buildings, sports stadiums, symbolic buildings, atomic plants, and other potential targets.

The extent of damage to the World Trade Center is almost beyond comprehension. Figure 8 shows an overview of the site and the location of the various buildings. We did not design the superstructures of Building 3 (Marriott Hotel) or of Building 7. Towers 1 and 2, which were totally destroyed, left behind utter chaos surrounded by towers of naked structural steel. The remaining steel towers were in some ways painful beyond belief, in other ways strangely beautiful. Building 3 collapsed down to a structural transfer level designed by us. Fortunately, the people who sought refuge in the lobby of the hotel, which was located immediately below the transfer level, survived. Buildings 4, 5, and 6 remained standing but were partially collapsed by falling debris; all three burned for about 24 hours. Although there was nothing special about the structural design of these buildings, the remaining structures stalwartly resisted the impacts of the wrecking ball. Building 7, after burning for nearly 10 hours, collapsed down to a structural transfer level designed by us. The below-grade areas under Towers 1 and 2 were almost totally collapsed; in areas outside of the towers they were partially damaged or collapsed.

In my mind, the loss of life and the loss of the buildings are somehow separated. Thoughts of the thousands who lost their lives as my structures crashed down upon them come to me at night, rousing me from sleep, and interrupting my thoughts at unexpected times throughout the day. Those who were trapped above the impact floors, those who endured the intense heat only to be crushed by falling structure, are merged with those who chose to take control of their own destinies by leaping from the towers.

The loss of the buildings is more abstract. The buildings represented about 10 years of concerted effort both in design and in construction on the part of talented men and women from many disciplines. It just isn’t possible for me to take the posture that the towers were only buildings . . . that these material things are not worthy of grieving.

It would be good to conclude this journey in a positive mode. We have received almost a thousand letters, e-pistles, and telephone calls in support of our designs. The poignant letters from those who survived the event and from the families of those who both did and did not survive cannot help but bring tears to one’s eyes. They have taught me how little I know of my own skills and how fragile are the emotions that lie within me. Yes, I can laugh, I can compose a little story . . . but I cannot escape.

Do those communications help? In some ways they do; in others, they are constant reminders of my own limitations. In essence, the overly laudatory comments only heighten my sense that, if I were as farseeing and talented as the letters would have me be, the buildings would surely have been even more stalwart, would have stood even longer . . . would have allowed even more people to escape.

Yes, no doubt I could have made the towers braver, more stalwart. Indeed, the power to do so rested almost solely with me. The fine line between needless conservatism and appropriate increases in structural integrity can only be defined after careful thought and consideration of all of the alternatives. But these decisions are made in the heat of battle and in the quiet of one’s dreams. Perhaps, if there had been more time for the dreaming . . .

Recognition must be given to the Port Authority of New York and New Jersey, who provided unparalleled support and guidance throughout the design and construction of the World Trade Center. Their understanding of the need to explore new avenues and break new ground reflected their sound professional and technical posture. We could not have asked for a more competent, more responsible, or more involved client. The men and women of our company who participated in the design and construction are without parallel. Their talents, energies, and good humor carried us through a most arduous journey. Dr. Alan G. Davenport (NAE) provided invaluable knowledge, insight, and support; his willingness to join us on this journey made many facets of the design possible. Minoru Yamasaki and his team, particularly Aaron Schreier, and the office of Emery Roth and Sons produced a wonderful architecture while making the entire process both fun and exciting. Richard T. Baum (NAE), of Jaros, Baum & Bolles, headed the HVAC (heating, ventilation, air conditioning) team and taught me much about these systems. Joseph R. Loring provided full professional services as the electrical engineer for the project.

In conclusion, the events of September 11 have profoundly affected the lives of countless millions of people. To the extent that the structural design of the World Trade Center contributed to the loss of life, the responsibility must surely rest with me. At the same time, the fact that the structures stood long enough for tens of thousands to escape is a tribute to the many talented men and women who spent endless hours toiling over the design and construction of the project . . . making us very proud of our profession. Surely, we have all learned the most important lesson—that the sanctity of human life rises far above all other values.

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