Fireproofing Blown Off Twin Towers
Fireproofing Blown Off Twin Towers
Report Details 9/11 Collapse in N.Y.
Wednesday, April 6, 2005; Page A03
Engineering solutions
Engineering solutions
In late 2001, engineers were working on designs for BP1 - a new tower for Canary Wharf, London. But following 9/11, they changed their plans. Together with steelwork specialists, they devised a pioneering system to strengthen beam and column connections.
Usually, each end of a floor beam is bolted directly to the face of a supporting column. But by pushing a short steel plate through the column, engineers found they could tie neighbouring beams together. Even with two adjacent columns removed, floor beams could hang from the remaining structure.
When the Twin Towers Fell
In Focus
October 09, 2001
 Image: NOAA BEFORE SEPTEMBER 11, 2001, the twin towers of the World Trade Center seemed a permanent part of the Manhattan skyline. |
When New York City's giant World Trade Center towers plunged to earth following successive suicide terrorist attacks on September 11th, the world was confronted with one of most shocking—and sickening—sights of modern times. The mechanisms by which these huge and seemingly solid edifices suddenly collapsed, snuffing out the lives of thousands, was the subject of a preliminary postmortem conducted last week in Cambridge, Mass. A panel of Boston area-based civil and structural engineers convened to discuss the fate of the superskyscrapers, struck by hijacked passenger planes, in front of an overflow audience on the campus of the Massachusetts Institute of Technology. Their starkly sobering analyses highlighted the vulnerabilities of ultra-tall buildings to fire and pointed out steps that could be taken to lessen them.
After first describing the highly redundant structural system that kept the 110-story twin towers standing for decades despite hurricane-force winds and a terrorist truck bomb, the engineers then delineated how that system was breached and finally overcome on that fateful day when America was attacked. The main culprits in bringing the famously lofty buildings down, they concluded, were the two intensely hot infernos that erupted when tens of thousands of gallons of aviation fuel spilled from the doomed airliners. Once high temperatures weakened the towers' supporting steel structures, it was only a matter of time until the mass of the stories above initiated a rapid-sequence "pancaking" phenomena in which floor after floor was instantly crushed and then sent into near free fall to the ground below. Significantly, the panel stated that any mitigating reinforcements and redundancies added to these buildings could have only delayed the inevitable failure, though they would have bought more time for the evacuation of the occupants. No existing or foreseeable economically viable skyscraper structure, they agreed, could have withstood this kind of cruel onslaught. Clearly, prevention is the best defense against this kind of assault.
"Though the twin towers were not much taller than their famous uptown predecessor, the Empire State Building, the World Trade Center rose during the late 1960s, a new era of construction characterized by rapidly erected, lightweight steel structures rather than heavy masonry walls," explained Robert Fowler, senior engineer at the structural engineering firm of McNamara and Salvia. Fowler was then a junior member of the WTC's engineering firm of record, Worthington, Skilling, Helle & Jackson, later renamed Skilling Helle Christiansen Robertson. "As the Trade Center was so much lighter in comparison to earlier designs, it was a watershed building in the history of skyscrapers," he added. Leslie E. Robertson, then the project manager, was the engineer most responsible for the superskyscraper's design, Fowler noted. He is currently principal partner at Leslie E. Robertson Associates, the current structural consultants to the WTC. The late Seattle-based architect Minoru Yamasaki designed the World Trade Center.
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How the Towers Kept Standing
As with all large buildings, the main structural engineering design criteria for the facility's 1,362-foot-tall south tower and 1,368-foot-tall north tower centered on two things: ensuring resistance to the gigantic gravity loads of the buildings themselves as well as to sideways or lateral forces caused by high winds and earthquakes, which can generate huge overturning forces at the bases. The former condition, Fowler explained, depends on specifying strong vertical columns that can efficiently transmit the mass of the building to the ground. The latter consideration concerns not only structural integrity but also "requires developing an acceptable comfort level for the occupants" by avoiding too much swaying. Opposition to lateral motion is controlled by "the design's structural mass [weight], the stiffness of its lateral members and the degree of structural damping employed," Fowler said.
"Though the WTC towers stood over 1,360 feet above the street level, the structures' bases were actually set 70 feet into the ground, and one had a 100-foot-tall antenna atop it, so with 205-foot widths, they had a lot of [exterior] area facing the wind," the engineer stated. He calculated that the approximate maximum wind shear force that a single face needed to withstand to be somewhere around 11,000,000 pounds. The gravity loads (weight) produced by the towers at their bases were on the order of 500,000 tons, Fowler said.
