NIST research staff (above) inventory the steel recovered from the collapsed World Trade Center buildings.
Images: Courtesy National Institute of Standards and Technology
The final draft report on the investigation into the fires and collapse of the World Trade Center (WTC) towers after the terrorist attacks of 9/11 is chock full of recommendations—30 in all—meant to address improvements to standards, codes, and practices, evacuation response procedures, and research needed to prevent failures of high-rise buildings in catastrophic events. But the recommendations have left some in the design community befuddled, including the AIA, with many saying the report’s findings are either too broad in scope or misguided because they are based on the result of a unique disaster.
Mandated by an act of Congress, the investigation by the National Institute of Standards and Technology (NIST) National Construct Safety Team took nearly three years and $16 million to complete. The final draft was released on June 23; public comment on the recommendations closed on August 6. “We believe these recommendations are realistic and achievable within a reasonable period of time and should greatly improve the way people design, construct, maintain, and use buildings, especially high-rises,” said the report’s lead investigator, Shyam Sunder, at its release in New York.
“The irony is that none of the recommendations are specific or quantitative in nature,” says Ron Klemencic, president of Seattle-based structural engineering firm Magnusson Klemencic Associates. Klemencic, who is chairman of the Chicago-based Council on Tall Buildings and Urban Habitat (CTBUH), says the investigators essentially “created a laundry list” of topics. “But for the money spent, one would hope the recommendations would have been more substantive.”
According to Carl Galioto, FAIA, a partner at Skidmore, Owings & Merrill in New York and architect of record for 7 World Trade Center and the Freedom Tower, “The report is not so much a conclusion, but a direction.”
Digital model of the World Trade Center.
The recommendations are divided into the following eight groups: increased structural integrity, enhanced fire resistance of structures, new methods for fire-resistance design of structures, active fire protection, along with improved building evacuation, as well as improved emergency response, improved procedures and practices, and education and training.
Many of the recommendations “are simply common sense in terms of fire and life safety,” while others are already common practices, says Raymond Clark, AIA, managing principal in the Chicago office of Perkins + Will. A case in point is recommendation No. 11, which advocates evaluating of the performance and suitability of advanced structural steel, reinforced and prestressed concrete, and other high-performance materials for use under conditions expected in fires. That’s already being done, says Clark, who is chairman of the Chicago Committee on High-Rise Buildings, an organization affiliated with CTBUH that is focused on the design, construction, operation, and maintenance of high-rise buildings in the Windy City.
The general nature of many of the recommendations is “due to the fact that they are taking this catastrophic event and trying to come up with specific recommendations,” says Clark. “Although the recommendations seem sensible in the context of the tragic result, there’s no correlation to other buildings.” He disagrees with recommendation No. 1, for instance, which calls for nationwide adoption of standards and codes to prevent progressive collapse. While sensible in theory, he says, changing the codes would not have prevented the collapse of the twin towers or the Alfred P. Murrah Federal Building in Oklahoma City, which collapsed in April 1995 when Timothy McVeigh bombed the U.S. Government complex. Klemencic agrees that preventing progressive collapse is a “nice statement,” but “we need to define what the hazards are going to be, and what type of building response to those hazards we’re willing to accept. Until we can do that, it’s difficult to do any rational engineering, and [attempts to change standards] become an otherwise emotional response.”
The AIA responded to several points in NIST’s report, saying the agency’s concerns about redundancy in fire protection are valid only in rare cases, and that specific fixes like making stairwells wider and hardening elevator shafts can actually backfire depending on the type of emergency that a building faces.
NIST conducted fire tests to assess the accuracy of computer models of the tower collapses.
Clark takes issue with recommendation No. 17, which calls for the design of skyscrapers to accommodate full-building evacuation of occupants if needed. “I don’t know how you can make that recommendation as policy without exercising reasonable judgment and taking into consideration the specific building,” he says. The recommendation contradicts the fundamental philosophies of building evacuation that have been in place and proven to save lives for decades. Standard practice is that of “hold in place,” says Clark, meaning that the floor on which the emergency is located and those around it are evacuated, while other floors are not because the building’s systems are designed adequately to protect occupants.
The design team for 7 World Trade Center and the Freedom Tower anticipated some of the recommended code changes in the report, as well as in New York’s recently enacted Local Law 26, which included changes to Big Apple codes relating to tall buildings, says Galioto. The team has incorporated many of the measures NIST noted into these projects. In the Freedom Tower, for instance, the designers are using the “structural frame” approach to fire-resistance rating, where columns and the girders that brace them have the same fire rating. The tower also will use a cementitious, medium-density spray-on fireproofing material that has more than five times the cohesion and adhesion as code requires, which meets NIST’s recommendation for development of fire-resistant coating materials and technologies. Both 7 World Trade Center and the Freedom Tower will also use redundant sprinkler systems, and the Freedom Tower will contain an emergency access core comprising a group of service elevators with equipment that is protected against water damage, which open onto a service vestibule that is fire rated and pressurized to mitigate against smoke intrusion.