Putting Together the Big Picture for the World Trade Center Disaster Investigation

N. Hanacek/NIST

Kathyrn Butler, Physicist, National Institute of Standards and Technology (NIST)

Imagine assembling a jigsaw puzzle of more than 14,000 pieces without an image on the box showing what the final picture will look like. Imagine that important pieces were missing and needed to be searched for. And imagine that this puzzle was four-dimensional, involving time as well as space.

This was the task for our small dedicated team in the wake of the World Trade Center (WTC) disaster on Sept. 11, 2001. In the weeks following the disaster, my colleagues in the Fire Research Division and I started to think about how we might assemble the story of what had happened. Some of our puzzle pieces were images captured by news photographers, reporters from networks and local television stations, and first responders; but others came from everyday people who — witnessing an unspeakable horror — raised a camera they just happened to have on hand that day. The images were from near and far away; of one building or both; at every angle imaginable; and from the moments after the first plane strike through the collapse of the towers. They were taken with professional-grade equipment and bargain-store cameras alike. Individually, they held isolated scraps of information, but we recognized that together they stored great knowledge.

We needed to be able to organize this material in such a way that the stills and clips could quickly and easily be called up to answer questions. NIST researchers would want to know such things as where and when fire or smoke was visible inside specific parts of the buildings, what time particular windows were broken, and which areas appeared to resist fire spread.

By the time NIST was officially assigned the responsibility for an investigation into the collapse of the WTC towers, we had already selected a digital asset management software system, thought out how a database might be used by researchers, developed search terms, and started to populate that database with early sets of photographs.

Visual materials started to pour in due to the dedicated work of Bill Pitts, who led the NIST image collection effort; Val Junker, a New York City-based photographer who had many contacts within that community; and the scores of professional and amateur photographers who had trained their cameras on the buildings.

To help organize the rapidly expanding collection, we hired students from Montgomery College — a nearby community college — who could devote several hours each week to this task. The students brought their talents and enthusiasm to the project, along with a variety of life experiences. Most, though not all, had a technical background. We discussed the difficulties of the assignment along with the satisfaction of filling in another piece of the puzzle. The work required dedication, and we hope that the students who participated know that we’re grateful for their contributions to the effort.

In order to carry out the task, a protocol was established to systematically incorporate every new photograph or video clip into the database. First, the material was logged in and copied to digital storage media. Videos were cut into continuous segments, with a break in the footage signaling an end to the clip. The most important but challenging task was to determine the time at which each photo and video was filmed. This required careful examination of the images to find some event that could be compared to media within the database whose time had already been established.

For digital cameras, determining the time one photo was taken allowed the times for all other photos from that camera to be established. For consistency, times were fixed relative to the moment of the second plane strike. In the end, we were able to fix the time of 44% of the photographs and 38% of the video clips within plus or minus three seconds. Finally, the database contents were categorized by location, time, viewpoint (near or far), key events such as the plane strike or collapse, the presence of smoke or fire, building damage, and other useful information. As the database grew, gaps were filled in and the overall picture became clearer.

By the time the many thousands of photos and videos that had been collected were incorporated into the visual database, a comprehensive picture of the entire event had been developed. Researchers could use it to get the answers to questions about what the situation on a specific floor was at a specific time.

The assembled puzzle was delivered to NIST researchers working on various aspects of the disaster, from impact to collapse. They supplemented it with information from other sources to enrich their analyses. The images covering the exterior façades, for example, were joined with first responder communications and phone calls from people within the buildings to build a more complete story of what transpired inside the towers.

The database was also used to develop a timeline for all eight sides of the two WTC towers, recording locations of fires and broken windows every few minutes. This was used as both input data and a reality check for the NIST-developed fire model FDS (Fire Dynamics Simulator) that was employed to understand how fire progressed through the buildings. FDS combined data from the building plans of the affected floors and heat flux information from fire experiments performed at NIST in which both intact and damaged WTC office mockups were set ablaze. The visual evidence provided boundary conditions and other information: Each broken window represented a new source of oxygen to feed the flames, while exterior areas that remained without visible fire suggested that interior walls were intact. As the model was moved forward in time, predictions of fire reaching the exterior walls of the building could be checked against the visual evidence; adjustments could then be made to keep the model on track.

Some of the videos were used to estimate the speeds of both planes upon impact. The database documented structural changes in the buildings, including column displacements and sagging floors. One of the videos in the database was used to assess the structural integrity of WTC 2 after impact by comparing the back and forth motions of the building’s windows after the second plane strike to the natural frequencies of the undamaged tower.

NIST’s investigation of the World Trade Center disaster took over three years and the work of more than 200 NIST employees, contractors and non-NIST contributors. Completing our puzzle and delivering the database was just one small part of that effort, but it provided crucial input to an analysis that yielded not only an understanding of what happened on Sept. 11 but information that could be used to prevent or limit the loss of life in future building catastrophes. Recommendations arising from the investigation have led to improvements in national and international building codes, standards and practices.

I have often been asked what it was like to work on this project. My overwhelming feeling is one of gratitude. It was truly a privilege to be able to bring my set of skills to bear on this most difficult event at a time when many Americans felt helpless. This is NIST at its best: providing opportunities for researchers to change the world for the better.

This post originally appeared on Taking Measure, the official blog of the National Institute of Standards and Technology (NIST) on August 25, 2021.

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About the Author

Kathryn Butler is a physicist in the NIST Fire Research Division. She is currently working on understanding the fire behavior of fences, mulch and woodpiles in wildland-urban interface (WUI) fires. Butler received a Department of Commerce Gold Medal for her work on the NIST investigation of the World Trade Center disaster and a NIST Bronze Medal for her work on the evacuation of people with mobility impairments during building fires.