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The Days After, The Days Ahead

Civil engineers assumed prominent roles in the aftermath of the terrorist attacks on the World Trade Center and the Pentagon and are leading the efforts to evaluate not only the performance of the structures involved in the two assaults but also the vulnerability of the nation's infrastructure to future attacks. Perhaps most important of all, they are helping to write the opening pages of a new chapter in American history.

By Anne Elizabeth Powell

he first 767 plowed into the north tower at 8:48 a.m., the second banked into the south tower at 9:03 a.m.: two commercial airliners commandeered into the twin towers of New York's World Trade Center by hijackers—their mission: the towers' destruction. American Airlines flight 11, en route from Boston to Los Angeles with 92 people aboard, struck the north tower; United Airlines flight 175, en route from Boston to Los Angeles with 65 people aboard, struck the south tower. Both planes exploded on impact, and the 24,000 gal (90,840 L) of fuel each was carrying stoked the infernos ignited by the crashes.

It was a ghastly event, now seared into the collective consciousness of the millions of people across the globe who witnessed it: a terrorist attack on two highly celebrated and densely populated cathedrals of American achievement. It was unthinkable. Unconscionable. An abomination of staggering proportions that stunned the civilized world.

And then the unimaginable happened: At 9:59 a.m. the south tower collapsed, and at 10:28 a.m. the north tower followed suit—dissolving in a cataclysm that would ultimately engulf as many as 5,000 human lives.

The atrocity unfolding on the outskirts of the nation's capital was every bit as horrifying: At 9:40 a.m. the Pentagon, too, was attacked by terrorists using a hijacked commercial jet as their weapon, and a fourth hijacked airliner was reportedly heading for Washington, its target unknown. American Airlines flight 77, a Boeing 757 en route from Washington's Dulles Airport to Los Angeles with 64 people aboard, slammed into Wedge 1 on the west face of the Pentagon, exploding on a diagonal path that penetrated Wedge 2. Portions of Wedge 1 were destroyed on impact and the remainder of this segment suffered heavy damage; Wedge 2 suffered significant fire and water damage. But the Pentagon survived the attack, and the lives lost numbered fewer than 200. That the Pentagon sustained the hit and ensuing fires with far less damage and loss of life than the terrorists might have anticipated was due in large part to the facts that renovation of Wedge 1—an undertaking that substantially fortified that segment—was within days of completion, and that Wedge 2, about to undergo renovation, had been vacated.

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Photo credit: Tech. Sgt. Cedric H. Rudisill/DoD/Getty Images

The fourth hijacked plane never made it to Washington. United Airlines flight 93, a Boeing 757 en route from Newark, New Jersey, to San Francisco with 45 people aboard, crashed in Shanksville, Pennsylvania, at 10:37 a.m. after passengers challenged the hijackers for control of the plane. All on board were killed.

Civil engineers were among the first on the scene at both disasters and have remained on the scene around the clock ever since. Initially they provided indispensable assistance in the rescue and recovery efforts. Now they are studying the disasters to determine what lessons can be learned from the destruction at both sites. Foremost among these engineers are members of the two building performance study teams assembled by ASCE to assess the collapses at the World Trade Center and the damage inflicted on the Pentagon. On October 9 ASCE established a critical infrastructure response initiative to formulate strategies and guidelines for assessing the vulnerability of the nation's infrastructure to future attacks.

The horrors of September 11 beggar our notions of evil, and the aftermath of these attacks is nearly as distressing as were the events themselves. The human toll exacted is still being tallied, and in both physical and emotional terms that tally is enormous. The recovery and cleanup processes are overwhelming in scope: So total was the destruction of the World Trade Center that it would no longer qualify as a place if not for the numbing reality that it is hallowed ground, a mass grave for the thousands of people funneled into the abyss.

In the wake of September 11 we have coined a term that encapsulates the events of that day, the horrors of that day, the losses, sorrow, anguish, fear, and rage of that day—the unspeakable hideousness of it all. That term is "September eleventh." September eleventh assaulted the American psyche in ways it had never before been assaulted. Yet the American spirit survived September eleventh fully intact and has held strong in the days since.

