After the Fall

The day after the September 11 terrorist attacks brought down the Twin Towers of New York’s World Trade Center (WTC), CNN news anchor Leon Harris conducted a telephone interview with Jerusalem-based architect Aaron Swirsky, who had worked in the office of Minoru Yamasaki as part of the project’s design team. Pushed by Harris to offer an opinion on what had caused the buildings to collapse, the clearly shaken architect speculated that it was the huge volume of jet fuel feeding the extraordinarily hot fire rather than the impact that ultimately caused the towers’ structural failure.

This opinion was echoed by numerous experts in subsequent weeks. Carl-Alexander Graubner, Professor of Engineering at Germany’s Darmstadt University of Technology, explained in an interview that “the impact of an airplane is relatively small in comparison with strong wind loads.” A panel of civil and structural engineers convened at the Massachusetts Institute of Technology in early October offered alternative theories to explain the buildings’ progressive collapse, but they differed only in detail, agreeing that it was the result of the steel structure being undermined by fire.

After offering his views, Swirsky added ruefully: “it was something completely unforeseen, so far as the design criteria were concerned.” Although many commentators concur, maintaining that buildings can’t be designed to withstand such catastrophic attacks, architects, engineers and other building specialists are examining design criteria and considering the implications of the Twin Towers’ collapse for the future of skyscraper design. Last month, the Council on Tall Buildings and Urban Habitat (CTBUH) convened a multi-disciplinary task force of architects, structural, mechanical and electrical engineers, vertical transportation specialists, fire protection and safety consultants, building owners and developers–as well as experts on building control systems, building security and blast and curtain walls–to discuss what actions might be taken to improve the emergency performance of buildings.

The CTBUH task force concluded that while it is impractical to design any building to withstand extreme acts of deliberate destruction such as those of September 11, there are measures that could improve the level of safety in tall buildings. These include re-examining egress strategies, providing multiply-redundant systems, integrating building control systems, working with performance-based building codes, and improving education and research.

Architect David Jansen, a partner in Mississauga, Ontario-based Adamson Associates, describes a similar strategy. Adamson Associates, working with Cesar Pelli & Associates as design consultants, acted as executive architects for the Petronas Towers at Kuala Lumpur City Centre (KLCC), currently the world’s tallest buildings. The same team also designed London’s Canary Wharf tower and the World Financial Center immediately south of the WTC site in New York (the Mississauga firm is overseeing remedial work on the World Financial Center, which suffered considerable damage to the building envelope but which remained structurally sound). In the aftermath of September 11, Jansen says, his firm has identified a range of issues for consideration, many of which coincide with the CTBUH task force’s list.

Jansen notes that one of the first steps his firm took following the attacks was to conduct a detailed worldwide code comparison to see how building standards might be improved. “The World Trade Centre didn’t have concrete cores; in New York, cores are built out of steel,” Jansen says, adding that this is a local tradition founded on what is “primarily a labour issue.” Professor Graubner noted that reinforced concrete stairwells might “have enhanced evacuation and conceivably resisted the fire longer.”

Jansen notes that providing concrete cores in lieu of steel, where appropriate, is just one item among many currently under review. These include encasing perimeter columns in concrete, enhancing primary security around buildings with bollards or other vehicle barriers, widening stairs to allow for increased exit potential, providing areas of refuge within hardened cores on all floors, and designing glazing systems to resist blasts. In addition to these architectural and structural measures, Jansen lists a host of mechanical and fire and life safety system considerations, including increased fireproofing, large water reserves for fire protection systems, and carbon filter or HEPA filtration devices to address bio-chemical threats.

Structural engineer Barry Charnish, a principal at Toronto’s Yolles Partnership Ltd., questions whether a concrete structure would necessarily have fared better under such an attack. “The burning temperature of the aviation fuel is so high,” he notes, “that it would affect concrete as well as steel. The change in temperature is so rapid that concrete could have exploded.” Charnish doesn’t believe that the use of steel is an issue in and of itself. Like Jansen, he stresses the importance of examining building codes to see how construction standards might be improved. As the structural engineer for Canary Wharf and the World Financial Center, Charnish has worked with a variety of codes, and notes that “British standards are more advanced in terms of progressive collapse and key element design.” Asked about the cost implications of adopting more stringent standards similar to the British provisions, Charnish maintains that they would represent only “three or four percent of structural costs”–a tiny fraction of overall building costs.

