The Challenge of Making Safer Structures
When recovery workers sifted through the 1.8 million tons of debris, personal belongings, and human remains at the World Trade Center, they were trying to achieve closure for grieving family members and friends of victims. On another level, though, they were just beginning to embark on a mission that could provide at least a glimmer of good from the tragedy: collecting evidence that would ultimately make high rises and other buildings more structurally sound and secure.
Three and a half years after 9-11, building codes are only just starting to reflect lessons learned from the World Trade Center (WTC) collapse. More movement is likely, however, as further concrete information is gathered about the design elements that did not perform as expected or that hindered rescue and evacuation efforts. One catalyst for these changes is a detailed report on the WTC collapse expected to be released this month from the National Institute of Science and Technology (NIST).
The report, more than two years in the making, is part of an ongoing NIST project that seeks to construct a set of well-grounded data to serve as a foundation for building high rises with improved structural integrity, better fireproofing, and enhanced evacuation capabilities.
In addition, the final NIST report is expected to address some issues not directly tied to 9-11 lessons, such as the effects of winds on tall buildings and the adequacy of wind-load standards for these buildings, says NIST's Dr. Shyam Sunder, who heads the World Trade Center investigation team. Recommendations will also address construction hazards and natural hazards, such as hurricanes.
The recommendations will be specific in terms of a building's expected performance, says Sunder, but not with regard to how that performance should be accomplished or what materials should be used. It will then be up to architects, designers, engineers, code writers (see "Model Makers," below), and local authorities to take the recommendations to the next level. The following overview looks at the progress made since 9-11 and at how future building designs are likely to evolve to meet the terrorist threat.
Despite a lot of talk by city building officials, architects, engineers, and security professionals, few cities have amended their building or life-safety codes as a direct consequence of 9-11.
The process of gathering and analyzing data from which code changes can be made is going "extremely slowly, unnecessarily slowly," says W. Gene Corley, an engineer who headed a building-performance study of the WTC collapse conducted by the Federal Emergency Management Agency (FEMA). The findings of that study were published in May 2002.
"There doesn't seem to be a sense of urgency," says Corley, who also helped analyze the remains of the Oklahoma City Murrah Federal building after its 1995 bombing.
Part of the problem, says Shuki Einstein, lead project architect for CH2M Hill, a global engineering and construction-management firm, is that there's no real pressure for building codes to change. Insurance companies might eventually alter that situation, he says, if they threaten to impose higher premiums on companies owning or occupying buildings that do not meet recommended criteria. "Then maybe building code standards will go up," Einstein surmises.
Absent code changes or economic incentives, builders and developers are understandably reluctant to make costly upgrades. Building developers certainly won't drive a trend toward better practices, says Einstein. "Developers are going to do the minimum to meet the building code."
Not everyone believes code changes are necessary. In a paper written for the American Institute of Steel Construction Inc., Nester Iwankiw and Lawrence G. Griffis advocate studying the behavior of buildings under abnormal loads, to create not code but a design tool. Sweeping code changes, rather than voluntary improvements dictated by individual risk assessments, might be an "overreaction," they write.
That go-slow attitude is not universal, however. As the epicenter of the 9-11 attacks, New York City was profoundly affected, and it has taken the lessons to heart.
As a result, developers there have been proactive. For example, Silverstein Properties, which owns the World Trade Center site, "is very interested in going well beyond building codes," says Carl Galioto, a partner at the architecture, engineering, and urban-design firm Skidmore, Owings & Merrill. "They've been an active leader and partner in developing new ideas," says Galioto, who is also a member of the New York building code commission.
Consider the new Seven WTC building. It was designed with a "robust structure" to protect against progressive collapse, Galioto says. (See "Pondering the Meaning of 'Progressive Collapse,' " below.) This measure addresses the fact that fires in the Twin Towers damaged vertical support members, leading to the buildings' collapse.
In addition, stairs and elevators are fully encased in a reinforced concrete core. This reflects FEMA's finding that the plasterboard that was providing fire resistance to the stairways at the Twin Towers was knocked off by the impact of the jets, rendering it useless and possibly hastening the buildings' collapse.