Among those who have led and continue to lead America beyond September eleventh are civil engineers. As this special report explains, they have worked both disaster sites day and night since September eleventh—first to gauge the stability of structures for rescue workers and then to evaluate the performance of structures during the disasters so that any lessons learned from the behavior of the structures can be used to advantage in future design and construction. Next they will consider the broader issue of the future of our infrastructure, addressing the problem of our deteriorating infrastructure and assessing the vulnerability of this infrastructure—and that of any future infrastructure—to both natural and man-made disasters.

This report, however, provides only an overview of their efforts. No article confined to a dozen or so pages could begin to articulate the full measure of their contributions. But it does make clear that they are indeed conspicuous among those writing the first pages in a new chapter in our history—the chapter that opened with September eleventh.

eptember 11, 2001, was the 60th anniversary of the commencement of the Pentagon's construction, and a number of those involved in the Pentagon's renovation program, which was well under way on that day, made a mental note that morning of the significance of the date. The thought crossed more than one mind that the original builders would have been pleased with the work now in progress. The Pentagon renovation program constitutes one of the most ambitious, complex, and challenging construction undertakings in contemporary history and, once complete, will make the headquarters of the United States military structurally sounder and more secure. (See "The Pentagon Project," Civil Engineering, June 2001.) All involved in the effort were proud that it was approaching its first major landmark: The renovation of Wedge 1—the first of the five structural wedges to be renovated—was just five days short of completion and most of its occupants had moved back in; Wedge 2 had been vacated in preparation for the renovation work soon to begin on that segment.

Each of the Pentagon's five wedges will undergo renovation as part of a $1.2-billion, 20-year-long undertaking. The plane struck Wedge 1 on a trajectory that also penetrated Wedge 2 (inset). However, because the renovation of Wedge 1 was just five days short of completion, the destruction and loss of life were far less than was the case at the World Trade Center site.

And then the plane hit.

The Pentagon trembled from the impact, just as the World Trade Center towers had trembled from the impact of each plane. But this strike would not inflict the scope of damage or loss of life sustained in New York. The World Trade Center towers rose 110 stories, and each tower had been struck at a highly vulnerable point—a point at which the impacted area might not easily have supported the weight above it, particularly following the structural weakening that most likely resulted from fire. The Pentagon, in contrast, is an immense, squat, five-story concrete structure that was struck in a segment that had just been reinforced. Still, the strike demolished a significant portion of Wedge 1, and the smoke produced by the fires shooting through the corridors of Wedge 1 and Wedge 2 made evacuation extremely difficult and perilous in some areas.

Lester M. Hunkele, a vice president of DMJMH&N and the program manager for the joint venture partners supporting the Pentagon renovation (PENREN) program—Los Angeles-based DMJM and Houston-based 3D/I—was at work on-site when the plane struck. "Immediately following the blast, we were told to go home," he recalls, "but what emergency personnel didn't realize at first were the critical roles we could play in the recovery effort. PENREN had contracting authority, access to all the necessary heavy equipment, and construction, architectural, and engineering expertise right there on the scene. Once we explained this, we got the go-ahead to set up an operations center immediately to support the FBI, the fire department—all of the rescue personnel.

"We tried to anticipate problems as fast as they were unfolding. For instance, we knew the work would continue nonstop for many days to come, which meant that we needed to get everything ready to run a night site—generators and lights—so that rescue efforts could continue uninterrupted. We hit the phones, and assembled everything that would be needed. Our modus operandi was, What do we think they're going to ask for next, and how can we get it here before they need it? The FBI agent in charge of on-site logistics subsequently told me that we turned what would have been a six-week effort into a two-week effort."

"Our on-site information resource center proved invaluable," says Stacie Condrell, the PENREN's planning and tenant relations group leader. "From the outset it was perilous for the FBI, the firefighters, the emergency rescue people—few of them knew the building they were entering, and these areas were dark and shrouded in smoke. The rescue teams needed to understand the interior layout in order to search the devastated areas as quickly and thoroughly as possible. We provided architectural plans to the operations center within two hours of the attack, and an off-site printing shop worked with us around the clock to make sure that plans were available whenever they were needed.

