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New Standard for Tall 'Targets'?

(enr.construction.com - 5/13/02)

By Nadine M. Post

The engineers who studied the World Trade Center disaster emphasize that the big and even sole new question raised by Sept. 11 is whether a separate design standard should be applied to supertall buildings because of their "target" potential and long evacuation times. Other issues raised in the aftermath of the terrorists' plane attacks on Manhattan's twin 110-story towers, including integrating fire and structural engineering, predate Sept. 11, sources say.

	(Rendering courtesy of FEMA)

The only clear item "that should be studied because of 9/11 is whether super high-rise buildings need to be designed differently," says Jonathan R. Barnett, professor of fire protection engineering at Worcester Polytechnic Institute, Worcester, Mass., and a member of the WTC Building Performance Study team.

Other questions, crucial or not, are not new, adds Barnett. These include the team's recommendations for further study of fire loads on critical structural members, the integration of fire and structural engineering and the limitations of spray- applied fireproofing on structural steel. Another concern is about flaws in test methods for the fire ratings given to building components and assemblies, including frames, shaft walls and other enclosures. "We need to study" these things, says Barnett, "but not because of 9/11."

The study, organized by the Structural Engineering Institute of the American Society of Civil Engineers, was released May 1 by its sponsor, the Federal Emergency Management Agency (ENR 5/6 p. 12). Its purpose was to examine damage, collect data, develop an understanding of the response of buildings affected, identify causes of observed behavior and identify studies that should be performed.

The report purposely avoids conclusions. The recommendations reflect not only the incredibly sensitive nature of the subject but the number of constituencies represented by the 27-person team, including specialists in blast, fire and structural engineering, and a host of government partners and professional associations. Outsiders have criticized the report as lacking teeth, but the intent to those involved was clear.

"It sets the stage for what comes later," says team member Bill Baker, partner in charge of structural engineering in the Chicago office of architect-engineer Skidmore Owings & Merrill.

The study's main message is that commercial and residential buildings should not be designed to resist airplane loads, because establishing the design criteria–the weapon's power–is elusive. Beyond that, the intent was to learn lessons from the tragic living laboratory. Recommendations will help owners interested in exploring construction that might mitigate the impact of assaults on buildings. Engineers recommend first doing a threat assessment of a building and then designing robust and redundant systems, including life safety, backup power and structural, and emergency exit stairs that are remote from each other.

Team members stress that these recommendations are not directed toward most buildings. "National model building codes do not include requirements to design for loads that might be imposed due to acts of war or terrorism," states the report. However, these loads may be included at the discretion of owners if they desire a higher level of protection.

The team recommends against designing buildings to resist airplane loads and recommends against changing any building or fire codes without further study. "One incident should not cause a change in a building code," says Barnett.

The study team and others in the industry advocate spending national defense dollars on eliminating the enemy rather than trying to codify the hardening of buildings, which they consider impractical, uneconomical and largely futile, because of the illusive and unpredictable nature of the assault. "Do we design against a 767, a 747, a future airplane that is even larger and carries more fuel, a string of airplanes, a truck bomb, a biochemical weapon or what?" asks one expert.

Team members think that designing critical components and connections of a frame for fire loads–not just wind, gravity and seismic loads–should become routine. This, they stress, was the case before 9/11. The calamity only brought attention to the issue.

The team grappled with many issues that seem contradictory. Barnett says he is often asked whether concrete would have lasted longer than steel in the fuel-triggered fire that covered some four to six floors initially, and was as intense as the heat generated by a nuclear powerplant. "The records, in terms of success of steel versus concrete [under fire loadings], when you look at the world outside 9/11 are equal," he says.

Study team members say there are no simple answers. For example, engineers often say stronger connections help resist progressive collapse. But in 7 WTC, strong connections are seen as a probable cause of the progressive collapse. Also, broken connections in 3 WTC are believed to be the reason that the hotel building did not collapse totally, saving the lives of those in the lobby.

	Click the image to see a larger version (Rendering courtesy of FEMA)

	Click the image to see a larger version (Rendering courtesy of FEMA)

There is much discussion about masonry versus gypsum shaft walls, first used in the WTC. Baker and Barnett were reassured that gypsum wall board did its job when it was standing intact around the ruined remains of 5 WTC, which collapsed after burning.

The team cautions that it may be impossible to ever recreate the exact sequence of the collapse. At some point the floors disconnected from the exterior wall, triggering the progressive collapse. But engineers don't know which failure came first–the core, the exterior wall or the floor system. "We may never know the precise sequence of events," says Jon D. Magnusson, team member and CEO of Skilling Ward Magnusson Barkshire Inc., Seattle.

The twin towers had a roughly 209-ft-square plan. Lateral loads were taken by a perimeter structural tube, made of columns spaced 40 in. on center and spandrel beams. The steel core, also made of columns and beams, was designed for gravity loads.

According to the study, floor construction typically consisted of 4 in. of lightweight concrete on 1.5-in. noncomposite steel deck. Outside the core, the decking was supported by a series of composite floor trusses that spanned between the central core and the exterior wall. Composite behavior with the floor slab was achieved by extending the truss diagonals above the top chord so they would act much like shear studs. Pairs of trusses spanned some 60 ft to the sides and 35 ft to the ends of the core. Metal deck spanned parallel to the main trusses and was directly supported by continuous transverse bridging trusses and intermediate deck support angles from the transverse trusses. The trusses were welded to the exterior framing.

Lessons learned in hindsight can be valuable to engineering and professional groups, says the report. One lesson is for local structural engineering groups to create emergency response plans for their areas that would be used to assist fire departments in emergencies so that the response can be improved. A draft plan, created by the Structural Engineers Association of New York, is included in the report. The document, along with the entire study, can be accessed on the Web at www.fema.gov.

Team members continue to stress that the event gave no clear signal about the adequacy of building codes. People, especially the families of the victims, "want simple answers and there are none," says Barnett.

Legislation Defines Tasks for Disaster Investigations

On may 9, house lawmakers were expected to introduce The National Construction Safety Board Act of 2002. The proposed bill, authored by Science Committee Chairman Sherwood L. Boehlert (R-N.Y.) and panel member Anthony David Weiner (D-N.Y.), encompasses many suggestions included in the engineers' report on the World Trade Center and seeks to avoid a recurrence of problems identified at a March hearing. The measure will be introduced in the Senate by New York's Hillary Rodham Clinton (D) and Charles E. Schumer (D).

The bill grants full federal authority in a disaster to the National Institute of Standards and Technology and is loosely modeled on the National Transportation Safety Board, which investigates airplane disasters. The measure "clearly spells out what their powers will be," says David Goldston, chief of staff for the science panel. NIST will have clear authority to enter sites, access documents, test materials, move evidence, as well as issue subpoenas. Currently, the Federal Emergency Management Agency has lead authority for disaster investigations in conjunction with NIST. The bill will seek $25 million per year for NIST, giving the director the resources to carry out this mandate.

The goal is to avoid the problems that surfaced during the World Trade Center investigation. Committee members pointed to reports that officials had problems securing blueprints of the twin towers and retrieving evidence before it was removed from the site. "This bill sets up a very clear process," says Goldston. Further, the bill makes it explicit that the investigative team has a legal obligation to keep the public informed as the investigation proceeds.

Although some officials advocated setting up a new entity to lead future investigations, committee officials decided that was not the best approach. This plan builds on existing expertise, will be quicker to set up and is more streamlined, says Goldston.

NIST will now conduct a multiyear follow-up investigation that could lead to changes in design and construction practices and building codes. That review also should focus on evacuation and emergency response procedures in a wide variety of buildings, asserts Boehlert.