“The real question,” adds Charnish, “is what are you actually designing for? If you start looking at providing places of refuge or bomb shelters on every floor, then you start having to ask at what point it gets uneconomical to build skyscrapers. But I think we will see changes to core design with respect to evacuation systems–avoiding scissor stairs, keeping exit stairs farther apart, that type of thing.” This is echoed by Professor Graubner, who notes that in contrast to the North American model, which collects exit stairs in a central core, in Germany “skyscrapers must have two separate exitways in different corners of the building.”

Fred Leber of Leber/Rubes Inc. Consulting Engineers, a Toronto firm specializing in fire and life safety and building code consulting, emphasizes that sophisticated evacuation and life safety systems must be complemented with thorough and well-considered emergency planning. “Training needs to be taken more seriously. Something as basic as fire drills can make a huge difference. In New York, lots of people may have stayed on an office floor from a comfort point of view–they may have found the prospect of descending down long, unfamiliar stairways scary. Proper emergency planning can help allay those fears and avoid panic in a real emergency.”

Leber’s partner, Jonathan Rubes, observes that total evacuation at the WTC “would not have been part of an initial strategy.” Tall buildings are designed for staged evacuation, with affected floors being evacuated quickly while people not in immediate danger can be let out more gradually. In a typical office fire, fuelled only by paper, furniture and building materials, says Rubes, “there may not be a need for a total building evacuation thanks to sophisticated fire protection measures.” He adds that evacuation of large buildings is not a simple proposition, and is governed to some extent by risk management considerations, noting that “people can get hurt in an unnecessary evacuation.”

Rubes, who sits on the Fire Safety and Occupancy Committee of the National Building Code, doesn’t believe that there will be any significant changes to the NBC or the Fire Code. “The requirements are already there–what’s important is compliance, especially with respect to system maintenance and emergency planning.” He adds that suggestions of increased fire ratings are well-intentioned, but misplaced. “The two-hour rating is based on a standard temperature curve. This is already overdesigned for sprinkler-cont
rolled fires, so increasing fire ratings is not the best place to spend additional dollars. It would be better to spend more money on commissioning, system maintenance and training. And here there are some very important questions. How do you control the evacuation of large buildings? Who decides, in a catastrophic situation, how a building is evacuated?”

Leber and Rubes note that in addition to implementing good emergency planning, many building owners will need to upgrade and update their systems. “We’ve always treated building code requirements as a minimum,” notes Leber. “For example, the code’s high building requirements apply equally to a 20-storey and a 75-storey building, even though in practice they represent quite different conditions. We’ve found that our clients are generally receptive to providing enhanced systems, especially after the 1993 bombing at the World Trade Center.”

Among these enhanced systems are portable command centres, allowing for the remote operation of computerized building systems in the event of damage or sabotage within the control room. Other strategies include redundancies in communications cabling and information systems and the decentralization of building controls. “In an emergency situation,” notes Rubes, “communication is extremely important.”

This may turn out to be a significant issue not only for tall building design, but also for the broader economic implications of September 11. A number of observers have noted that the events will have profound financial repercussions, sinking an already flagging U.S.–and, by extension, global–economy. Asked about the attacks’ overall impact on the construction industry, Alex Carrick, Chief Economist at CanaData, noted that particular sectors are likely to respond in different ways. “Immigration accounts for a significant portion of demand for new housing starts, so if we see a clampdown on immigration that would likely affect the housing market. Buyers may also be jittery about high-rise condominiums. With respect to office space, we may see increased decentralization, with companies opting to locate in low-rise suburban offices rather than in towers in the financial downtown.”