"Formal, informal, whatever the route—once a need was articulated, we figured out a way to meet that need. For example, a lot of our staff knew people who were missing—they had worked with them in designing their spaces, they had moved them in. That afternoon and evening they went through all of the floor plans, writing every name on every desk on the plans so that various agencies had as much information as possible in trying to develop the lists of the missing, the survivors, and the victims. It was a very difficult process, but they knew it could mean the difference between life and death in many cases."


World Trade Center

This study is being organized and led by ASCE and its Structural Engineering Institute, with participation by a coalition of engineering societies. The organizations participating thus far are as follows:

American Institute of Steel
Construction, Inc. (AISC)
American Concrete Institute (ACI)
Council on Tall Buildings and Urban Habitat (CTBUH)
International Code Council (ICC)
National Fire Protection
Association (NFPA)
Society of Fire Protection

Engineers (SFPE)
Structural Engineers Association of New York (SEAONY)
National Council of Structural
Engineering Associations (NCSEA)

Team Members

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

William Baker, P.E., S.E.
Partner, Skidmore Owings & Merrill LLP
Expert in tall-building design

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 Association
Expert 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. Seattle
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
University of Maryland
Expert in fire resistance analysis

James A. Rossberg, P.E.
Director, Structural Engineering Institute
ASCE 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.


Paul F. Mlakar, Ph.D., P.E., Lead
U.S. Army Corps of Engineers
Engineering Research and Development
Vicksburg, Mississippi
Expert in blast-resistant design; investigator, Murrah Federal Office Building study

Mete Sozen, Ph.D., P.E.
Professor, Department of Civil Engineering
Purdue University Lafayette, Indiana
Expert in concrete and concrete structures

Donald Dusenberry, P.E.
Principal, Simpson Bumpert & Heger
Arlington, Massachusetts
Expert in blast effects and structural design

James R. Harris, Ph.D., P.E.
Owner, J.R. Harris & Company
Expert in structural engineering and
concrete design and construction

Gerald Haynes, P.E.
Fire Protection Engineer
Arson and Explosives Division
Bureau of Alcohol, Tobacco, and Firearms Washington, D.C.
Expert in fire protection

Long Phan, Ph.D., P.E.
Research Structural Engineer
Building and Fire Research Lab
National Institute of Standards and
Gaithersburg, Maryland
Expert in concrete structural and fire engineering

The PENREN's contracting capabilities also facilitated the early cleanup efforts. Explains Ed Pickens, the senior construction scheduler: "On that first day we discussed with the FBI where to place the dumpsters needed to cart away debris. We discovered, however, that the dumpsters had to be brand new because the debris was evidence and could not be contaminated in any way. So we had to deliver numerous new dumpsters to the site immediately. And then I informed the FBI that we were going to have to build a road for the trucks carrying the debris because the ground around the heliport—the area closest to the blast—was too soft.

"The FBI authorized construction of the road, and I called a contractor, who got the gravel, and we got things moving. We discovered, however, that the delivery trucks were moving too slowly because of security checks, so we set up a system whereby FBI agents rode the trucks to expedite delivery. We had that road built in two hours. And that's how this effort progressed."

The World Trade Center site was far more complex and dangerous, however—rescuers had to cope with 1.2 million tons (1.09 million Mg) of tangled debris in which, wrenchingly, perhaps thousands of people lay either trapped or dead, and they had to determine the structural stability or instability of surrounding buildings. Within just a day of the collapse the city of New York hired LZA Associates and Thornton-Tomasetti Engineers—both divisions of the Thornton-Tomasetti Group, Inc., of New York City—to assemble a team of engineers and contractors to inspect the World Trade Center and buildings surrounding the site to help ensure the safety of rescue workers.

George J. Tamaro was among the first team members on the site. A partner in the firm of Mueser Rutledge Consulting Engineers, of New York City, and a former staff engineer for the Port Authority of New York and New Jersey who helped construct the World Trade Center's foundation, he began work at the site on September 12. "We started off in an emergency mode," he recalls, "advising the fire department, police department, and all of the city agencies on what we knew about the site—where the dangerous conditions existed, where there were platforms, where you could put cranes, where you couldn't put cranes, and what risks there were related to the slurry wall if indiscriminate excavation were to take place from within the perimeter of the slurry wall. It was kind of an emergency response in an advisory capacity to keep city officials and all of the uniformed personnel informed on what they should be concerned about.