Seven WTC also has spray-on fireproofing five times more adhesive than called for by existing standards, says Galioto. It has a redundant water supply for sprinkler systems as well.
Another fire-safety feature in Seven WTC is that fire stairs are located at opposite ends of the building. (The FEMA report noted that the suicide pilots on 9-11 were able to slice through two sets of emergency stairs because the stairs were close together.) And at 66 inches, stair width goes well beyond code.
Voluntary widening of fire stairs in other buildings is an item under discussion by experts, not just to help with evacuation but to allow space for the counterflow of emergency responders. Many building owners may, however, balk at widening stairs, says architect Barbara Nadel, author of the book Building Security: Handbook for Architectural Planning and Design, because it reduces rentable space.
Moreover, Seven WTC and Freedom Plaza across the street have photoluminescent paint or electroluminescent strips on exit stairs even though they are not yet mandated by code. Freedom Tower will have a third stairwell dedicated to emergency response. And the stairwells in Seven WTC have triple redundancy for stair lights: First, there is emergency lighting to back up regular lighting; then there is battery power; and, if those fail, the photoluminescent paint is in place.
Galioto's firm is also studying how and whether it could implement some type of stair pressurization to reduce smoke in the stairwells in the event of an emergency. Higher air-pressure levels inside stairwells than exists on the floors reduces smoke migration through closed doors, he explains.
Such measures are feasible at the WTC site, but are they cost-effective for other properties? According to Galioto, the overall cost of extra security at Freedom Tower and Seven WTC "will be in the single-digit percentages of construction costs," not including the dedicated emergency stairwell and the wider stairs. These buildings can serve as an example for other iconic or high-risk buildings beyond Ground Zero, he says.
Code. While firms like Galioto's have voluntarily incorporated new life-safety features into their building designs at Ground Zero, the city of New York did not want to rely solely on the goodwill of private industry.
To that end, New York City's Department of Buildings formed its own building code task force, which gathered input from government agencies, design and engineering societies, the construction industry, real estate associations, and other experts. The task force also considered earlier recommendations from FEMA, but it preferred not to wait for this year's NIST report before taking action.
In February 2003, the task force issued 21 recommendations. For example, the task force proposed a requirement that new commercial high rises have elevators that open onto a "smoke-stop vestibule."
In addition, it suggested that existing buildings be required to have inspections, when spaces are being renovated, to assess existing spray-on fireproofing. The task force further recommended that nonresidential high rises be barred from using open-web steel joists. These structural members, which support floors and ceilings, have too little surface area for adequate spray-on fireproofing to be applied.
These proposals echoed the earlier FEMA report findings about the adequacy of fireproofing at WTC buildings. The FEMA building analysis of the WTC remains had found, for example, that some important structural members in the towers lacked adequate--or any--fireproofing.
New York City has codified most of these recommendations through a local law that came into effect in June 2004. As these new codes are implemented, they may serve as a testing ground and a model for other localities.
Four of the provisions apply retroactively to existing buildings but allow considerable time for compliance. For example, office buildings that are 100-feet tall or higher must have adequate sprinkler systems throughout the premises, but building owners have until July 1, 2019, to comply.
For installation of photoluminescent markings on exit signs and stairs, however, high rises have only until July 1, 2006. They have a year beyond that to install additional signs where the exit path is not clear and to provide a battery or generator as a backup power source for existing illuminated exit signs.
The changes are lauded by fire-safety experts, though some criticize the lengthy time allotted for compliance. One positive step, according to Vincent Dunn, a high-rise fire-safety expert and former New York City fire chief, is a provision requiring stair and elevator enclosures to be constructed of "impact-resistant materials."
At the same time, he notes that few of the new rules apply to existing buildings. "There are thousands of existing high rises," he says. "What happens to them?"
He points out that many New York high rises lacking sprinklers have open floor designs with 10,000 to 20,000 square feet of open space. A single fire hose can only handle 2,500 square feet, he says. "It's very hard to extinguish these fires."