To handle these immense forces, the engineers "designed the World Trade Center essentially as a large beam section," explained another panel member, Robert McNamara, president of the engineering firm McNamara and Salvia. Called structural tubes in the business, each twin tower was strongly framed in structural steel. The frame comprised inner and outer rectangular box tubes consisting of closely spaced steel box columns connected by steel spandrel members or truss beams that supported 40,000-square-foot cross-braced floors, each nearly an acre in area, the empaneled engineers said. This configuration created a complete exterior tube around the building and a center tube down the middle.
The 90-foot-long central core, formed of massive vertical steel columns that held most of the building's weight, contained elevator shafts, stairways and utility spaces, they said. The core's columns were thicker toward the base to support huge accumulated gravity loads. The outer perimeter tube, a tight prefabricated latticework with 61 14-inch steel box columns (spaced 39 inches on center) on each building face, provided all the bracing resistance against lateral and twisting forces from wind and seismic action. This exterior grid served as a moment frame, providing a large moment arm (of torque) against overturning and deflection forces. The outer tube bore part of the gravity-induced downward load as well as, they noted.The huge inner and outer rectangular tubes "needed to be protected to maintain their structural integrity, so the floors acted as reinforcing diaphragms or bulkheads [the term used in shipbuilding]," said panel member Jerome Connor, professor of civil and environmental engineering at M.I.T. The office floors, which each comprised a 35- to 60-foot clear span from the core to the exterior grid, were panelized structural members supported by open web joists with steel decks above them, he said. The horizontal truss struts, bolted and welded to the exterior grid and the core column structures, included viscoelastic stringers that provided increased damping to help make the structure less lively in the wind, according to Connor. Each steel floor deck was covered with four inches of concrete. "With almost an acre of area for each floor and figuring about 100 pounds per square foot of area," he estimated that "each floor system weighed about 3,200,000 pounds."
Why the Towers Fell
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With all of its structural redundancies, "the World Trade Center was probably one of the more resistant tall building structures," McNamara said, adding that "nowadays, they just don't build them as tough as the World Trade Center." His statement is bolstered by the fact that the support structures of both twin towers withstood the initial hits of the two kamikaze airliners despite the breaching of many levels of framing. After the deletion of key structural members from about the 90th to 96th floors on the north face of the north tower, One WTC, and from about the 75th to the 84th floors of the south, east and north faces of the south tower, Two WTC, the buildings' skeletons found alternative paths to take the loads. Each impact and following explosion imparted first a large local lateral force and then an omnidirectional force to the structures, together causing massive initial damage to the columns and floor systems at the elevation of the crash.
Despite shocks and explosions estimated to be equivalent to that of the 1995 truck bombing of the Alfred P. Murrah Federal Building in Oklahoma City (about 400 tons of TNT), the towers remained upright. "The buildings displayed a tremendous capacity to stand there despite the damage to a major portion of the gravity system, and for an hour or so they did stand there," McNamara said. "The lateral truss systems redistributed the load when other critical members were lost. It's a testament to the system that they lasted so long."Newspapers and TV newscasts reported that the twin towers had been designed to withstand a collision with a Boeing 707. The events of September 11th show that this was indeed the case. "However, the World Trade Center was never designed for the massive explosions nor the intense jet fuel fires that came next—a key design omission," stated Eduardo Kausel, another M.I.T. professor of civil and environmental engineering and panel member. The towers collapsed only after the kerosene fuel fire compromised the integrity of their structural tubes: One WTC lasted for 105 minutes, whereas Two WTC remained standing for 47 minutes. "It was designed for the type of fire you'd expect in an office building—paper, desks, drapes," McNamara said. The aviation fuel fires that broke out burned at a much hotter temperature than the typical contents of an office. "At about 800 degrees Fahrenheit structural steel starts to lose its strength; at 1,500 degrees F, all bets are off as steel members become significantly weakened," he explained.