"From there we progressed into a discovery mode. Our people would accompany the rescue people into a building, into the basements, and begin to explore and determine the conditions of the basements so that we would be able to better understand where we needed to restore tiebacks for the lateral support of the slurry wall and where it was necessary and where it was not necessary because we had sufficient floor framing in place. Concurrent with that, we were working with the New York City Transit Authority in shoring up and investigating the conditions of the subways to determine which lines were safe—and could go back into operation—and which lines required immediate shoring to prevent further collapse. We've got several things going on concurrently, and we're also working with the Port Authority on the plugging of the path tubes in New Jersey.

"We have gotten as far as we can in plotting the conditions of the existing structure. And we're now preparing drawings and specifications for tiebacks and the next phase, which would be the excavation from within the bathtub. We've moved from an emergency mode to a planning mode to a design mode. And the design mode will involve the design of a very difficult excavation because the interior is filled with massive pieces of steel in random configurations."

By September 13, Eugenia Roman, a young structural engineer with Hardesty & Hanover in Hoboken, New Jersey, who is the president of ASCE's North Jersey Branch, was being inundated with e-mails from fellow ASCE members in search of some "place" where they could offer assistance to those working at the World Trade Center site. After seeking advice from V. Richard Mariani, the president of ASCE's New Jersey Section, on how to marshal this interest into some form of effective response, Roman sent an e-mail to everyone on the New Jersey membership distribution list. She suggested that anyone interested in providing assistance to those working at the World Trade Center site immediately inform her via e-mail so that she could form

WTC Site Photograph
The manipulated photograph of the World Trade Center site produced by Eugenia Roman's team of volunteers was created expressly for use in the rescue efforts.

ulate a list of contacts. "Again, I was immediately immersed in a sea of e-mail responses from the general membership asking to be put on the list," she says. "In all, one hundred twenty-five engineers—both members and nonmembers of ASCE—responded. I was deeply moved by the outpouring of help and support."

On September 14, as the list was being compiled, Roman received a phone call from Bryan Juncosa, a structural engineer with Atlantic Engineering in Kinnelon, New Jersey, and a member of the New Jersey Task Force 1 Urban Search and Rescue Team, which operates under the auspices of the New Jersey State Police Office of Emergency Management and is certified by the Federal Emergency Management Agency (FEMA). Juncosa needed help. Numerous FEMA teams were mobilized to assist in the rescue and recovery efforts, but Juncosa's was the first on the scene. (There are only 2 engineers on each 62-member team.)

"I informed him that we could help—that people from all aspects of engineering had contacted me, offering their services and their companies' resources in any effort," says Roman. Juncosa asked Roman to organize a meeting at her Hoboken office the next day, and on short notice she managed to assemble a handful of people to meet with Juncosa. He explained at the meeting that the most pressing need of his task force was to somehow superimpose onto aerial photographs just taken of the site the outlines of structures, sublevels, air shafts, elevators, utility lines, et cetera—anything that might prove helpful to rescuers searching through the rubble for victims. Because of the extraordinary scope of the rubble, rescue workers had no idea where, specifically, the buildings had been located before the attacks and therefore had no way of establishing where victims might be located. Juncosa believed that with the manipulated photographs rescue workers would be able to gain some degree of orientation—understand which components of structure and infrastructure had once stood on a particular location—and thus determine where some of the victims might lie.

"The reason we needed this," says Juncosa, "was because when we're searching and we find a void—which might have a two-foot opening and go down sixty feet—we want to know if that's to a five-hundred-car parking garage with possible victims or if it's to an air shaft.

"We were the first team there, and because there are only two engineers on each team, we were in short supply. I was up twenty-four, thirty hours straight. I had to do all this mapping and engineering, and then I would be on the pile and be called all over the place. We needed engineering help.

"The reason we needed the manipulated photo was that this is a fourteen-acre site and we need to sort things out—find things. So we superimposed the grid—which is a very wide grid with four-hundred-foot quadrants that could be narrowed down to fifty feet. The idea was that if you found a victim, you could tell where they came from."