Chicago. New York City has not been the only municipality to take seriously the need for changes after 9-11. Changes to Chicago's municipal code, reflecting more stringent high-rise-safety requirements, quickly followed 9-11. But the Chicago Department of Buildings code changes focused more on planning and the human factor than on structural building issues.
The Chicago provisions tie fire-safety measures to building height. For example, all office buildings 80-feet tall or more must have an emergency evacuation plan in place, but only those 540 feet or higher must file the plan with the city's office of emergency communications.
The tallest buildings, those more than 780 feet, must designate a fire-safety director, deputy fire-safety directors, a building-evacuation supervisor, fire wardens, and an emergency-evacuation team. Slightly shorter buildings, over 540 feet and up to 780 feet, need not have fire wardens. Buildings between 275 feet and 540 feet need only designate a fire-safety director and deputies.
The provisions include specific staffing requirements; a building's fire-safety directors or deputies, for example, must be on site when more than half of the building's regular occupants are present.
Other elements of a valid evacuation plan include evacuation procedures, the posting of a core floor plan, maintenance of a list of occupants needing evacuation assistance, and regular staff participation in safety drills. Stairwells must also be marked to identify floor number and the site of re-entry locations.
Not all of the Chicago code changes are personnel related, however, and some are driven by events other than 9-11. For example, an ordinance passed after the October 2003 fire at the Cook County Administration Building, which killed six people, prohibits the locking of reentry doors in stairwells. And just days after a December 2004 high-rise fire, the city council voted to make owners of pre-1975 commercial high rises install sprinkler systems (a 1975 provision requires only new buildings to have sprinklers). But the impact of that change is blunted by a 12-year grace period for compliance.
In general, the focus of code changes and design initiatives has been on how to harden buildings against structural damage that might be caused by blasts or fire and how to facilitate evacuation and emergency response.
"All of the effort seems to be going toward the physical structure," says James Woods of the Building Diagnostics Research Center, who also chaired the American Society of Heating, Refrigerating, and Air-Conditioning Engineers' ad hoc Committee on Building Health and Safety under Extraordinary Incidents. "As far as protection against chemical, biological, and radiological, not so much."
That's a mistake, Woods contends, because infection control is viable. Biological labs have those measures in place already, he says. And companies such as U.S. Global Aerospace manufacture HVAC "nanofilters" that capture minute agents such as anthrax.
Countermeasures that could be designed into a structure include sensor technology--for early detection--and better design and use of HVAC systems and filters to reduce the spread of toxic agents, says Woods.
Scientists at the Georgia Institute of Technology are working on chemical and biological sensors that can be built into ventilation systems. Upon sensing a harmful agent, the sensor could trigger a shutdown of the system.
A few building designers and engineers are also addressing this threat, says fire expert Dunn. Instead of central air conditioning, for example, they have designed some buildings with air conditioning systems--and ductwork--that is segmented by zones that cover just one or two floors instead of 10 or 20.
Central-air systems that serve many floors have shaftways and duct systems that penetrate fire-rated walls, floors, partitions, and ceilings. Extra air systems drive up the cost, but they also potentially limit the spread of toxic agents or smoke.
Another way in which some building designers are going beyond hardening of the structure to reduce the terrorist threat is by adapting their own version of crime prevention through environmental design (CPTED). The concept might be called antiterrorism through environmental design.
The objective is to lay out a new building to facilitate, rather than fight with, security's access control needs, limit injuries from attacks that do occur, and allow for quick evacuation in an emergency. Ron Couch, Stanley Security Solutions' chief operating officer for security integration, says that he is educating architectural and engineering groups on designing buildings with these CPTED-related principles in mind to enhance security.
In New York, says Galioto, new buildings are designed with these principles in mind. Architects, designers, and engineers consider occupant evacuation, checkpoints for delivery people, and mailroom sites early in the design phase, he reports. For example, lobbies sometimes must accommodate larger reception desks for package and briefcase screening.
With the exception of the major metropolitan areas, especially New York and Chicago, action to incorporate the lessons of 9-11 into future buildings is virtually nil, says Jon Magnusson, an engineer who sits on the NFPA High Rise Building Safety Advisory Committee.