According to Carrick, one area that will be adversely affected is the hotel and hospitality sector, but he notes that in times of crisis, there is also an “escapism factor–people seeking to forget their worries. This means that casino construction will probably continue,” he adds, although in the month following September 11 border town casinos depending on American visitors have suffered significant reductions in attendance. One area that may experience a boom is engineering infrastructure, especially, notes Carrick, “if we see a move toward creating ‘Fortress North America’ with respect to increased energy self-sufficiency. We could see huge investment in pipelines and other oil, gas and electricity projects. The question here is whether funds diverted into military spending will be taken away from infrastructure projects.”

Another likely result of the destruction of the Twin Towers will be a resurgent interest in the use of advanced communications technology, not only for building security, but also for conducting business. Carrick believes that the demand for security and communications systems could re-energize the flagging technology sector. “Governments will be spending on security and intelligence equipment, and there is also bound to be additional interest in teleconferencing, reducing the need for business travel.”

This is a view shared by Douglas MacLeod, Director of Projects for Alberta’s NetEra Alliance. He notes that “in the aftermath of September 11, safety has become a prime concern in any building project and increasingly architects will be asked to incorporate new information technologies for surveillance and communications as key components of their designs.” He also agrees with Carrick that with airline security in question, businesses are turning to videoconferencing as an alternative to in-person meetings. In the month following September 11, for example, “shares in Polycom Inc., a videoconferencing products manufacturer, climbed 56%. The current trend in this area is to produce inexpensive systems that run off commercially available personal computers and are connected through broadband Internet connections. Far from the room-sized installations of the past, these units are small, mobile and often cost well under $10,000.” MacLeod cautions, however, that “architects should be aware that proper lighting and careful placement of speakers and microphones can have a dramatic effect on the quality of the conferencing.”

MacLeod also reports that “within hours of the attacks, the U.S. House of Representatives had placed an order for 470 Blackberry pagers manufactured by Research in Motion of Waterloo, Ontario.” He argues that this increased interest in communications technology “underscores the fact that in the wake of these terrorist strikes, technological trends–both good and bad–are accelerating rapidly.” The flip side of improved communications, he notes, is increased surveillance through technology. MacLeod points out that “a recent Globe and Mail/CTV/Ipsos-Reid poll indicated that 80% of Canadians are prepared to provide their fingerprints for a national identification card. Similarly, the United States Secretary of Transportation’s Rapid Response Team on air travel security has recommended that information technology, including ‘smart’ cards, be used to increase security. Biometric scanning devices that read fingerprints or eye patterns are also being considered.”

Widespread use of videoconferencing and biometric scanning has dramatic implications for communications infrastructure. MacLeod notes that “effective videoconferencing requires considerably more bandwidth than a cable modem can provide. Such requirements may speed Canada’s plans to implement the recommendations of the National Broadband Task Force and provide high-speed access to every home, school and office in the country.” This observation has been borne out by the federal Cabinet’s recent approval of a $1 billion plan to provide broadband access to every community in Canada.

Whatever measures will be implemented as a result of the attacks, David Jansen notes that an important dimension to keep in mind is the psychological one. Part of what will happen, he predicts, is that while enhanced visible security measures–like armed guards in office lobbies–will have “a placebo effect,” providing a degree of reassurance, real benefits will accrue through changes to building standards. “After an initial reaction and stall in tower construction, we’ll go again.” Barry Charnish agrees, and notes that talk of reconstruction at the WTC site began almost immediately following the collapse of the Twin Towers. “I’m hearing two stories,” he adds, “one that has the towers rebuilt in the same configuration, only taller, and another that resurrects an earlier scheme to build four 50-storey towers instead.”

Most commentators agree with the CTBUH task force statement that overall, tall buildings have excellent safety records, and the Twin Towers “performed heroically,” allowing more than 20,000 people to evacuate. In a statement following its meeting in Chicago, the task force noted that “It is important to understand that the attack on the World Trade Center was not about tall buildings, it was about terrorism.” In a similar vein, Professer Graubner notes that “the problem of security is not concerned just with skyscrapers”: sports stadiums and arenas could also be susceptible to attack. “The potential danger here also involves tens of thousands of people. The question, then, is not how to make skyscrapers more secure, but rather how terrorist attacks are effectively hindered.”

The CTBUH task force will be presenting their recommendations at next month’s Building for the 21st Century conference in London, England. For more information visit