Physical Damage Assessment
Pentagon Renovation Program

Those who attended the meeting in Roman's office swung into action. Catherine Britell, a structural engineer with CUH2A, Inc., in Princeton, New Jersey, had worked as a structural site engineer with New York City-based Leslie E. Robertson Associates (LERA) at the World Trade Center in 1966 and 1967. She was able to obtain copies of architectural plans from lera for the plaza and subgrade areas of the World Trade Center complex. lera engineers helped Britell explain the layout of the subgrade spaces to rescue workers. Richard Cassin, a senior structural engineer with dmjm+Harris, in New York City, had experience with and access to utility plans in the area of the World Trade Center. (Some of the underground utility information was found in plans prepared by Goodkind & O'Dea, based in Rutherford, New Jersey, for the New York State Department of Transportation for segment 2 of the Route 9A Reconstruction Project.) Anthony Valente, a project manager for Houston-based A.I. and Associates, was extremely adept at manipulating several of the computer programs with which the needed images were produced. Peter Clayton, a senior computer-aided design and development (CADD) designer with DMJM + Harris, converted the available underground utility information into a cadd overlay that was superimposed on the aerial photograph. With the support of Roman's supervisor, Joseph Solis, a project manager for Hardesty & Hanover, and help from Harry Seepaul, an engineering technician for Hardesty & Hanover, the group was able to produce an image that incorporated the aerial photograph and a scaled schematic of the buildings overlaid with a grid, which delineated quadrants of the disaster site. The image was delivered at 2 a.m. on September 16.

Assignments continued to be called in after the rescue team saw what "The Engineers" could produce. As a consequence, several more people joined the group. For example, Todd McNamara, a structural engineer and assistant project manager with M.D. Carlisle Construction Company, in New York City, and Jaimin Amin, a structural engineer with Arora Associates, in Lawrenceville, New Jersey, were instrumental in reading the intricate building plans to determine access locations to the sublevels below the World Trade Center complex—elevator shafts and stairwells, for instance. Jeffrey Case, also a structural engineer with Arora Associates, John Rossi, the director of surveying for Arora, and Seepaul manipulated scanned as-built prints to produce cadd drawings that indicated the limits of the slurry wall around the World Trade Center complex. They also produced cross sections of 5 World Trade Center and floor plans of each of the sublevels (B1 to B6), denoting the critical access locations. Aleksandr Babin, a cadd designer with Barbara Thayer P.C., in New York City, also assisted in manipulating and modifying the schematics in order to produce AutoCad drawings.

Click for larger mage

"Our work proved to be critical to the rescue efforts at the World Trade Center," says Roman. "When we actually had the opportunity to meet some of the firemen and rescuers—some of whom were seriously injured in the rescue efforts—they came to shake my hand to thank us for the work we did. They said we saved their lives. They said we kept them safe. They said we guided them through the darkness. Members of rescue teams from all over the country came to see what New Jersey Task Force 1 had to offer, and they were all amazed. The effort that we were able to muster and the work we were able to produce was unmatched by any other agency involved in the effort.

"I am at a very early point in my career, and I cannot believe that I was able to facilitate something so monumental. I say that not out of conceit, but out of amazement. This experience has left an indelible mark on my life. I have seen Ground Zero firsthand—I have seen the destruction and terror that most people have seen only through the lens of a camera. I have seen rescue workers appear as small as grains of sand amidst the twisted steel and concrete that is piled stories high. I have seen hundreds of construction workers lined up, ready to lift the steel and rubble away with their bare hands, hoping to find a survivor. The efforts we put forth on behalf of the rescuers made us feel like we were able to help. And that has meant the world to us."

In fact hundreds of engineers volunteered to assist in the rescue and recovery efforts. The Structural Engineers Association of New York (SEAONY) coordinated much of the volunteer effort, initially assembling five teams of engineers from more than 20 New York-based firms. The teams worked with the New York City Office of Emergency Management, the New York City Department of Design and Construction, and the New York City Building Department in eight-hour shifts around the clock to help assess the integrity and stability of buildings just beyond the area of the collapses—an area bounded by Chambers Street to the north, Rector and Wall streets to the south, the Hudson River to the west, and William Street to the east. Engineers used a modified ATC-20 system, an evaluation system developed by the Applied Technology Council for use in on-the-spot evaluations regarding continued use and occupancy of buildings damaged by earthquakes. Engineers surveyed 400 buildings in two days.

