Reconstructing the Fires That Brought Down WTC 1, 2 and 7

Everyone has a story about where they were when they heard about the disaster universally called “9/11.” Mine begins in Ottawa, Canada. As chance would have it, I was there for a meeting of the Fire Safety Engineering Subcommittee of the International Organization for Standardization. One of the prime subjects was standards for the structural performance of buildings during a fire. Shortly before lunchtime, Rita Fahy of the National Fire Protection Association, a member of the subcommittee, entered the room and, barely able to speak, managed to convey that airplanes had crashed into the World Trade Center towers and they had collapsed.

Amid an aura of unreality, we adjourned the meeting and charged back to our hotel rooms to learn more. By the end of the afternoon, we also learned of the attack on the Pentagon and the thwarted attack that led to a plane crash in a Pennsylvania field. Running through our minds were thoughts like: How could this happen? All those people started their day normally, not realizing they would not be returning home that evening. Can a steel structure really collapse?

For the better part of the next year, we all learned more and more about that day: who planned the attack, how the pilots were trained, and some information about the World Trade Center itself, but little about how the three skyscrapers came to collapse.

Somewhere in this interval, I had a long conversation with Jack Snell, the director of the Building and Fire Research Laboratory at the National Institute of Standards and Technology (NIST). The subject was the possibility of the creation of an entity that would deal with construction failures in much the same way the National Transportation Safety Board investigated airplane and motor vehicle crashes and the Chemical Safety Board investigated disasters in the manufacture and transportation of chemicals. An idea was percolating through Washington for such an entity, and on Oct. 1, 2002, the National Construction Safety Team Act was signed into law.

In anticipation, NIST had already begun its investigation into the collapses a month earlier. The first project under the Act was to be a reconstruction of how the two 110-story towers (WTC 1 and WTC 2) and the 47-story tower across the street (WTC 7) collapsed, along with guidance for preventing or mitigating future such disasters. After this was decided, Jack asked me to lead the reconstruction of the fires, one of the eight projects that made up the investigation. The overall leader was Shyam Sunder, chief of the Structures Division.

Let me say a few words about my path to this point. I had been interested in math and science since high school and studied math, chemistry and physics at a small liberal arts college in my home state of Connecticut. I then earned a Ph.D. in physical chemistry. My thesis and postdoctoral research were in upper atmospheric chemistry. However, my adviser was known for his work in combustion, and that led to my beginning a career in fire research at the Naval Research Laboratory.

A few years later, I was offered a position in the new Center for Fire Research at the National Bureau of Standards (now NIST). Fire research is a multidisciplinary field, and I soon learned about engineering and human behavior from my colleague (and carpool mate during long commutes) Bud Nelson, the dean of fire protection engineers. By 2001, I had been chief of the Fire Science Division for 18 years and had recently become a senior research scientist to better serve as the manager and technical leader of two large, interdisciplinary and interagency projects.

I quickly learned that the investigation had a head start. Bill Pitts was already collecting videos and photos of the WTC site taken by New Yorkers, tourists, TV stations, etc. John Gross and Terri McAllister had been to the site as members of the American Society of Civil Engineers team. Howard Baum and Ron Rehm had modeled the combustion of the jet’s fuel following impact with WTC 1.

My first realization was of the magnitude of what we were going to do. There had never been a reconstruction like this. There were such heavy questions: How were we going to get the information needed to model the fires with a high degree of confidence? Would we be able to track the movement of the nearly 14,000 occupants of the towers? With nearly all the physical evidence in a pile of rubble, and with the removal already underway, how would we be able to discern the failure sequence in each of the three buildings? For most of us, this was like heading out to sea in a rowboat with only a life preserver and some swimming lessons.

Well, we did it! To recreate the fires, we obtained tenant floor plans of the two towers, as well as information about how they were furnished. We learned about the structures of the airplanes and the masses of jet fuel, cargo, etc. that were onboard. We conducted burns of renditions, or mock-ups, of parts of the buildings, generating input for the complex models of fires that were moving differently and simultaneously on multiple floors. We constructed a visual archive of almost minute-by-minute views of the buildings. We developed computer models to impose the heat from the fires on the structural components in order to track the weakening of the steel, concrete and connections. From all this, we were able to model the destruction the airplanes caused, how the buildings responded, and how the fires eventually led to the towers’ collapsing, taking with them the lives of nearly 3,000 people. We were excited (and relieved) at the agreement between the science, the sequence of photographs, and the eyewitness accounts.

As we closed in on our understanding about what happened on 9/11, we began considering how to present our findings. We were already preparing a set of NIST technical reports on each of the eight technical projects and supporting reports on each of the subprojects. Meanwhile, many of us had read the report of the 9/11 Commission, which documented the events leading up to that day. It was several hundreds of pages long and could not be read and digested in one sitting. The whole country (and much of the entire world) wanted to know how this could have happened, and whether it could happen to a building in which they lived, visited or worked. I felt we had to tell this as a story for the nontechnical audience and provide technical backup for the professionals who might later need to use our findings as they designed, built and renovated tall buildings.

And this is what we did. I was asked to digest over 10,000 pages of technical descriptions and findings into what became the 47-page narrative making up part one of NIST NCSTAR 1. The families of the deceased, the nation and the world would now know how the impossible had turned into a horrible reality. I’ve done a lot of writing before and since then, but nothing satisfies me more than picking up this report and reading it yet one more time.

But the investigation was far from over. We still needed to understand how WTC 7 collapsed without being hit by an airplane. There were no casualties since the building had been evacuated hours earlier. The structural damage to the building due to debris from the collapse of WTC 2 was small enough that the building still stood for seven more hours. This was an “everyday” type of building and fire. Its collapse was simply a result of fires that could not be contained or quenched finding the weaknesses of a steel building.

While many of the tasks of the team had already been completed, the intensity of effort of the fire and structural teams just changed from writing mode back to reconstruction mode. We were all tired, but this technically different phase kept our juices flowing. I remember Bill Pitts showing us how the photographs and videos of the windows showed how the small initial fires moved and grew. I remember John Gross coming into a team meeting with an analysis of how the convergence of those multifloor fires at column 79 could lead to its collapse. I remember Terri McAllister and our structural modeling contractors babysitting the simulations of WTC 7 that ran 24 hours a day for six months. Once again, the science agreed with the observations. We told this story in the 23 pages that make up chapters 1 and 2 of NIST NCSTAR 1A.

Was this reconstruction of the fires a success? My answer is yes. Our reports resolved how the fires grew and how they led to the structural weakening of the three buildings on the actual timeline of the disaster.

Furthermore, while most fire investigations have involved code and engineering experience, this investigation demonstrated the value of adding expertise in fire science, structural mechanics, materials science and human behavior. This broadened project team anticipated the need to go beyond the state of the art, with the full range of talent being up to speed when their contributions were called for.

At the practical level, this effort greatly increased the number of people with experience in complex fire investigations. The in-depth, publicly available documentation will serve as a template of what it takes to get to the heart of such a disaster.

I firmly believe that the authorities made the right choice in having NIST conduct the investigation. There was no other organization that could have brought together the mixture of expertise, the number of pertinent organizations, and the commitment to reaching and checking the technical outcome, to providing the needed documentation of the findings, and to maintaining standing and credibility in the presence of skeptics.

NIST's World Trade Center Investigations