The reason for inaction is not hard to ascertain: "The risk is perceived to be relatively low," says Woods. In addition, outside of the cities that take the risk seriously, awareness of the changing standards is also very low.
Architect Nadel speaks around the country about changes made to the New York building code. "I get the real impression that architects, engineers, and design professionals are totally unaware" of those changes, she says.
Woods has likewise crisscrossed the country talking about these issues. He too finds that interest in further protecting buildings "is just not there."
That may be a mistake, cautions Nadel. She points out that the standard of care that courts will impose for building protection might be increasing as a result of the code changes.
"If it's out there and design professionals are changing best practices, then someone in another part of the country might be expected to know," Nadel warns. They are responsible for identifying these trends and conveying them to their clients, she says. Failing to do so could raise liability issues, although as yet there is no case law to show how courts view the issue.
Given that terrorism is still seen as a remote threat for most building owners, calls for better building designs, such as those forthcoming from NIST, may continue to fall on unreceptive ears. But, notes Nadel, companies do understand the threat posed by natural disasters. "If we can get their attention dealing with that, they might deal with some of the terrorism issues."
Pondering the Meaning of "Progressive Collapse"
This article discusses the term "progressive collapse" and how architects, designers, and engineers are trying to put up buildings that are more resistant to the phenomenon. Progressive collapse is defined by the U.S. General Services Administration as a situation in which "local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse."
But some experts say that building designers often confuse this phenomenon with "disproportionate collapse" (a situation in which huge damage is out of proportion to the minor triggering event) and that buildings should be designed with prevention of disproportionate collapse, not progressive collapse, in mind. In a paper on the topic, engineer R. Shankar Nair says that "progressive collapse" is almost a meaningless term, because virtually all collapses could be called progressive in that they follow a sequence or progression of events.
Although the events of September 11 have led to a flurry of work on preventing progressive collapse, Nair writes that "the engineering imperative should be not the prevention of progressive collapse but the prevention of disproportionate collapse (be it progressive or not)." Ironically, the Twin Towers collapse was not disproportionate, according to Nair, because each collapse was precipitated by a large crash and fire.
Stairway to Structural Security
The legacy of the World Trade Center collapse could be safer skyscrapers. Here's what tomorrow's high rises--at least those for which a risk assessment dictates special security precautions--might look like.Greater structural support to prevent progressive collapseWider stairs for evacuationStairs and elevator shafts encased in concrete or steelBetter, longer-lasting fireproofingHard-to-access air intakesFull sprinkler systemsDetailed evacuation plans and routesStairwells on opposite sides of buildingElevators for evacuation or emergency responseSensor-activated electronic evacuation guidance"Smart" bricks that detect structural weaknessBackup battery power for stairwell lightingAntismoke stair pressurizationMultiple HVAC systemsChemical and biological sensors in HVAC systemsPhotoluminescent paint or electroluminescent strips on exit stairs
Although building codes are written at the local municipality level, many cities turn for guidance to either the International Code Council (ICC) or the National Fire Protection Association (NFPA). The ICC's building code has been widely employed throughout the United States, while its newer NFPA counterpart is just getting its footing.
A New York mayoral commission consisting of industry, real estate, labor, and government leaders overwhelmingly chose to adopt and tailor the ICC code. The selection was based on 16 areas of comparison, such as code organization, performance history, ease of understanding, and adaptability to New York's situation. The ICC outscored the NFPA on each factor.
The ICC promulgates the International Building Code (IBC), one of many model construction codes that the organization has developed. The IBC establishes minimum regulations for the design and installation of building systems through prescriptive and performance-related measures. It addresses such issues as use and occupancy, safety, engineering practices, and construction technology. Through the IBC, the ICC will play a major role in determining how many more of the lessons from 9-11 are translated into changes to building codes--ultimately transforming new buildings--across the country.
The ICC has as yet generally not incorporated 9-11 lessons into the IBC. The group is waiting for the final NIST recommendations.