As of midnight on Friday, September 14, seaony had dispatched 24 teams of volunteer structural engineers to the World Trade Center site to assess buildings surrounding the site. An additional 24 teams had signed up to work that Saturday and Sunday, for a total count of roughly 150 volunteers.

The U.S. Army Corps of Engineers also played a significant role, coordinating its efforts with FEMA to provide immediate disaster response support. This support included providing technical assistance for debris removal, electric power assessment, and structural assessments. Corps members also provided technical assistance for debris removal at the Pentagon. Under FEMA's management, Corps personnel provided assistance to supplement FEMA's and New York City's geographic information system mapping and analysis capabilities. Thermal imagery illustrated the location of fires still burning and pinpointed particularly dangerous hot spots. The maps generated were used in safety briefings. Additionally, Corps personnel undertook dredging to increase the depth of the berth at Pier 6 so that debris could be more efficiently transported to the landfill in State Island where it is being examined.

Engineers continue to provide assistance and advice 24 hours a day.

e have to focus on why the towers remained standing for as long as they did, not on why they collapsed," says Ron Klemencic, the president of Seattle-based Skilling Ward Magnusson Barkshire and the president of the Council of Tall Buildings and Urban Habitat. He notes that the council has received e-mails from every corner of the world and that he has been struck that so many people admired from afar the fact that the towers stood for as long as they did. "If subjected to identical attacks, most tall buildings in the world would not have stood," he observes. "And that is a really important thing for people to remember."

At this point, the analyses of the collapses of the twin towers and of the damage to the Pentagon are still in the data-gathering phases. And while it is certainly true that these are not typical forensic studies—that is, not studies of buildings that collapsed because of inherent flaws—they are extremely important. For the details of the structural collapses that will be revealed by these studies will almost certainly provide information that can be applied in the future to the design and construction of new buildings and the retrofit of existing structures.

However, an important point underscored by a number of those involved in these analyses is that constructing buildings to withstand the type of assaults that occurred on September 11 is not a realistic objective. The point to bear in mind is that the attacks shouldn't have occurred—that the hijackers should never have gained access to the planes. Nevertheless, there are no doubt lessons to be learned from both disasters—particularly with respect to fire protection.

ASCE has assumed the lead in both analyses, establishing two building performance study teams—one for the World Trade Center, one for the Pentagon—to conduct exhaustive studies that will include extensive analysis of the structural remains at each site. Both teams were organized by ASCE and were led by ASCE and its Structural Engineering Institute. A crucial partner in the World Trade Center study is FEMA, which is providing support through funding, logistical assistance, and production of the joint ASCE/FEMA report. A coalition of engineering societies are participating in the studies. The findings of the studies—each of which is expected to take between six months and a year to complete—will be published in a joint ASCE/FEMA report that will be made available to ASCE members and the public. The report results will also be available to support FEMA and other government groups during the rebuilding effort for the

Pentagon and possibly for the World Trade Center and to provide guidance in the future design and construction of tall buildings or those that have been given landmark status.

W. Gene Corley, who leads the World Trade Center study team, explains the primary focus of the investigation: "Despite the terrible mess that always occurs after any kind of a collapse, it has been my experience that when it is examined systematically we have always found the information we needed to pin down the mechanism that caused the collapse. In this case, the things that we are going to be looking for include the portions of the building that were damaged by the impact. Specifically, we want to know how many columns were destroyed—or at least diminished to the extent that they had a greatly reduced capacity—and then we will also be looking for any evidence of what the fire did. It would surprise me if there wasn't some damage to the fire protection system, but no matter what, the buildings lasted roughly an hour and an hour and a half before collapse occurred. Whether or not the fire protection was damaged, it still helped. I think we are going to find that the fire protection had to do an awful lot to afford people that much time to get out of the buildings."

