Page N1.2 . 17 October 2001                     
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    Engineering Forensics of Collapse


    One of the most ambitious efforts is supported by $300,000 in grants by NSF to eight separate research teams from universities across the United States. The awards, which range from $10,000 to $100,000, support forensic investigations to:

  • evaluate the performance of the structural steel in the World Trade Center buildings

  • develop a land-based laser system to "map" the disaster sites

  • collect data at the World Trade Center using global positioning system (GPS) technology, digital cameras, and hand-held computers

  • study the seismic effects of the World Trade Center collapse on surrounding structures

  • evaluate the performance of fire-protection materials and systems

  • understand the interdependence of infrastructure systems, such as how electrical power loss affects control systems

  • study the coordination of emergency response teams.

    The awards were made only days after the disaster, and many of the investigators were already involved in ongoing NSF-supported research. Amber Jones, of NSF's Office of Legislative and Public Affairs explains: "The proposals and awards were processed faster than normal due to the perishability of some of the data, as material is removed from the sites and destroyed or recycled."

    A Race Against Time

    Indeed, critical evidence of how the trade center collapsed may be lost already. In the immediate aftermath of the disaster, haulers began to cut up and truck pieces of the towers' 300,000 tons (272,000 tonnes) of structural steel to recycling centers because the pieces purportedly did not hold any evidence important to criminal investigators.

    According to a report in the New York Times, city officials who were focused on recovery efforts overlooked the possibility that the debris could hold clues of interest to forensic engineers. A request to the city by ASCE to study the structural steel did not reach officials until almost three weeks after the disaster.

    In fact, one of the NSF investigators accidentally discovered critical pieces of the trade center's structure when a flatbed truck loaded with twisted columns and beams happened to park in front of his hotel in New York, according to the Times.

    Abolhassan Astaneh-Asl, professor of civil engineering at the University of California, Berkeley, has since had an opportunity to examine pieces of the steel structure and to train workers unloading steel for recycling to look for tell-tale signs of destruction. Based on the data now being collected, Astaneh-Asl hopes to construct a computer model of the trade center buildings and how they fell.

    Clues about the Collapses

    The evidence thus far appears to support initial theories about why and how the twin towers collapsed: the steel structure made up of 61 columns on each side of the square towers, steel columns around the elevator core, and steel truss beams supporting the floor sustained the impact of the jetliners crashing into the buildings.

    However, it was the explosion and burning of jet fuel, perhaps 18,000 gallons or 50 tons* (68,000 liters or 54 tonnes, some three quarters of the aircraft's capacity) of it at each building, that gradually weakened the steel structure to the point of collapse. Gravity did the rest to bring the towers down. This seems to explain why the buildings did not fall at the moment of impact but suffered collapse an hour or more later.

    Astaneh-Asl has uncovered pieces of structural steel that bear gouge marks material removed as the Boeing 767s tore through the buildings. One column shows the impact of what Astaneh-Asl believes to be an airplane wing slicing through the steel; another, the curve of a plane's nose. These marks, Astaneh-Asl explains, could be made only by a sharp object moving very fast. (One estimate clocked the impact speed of the jetliners at 300 miles (480 kilometers) per hour.)

    He has found pieces of bent steel with bolted connections intact, which indicates that the material gave way when its strength was compromised by the inferno.

    In the range of 900 to 1100 degrees Fahrenheit (500 to 600 degrees Centigrade), structural steel loses about half its tensile strength, and so begins to deform and buckle under loads. At 1400 degrees Fahrenheit (800 Centigrade), only about 10-20% of the strength of steel remains. The actual melting point of steels is nearly twice as hot, at temperatures difficult to reach outside the specialized conditions of a foundry or forge.

    According to Ted Krauthammer, professor of civil and environmental engineering at Penn State University, it has been estimated that the jet fuel fire in the towers ranged from 1000 to 3000 degrees Fahrenheit (550 to 1,670 degrees Centigrade). This is higher than temperatures usually expected in office building fires, which are typically fueled by paper and furnishings.

    However, without precise information on the dispersion of the airplane fuel, including how much was consumed in the initial fireballs, estimates from external observations cannot be very meaningful. If the data collection efforts currently in progress are successful, it should be possible to derive much better maximum temperature estimates from close observation of many individual building component from different locations in the structures.