A draft of the NIST report is expected to be available this month. It will be finalized a few months later, after public comments are received and analyzed. At that point, ICC will look hard at adding post-9-11 provisions, says Mike Pfeiffer, vice president of codes and standards for the council.
In preparation, the ICC has established an ad hoc Terrorism-Resistant Buildings Committee. The committee comprises code officials, engineers, architects, and other interested building and security professionals.
A few code changes related to 9-11 were submitted in the 2003-2004 code-change cycle, Pfeiffer says. All but one was defeated, however, which may signal a tough road ahead for proponents of other changes.
The one code change that did get added to the IBC concerned fire-resistance ratings for buildings. It requires that the various components of buildings 420 feet and higher (about 42 floors including floor and roof construction) have a minimum three-hour structural fire-resistance rating, whether sprinklers are present or not. Previously, a building with sprinklers needed only a two-hour rating, explains Pfeiffer.
The rationale, according to an ICC statement, is that "fire department apparatus is generally not capable of supplying adequate water in terms of pressure and flow to floors located above 420 feet in height." Thus, these buildings "must be able to stand on their own."
Of the rejected proposals, one sought to widen stairs in buildings 20 stories and higher from 44 inches to 72 inches, Pfeiffer says. But there was no documentation to support the rationale for the 20-story threshold or the benefits of the specific increase in width, he says.
Another proposal would have required 25-story-and-higher buildings to encase stairwells in either concrete or masonry. Again, no specific data supported the change.
A final change would have set a standard for progressive collapse, which means that failure of one structural member leads to failure of another, and so on progressively. But it was "unclear what the expectation was," reports Pfeiffer, making it difficult to write a definitive provision.
ICC is not the only standard setter in this arena, as noted earlier. Another significant player is the National Fire Protection Association, which writes both the Life Safety Code (LSC) and the Building Construction and Safety Code (BCSC).
Like the IBC, the NFPA's codes serve as models for local authorities. The LSC has been adopted by far more jurisdictions than the BCSC has, however.
Both the LSC and the BCSC will likely reflect the recommendations of the NIST investigators, says Robert Solomon, the NFPA's assistant vice president of building and life-safety codes. "We want to see what the NIST report says before we change," says Solomon. "We don't want to change for the sake of change."
Similar to the ICC, the NFPA has established a dedicated group to focus on these issues, in its case the High Rise Building Safety Advisory Committee. According to the committee's charter, collapse, evacuation procedures, and fire resistance are among the topics being addressed.
The committee is also expected to address specific issues such as redundant water supplies, fireproofing materials, and the routing of lines between storage tanks and generators, says Solomon. In addition, it might look at related issues, such as how to prevent metal-detection equipment from interfering with emergency egress.
Some suggested changes designed to improve evacuation are already working their way into the code. For stairways handling more than 2,000 occupants, Solomon says that it is likely that the minimum required width of stairwells will increase for both the life-safety and building codes. Width is measured in this case between the handrails.
The NFPA has also been exploring alternative escape devices. They range from parachutes to platforms that traverse the side of a building like a window-washer's unit to zip-cord systems for high-wire escape to neighboring buildings.
Solomon emphasizes that the NFPA doesn't endorse these devices, "but we're acknowledging they exist.... We have some obligation to at least mention them in the code."
Not only won't the code require use of the systems in any building or structure, but the equipment also won't receive credit in the code as an "alternative to the fundamental and basic requirements for the means of egress," he says.
"It introduces the concept of the devices and lists a series of considerations that need to be accounted for." For example, the code might state that a system should not pose a safety hazard to anyone in the vicinity.
The NFPA may revisit the topic in more detail in the future. It is waiting to see whether the standards developer ASTM International, which is currently working on technical criteria for these devices, comes up with viable standards.
Some possible options are considered too risky to make it into the latest version of the NFPA codes. For example, the high-rise community has long studied the use of elevators that can be used during fires, but the technology isn't quite there yet, says Solomon. Any conclusions that might lead to inclusion in the code are still two or three years away, according to Solomon. NIST is spearheading this research as well.
Michael A. Gips is a senior editor at Security Management.