Although there can be no substantive discussion of the causes of the twin towers' collapse until the building study team's report is published, it is widely believed that they were felled by a progressive collapse. The towers were classic tube structures, which derive their strength from tightly spaced perimeter columns. The towers were a pair of 209 ft (63.7 m) squares that encompassed almost one acre (0.4 ha) of space on each of the 110 floors on each tower, and represented an advance in skyscraper design and technology in that they used the exterior walls as the load-bearing walls. The majority of the structures' steel was on the exterior rather than the the interior; the only interior columns were in the central core, which contained the elevators.

The first plane struck the north tower at the 96th floor; the second plane impacted the south tower at the 87th floor. Both crashes destroyed a significant number of perimeter columns on several floors, weakening the overall structural systems, which were further undermined by the intense heat of the ensuing fires.

While it may not be known for several months precisely how and to what extent fire contributed to the collapses, the question of how these events will affect building codes certainly merits asking. "In my opinion, unusual buildings should always involve a fire protection engineer," says Jonathan Barnett, a professor of fire protection engineering at Worcester Polytechnic Institute and a member of the World Trade Center study team. "When we get to more complex buildings we always bring in the structural engineer. We should also bring in a fire protection engineer. The problem is that we don't think about doing that, and structural engineers aren't trained in fire. Fire protection engineers aren't trained in structures. Architects have a little bit of training in both. We just don't have the mechanism yet.

"In April 2000 the International Building Code was published as part of the International Code Council's International Code Series. It's not just for fire, but it includes structural performance. I think that if you want to look at unusual buildings—very large, very tall buildings—you have to ask, What kind of performance do we want? I think the future has got to be about performance. New Zealand has had a performance-based code since the early nineties, and Australia and the U.K. have allowed such design throughout the last decade. Europe is slowly moving in that direction, and we must do so as well if we're going to continue to build unusual buildings—which of course we will."

Egress is another issue that will almost certainly be raised in the final analysis of the towers' collapse. Many thousands of people escaped unharmed from the World Trade Center towers, and evidence to date suggests that the stairwells were for the most part clear when the towers collapsed. But it is worth examining the building codes of such countries as the United Kingdom, China, and the Philippines. British code now requires separate stairwells and freight elevators for rescue personnel as well as wide stair landings in order to eliminate crowding during egress. And Chinese and Philippine codes require open-air floors every 10 to 12 stories so that building occupants can breathe while awaiting rescue personnel.

enovations strengthen structures, enhance productivity, and extend the life cycles of buildings. But in the case of the tragic events that unfolded at the Pentagon on September 11, the almost completed renovation of Wedge 1 also saved lives.

Buildings are simply envelopes constructed to house and protect those who live or work within them. And no matter what their architectural, historical, or cultural significance may be, buildings can be replaced; human lives cannot. The damage inflicted on the Pentagon during the terrorist assault is extensive: Of the 6.6 million sq ft (613,140 m2) of floor space the structure encompasses, nearly 700,000 sq ft (65,030 m2) was affected.

Investigators have reported that the data recorder of American Airlines flight 77 indicates the plane was traveling at 345 mph when it struck. The work spaces of several thousand people were destroyed, and yet fewer than 200 people lost their lives—64 of whom were on the plane. Why so few?

"One major reason is that the plane struck the recently renovated Wedge 1, although the fire damage does extend into Wedge 2," says Hunkele. "This renovation involved several components that saved numerous lives. First, the walls had been structurally reinforced. This reinforcement stopped the plane from penetrating farther. And the strengthening of the walls enabled the internal structure of the penetrated rings to remain standing for a while. In an explosion, shards of flying glass can be deadly. The blast-resistant windows on the exterior ring minimized glass-related injuries. Additionally, the fire that followed the crash was held in check by the new sprinkler system and by the accordion fire doors that slammed shut immediately following the crash. These doors, along with fire dampers in the ducts, stopped smoke from circulating throughout the building. These five elements of the renovation helped to protect those who were in the immediate vicinity of the crash and give them additional time to escape from the building.

"Following every disaster—be it an earthquake, a hurricane, or, as in this case, an act of terrorism—there are lessons to be learned. And those lessons will translate into better construction and renovation methods that will create an improved and more secure Pentagon. This is not a silver lining in the dark cloud of that terrible act. It is a simple fact. The renovation of the Pentagon has already helped to save many lives."