    Prior to this disaster, it had been claimed that fire had never caused collapse in a properly sprinklered building. Were the World Trade Center Towers properly sprinklered? More needs to be discovered and reported about the actual types and conditions of steel fireproofing in the buildings. Similarly, more needs to be discovered about the types and status of fire suppression systems.

    Did 600 ton water-filled mass dampers at the tops of the towers hasten the collapse? Did they, or could they in future designs, contribute to fire suppression? Did 10,000 bonded-rubber viscous damped steel connections play any special role in the initiation or progression of collapse?

    Engineer Eugene Corey, lead investigator on the ASCE's team, notes that the delay between impact and collapse is important to study. In an address last week at ASCE's convention in Houston, Texas, Corey noted that engineers need to be "looking for things that might increase the time [for occupants] to get out."

    In the case of the World Trade Center towers, this time allowed tens of thousands of people to escape but left some thousands more trapped behind. An initial report on the ASCE's findings should be available by April 2002.

    Revamping Design Guidelines

    On October 16, the Council on Tall Buildings and Urban Habitat released a statement by a task force made up of 24 industry experts on enhancing the emergency performance of buildings. Among the panel's recommendations:

    Update the standards that contain the varied approaches of egress processes, systems, shelters, stairwells, and elevators to increase awareness, understanding, and probability of exiting a building.

    Design building systems with multiple sources and independent distribution routes to better withstand disruptions caused by extreme events.

    Integrate the numerous systems (structure and infrastructure, electrical, security, building, and utility management) to provide on-site and remote information about the building and its occupants to the appropriate authorities.

    Institute guidelines for regularly explaining safety procedures, as is done on airplanes and in schools, to better educate building managers about decision-making and communications during an emergency.

    "It is important to understand," noted the task force, "that the attack on the World Trade Center was not about tall buildings, it was about terrorism."

    Designing office buildings to withstand the catastrophic events of September 11 may be structurally feasible, but might not be economically practical. Along with how the buildings performed and failed, attention should now focus on improving evacuation via conventional methods and alternatives to getting people (able-bodied and disabled) out of buildings at risk.

    There may be lessons to learn about fire suppression systems and their performance in severe conditions. And surely if society makes the commitment, technology for fighting fires in tall buildings can be advanced beyond sending men on foot up thousands of steps carrying 100 pounds or more of equipment.

    The various investigations underway should yield very important insights into how we can make buildings safer havens, and easier to escape as well.

    Michael J. Crosbie is editor-in-chief of Faith & Form, an associate with Steven Winter Associates, and a contributing editor to ArchitectureWeek.

    * Number of tons (and tonnes) corrected 2002.0330

    Discuss this article online at

    Related Stories:
    Rebuilding in New York
    Pentagon Battered but Firm
    Beyond Disaster



  • ArchWeek Image

    Details of the broken concrete and severed steel rebar may provide clues to the nature of the World Trade Center collapse. (September 24, 2001)
    Photo: Michael Rieger/FEMA News Photo

    ArchWeek Image

    Workers with heavy equipment try to separate rubble from instructive evidence as debris is hauled away. (September 24, 2001)
    Photo: Michael Rieger/FEMA News Photo

    ArchWeek Image

    How the steel twisted during failure may help architects design safer buildings in the future. (September 26, 2001)
    Photo: Michael Rieger/FEMA News Photo

    ArchWeek Image

    The World Trade Center disaster site, viewed from above. (October 4, 2001)
    Photo: Michael Rieger/FEMA News Photo

    ArchWeek Image

    Aerial view of the World Trade Center disaster site. (October 4, 2001)
    Photo: Andrea Booher/FEMA News Photo

    ArchWeek Image

    RAM Steel is an example of structural analysis software that can analyze a 3D lateral load resisting system. This type of software will be used in reconstructing building failure modes.
    Image: Martin Design, Inc.

    ArchWeek Image

    Fire safety provisions in the World Trade Center allowed many but not all to escape. How much would tougher European safety standards have helped?
    Photo: Curtis J. Gibbs

    ArchWeek Image

    Computer models of many types will be used in reconstruction. This downloadable simple massing model offers an overview of building forms.
    Image: Mike Wheeler, Artifice, Inc.


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