A three-year-long, $258-million renovation effort was destroyed in the assault, but as Hunkele points out, buildings can be replaced whereas human lives cannot. That the renovation of Wedge 1 prevented more extensive damage and loss of life has been widely reported in the media. Approximately 25,000 people are at work at the Pentagon on any given day—roughly 5,000 people per wedge. It was indeed fortunate that the structural reinforcement and fire safety enhancements included in the renovation of Wedge 1 had been completed; the benefit was that thousands of people escaped to safety. It was equally fortunate that Wedge 2, which was next in line for renovation, had been vacated. That fact also saved thousands of lives.

In his news briefing of Saturday, September 15, W. Lee Evey, the program manager for the Pentagon renovation, summed it up thus: "I'm here to tell you that had we not undertaken this effort, this could have been much, much worse." Although opinions differ as to whether the World Trade Center will be rebuilt, Wedge 1 of the Pentagon will be reconstructed in accordance with the renovation guidelines, and the renovation of the other four wedges will follow. And certainly any lessons learned from the forensic analysis of the assault will be applied. As Paul Mlakar, the head of the Pentagon building response study team, observes, "We have a very interesting opportunity here to contrast the response of the two very different sections."

n the words of George Tamaro: "We're going to become very introspective—you know, What do we do now? How do we prevent this from happening again? I think we prevent it from happening by having better foreign policy and better intelligence. It's more a matter of counterterrorism activities than a matter of designing something significantly different. I also think that engineers—particularly civils, who have gotten the predominant exposure here—are going to be perceived differently. I think if you took a vote right now on the importance of various professions you'd find that civil engineering has gained significantly in terms of people's perception of the profession's value to society."

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Six weeks after the terrorist attacks fires still burn at the World Trade Center site. Photo credit: Greg Brouwer

Civil engineers' contributions to the rescue, recovery, and cleanup efforts following the September 11 disasters have been detailed daily by the media, and Tamaro is probably correct in his assessment of the public's current perception of the engineering profession. But the truth of the matter is that civil engineers have always contributed significantly to society—indeed, the accomplishments of America's civil engineers are, in a word, remarkable. Thanks to American civil engineers, America boasts the greatest infrastructure in the world.

Tamaro's take on how we prevent or mitigate future terrorist attacks is also sound. However, there is no question that America is now studying the vulnerability of its infrastructure to disaster and that civil engineers are playing very visible roles in this assessment.

Fifty years ago, in the years immediately following World War II, the United States was hard at work constructing what would be the best infrastructure in the modern world. In a very real sense, civil engineers defined postwar America by their construction of this infrastructure. Now, however, America must confront two very significant problems with respect to its infrastructure: One, it is rapidly deteriorating, and the need for rebuilding it is a national issue that must be quickly addressed; two, the events of September 11 made it abundantly clear that security will figure much more prominently in planning, design, and construction.

While it is true that there is no ironclad way to protect the complex infrastructure of a nation as large as the United States against terrorist attacks, there are certainly numerous ways to mitigate the effects of such attacks. And it is in this arena that civil engineers will help define America in the 21st century.

On October 9 ASCE's Board of Direction launched a critical infrastructure response initiative, assuming a leadership role in addressing infrastructure vulnerability. The objectives of this initiative are to assess the vulnerability of all components of the nation's infrastructure, use the vulnerability assessments to prioritize infrastructure renovation, determine research and development needs for new approaches to protecting critical infrastructure, develop retrofit designs to mitigate damage from disasters, develop new design procedures—including codes and standards—and improve disaster preparedness and response.

Post-September eleventh America feels vulnerable—and very much dislikes feeling vulnerable. Which means that America's progression into the 21st century will be focused on protecting America's freedom and security. And just as civil engineers built post-World War II America, so too will they build post-September eleventh America. From the rubble of New York and Washington will emerge a better America—an America that is more secure, more certain than ever that the American way of life will be preserved for many future generations.

Greg Brouwer, Jeff L. Brown, Brian Fortner, and Laurie Shuster contributed to the reporting of this article.

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