Some have raised questions about the degree of fire protection available to guard the structural steel. According to press reports, the original asbestos cementitious fireproofing applied to the steel framework of the north tower and the lower 30 stories of the south were removed after the 1993 terrorist truck bombing.Others have pointed out the possibility that the aviation fuel fires burned sufficiently hot to melt and ignite the airliners' aluminum airframe structures. Aluminum, a pyrophoric metal, could have added to the conflagrations. Hot molten aluminum, suggests one well-informed correspondent, could have seeped down into the floor systems, doing significant damage. "Aluminum melts into burning 'goblet puddles' that would pool around depressions, [such as] beam joints, service openings in the floor, stair wells and so forth...The goblets are white hot, burning at an estimated 1800 degrees Celsius. At this temperature, the water of hydration in the concrete is vaporized and consumed by the aluminum. This evolves hydrogen gas that burns. Aluminum burning in concrete produces a calcium oxide/silicate slag covered by a white aluminum oxide ash, all of which serve to insulate and contain the aluminum puddle. This keeps the metal hot and burning. If you look at pictures of Iraqi aircraft destroyed in their concrete shelters [during the Persian Gulf war], you will notice a deep imprint of the burned aircraft on the concrete floor.
Though the Boeing 767s airliners that hit the towers were somewhat larger than the Boeing 707 (maximum takeoff weights: 395,000 pounds versus 336,000 pounds) the structures were designed to resist, the planes carried a similarly sized fuel load as the older model—about 24,000 gallons versus 23,000 gallons, according to Kausel. "Most certainly," he continued, "no building has or will resist this kind of fire." The sprinkler system, which was probably compromised, would have been are useless against this kind of fire, he said, adding, "The World Trade Center towers performed admirably; they stood long enough for the majority of the people to be successfully evacuated."
Kausel also reported that he had made estimates of the amount of energy generated during the collapse of each tower. "The gravitational energy of a building is like water backed up behind a dam," he explained. When released, the accumulated potential energy is converted to kinetic energy. With a mass of about 500,000 tons (5 x 108 kilograms), a height of about 1,350 ft. (411 meters), and the acceleration of gravity at 9.8 meters per second 2, he came up with a potential energy total of 1019 ergs (1012 Joules or 278 Megawatt-hours). "That's about 1 percent of the energy released by a small atomic bomb," he noted.
The M.I.T. professor added that about 30 percent of the collapse energy was expended rupturing the materials of the building, while the rest was converted into the kinetic energy of the falling mass. The huge gray dust clouds that covered lower Manhattan after the collapse were probably formed when the concrete floors were pulverized in the fall and then jetted into the surrounding neighborhood. "Of the kinetic energy impacting the ground, only 0.1 percent was converted to seismic energy," he stated. "Each event created a (modest-sized) magnitude 2 earthquake, as monitored at Columbia University's Lamont-Doherty Observatory, which is located about 30 kilometers away from New York City." Kausel concluded that the "the largest share of the kinetic energy was converted to heat, material rupture and deformation of the ground below."Despite the expert panel's preliminary musings on the failure mechanisms responsible for the twin towers' fall, the definitive cause has yet to be determined. Reportedly, the National Science Foundation has funded eight research projects to probe the WTC catastrophe. The American Society of Civil Engineers is sponsoring several studies of the site. Meanwhile the Structural Engineering Institute of the American Society of Structural Engineers has established an investigative team to analyze the disaster and learn from the failure. W. Gene Corley, senior vice president of the Construction Technology Laboratory in Skokie, Ill., is said to be heading the ASSE study team through its initial phase of data gathering, and then William Baker, a structural engineer at the Chicago-based firm of Skidmore Owings & Merrill in Chicago, will lead the following analysis phase. The Structural Engineering Institute is to partner with the American Institute of Steel Construction, the National Fire Protection Association and the Society of Fire Protection Engineers. The Federal Emergency Management Agency has been invited to join as well.
How the Towers FellGiven the lack of firm conclusions regarding how the collapses occurred, the M.I.T. panel participants asked their audience to consider various theories they put forth. In general, it was agreed that as the structure warped and weakened at the top of each tower, the frame, along with the concrete slabs, furniture, file cabinets and other materials, became an enormous consolidated weight that eventually crushed the lower portions of the structure below. The details of how the frame members failed remain under contention.
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Professor Connor's theory focused on weaknesses in how the vertical and horizontal structural members were tied together. During construction, he explained, each prefabricated floor system was lifted into place by a crane and "supported at the ends like a hinge, where they were bolted and welded to the inner and outer framing tubes" so that part of the gravity load went through the core and the other part through the exterior structure. "The floor trusses sat on beams and were tied down so the core was locked to the exterior," he said. "It was an unusual system and very lightweight. If you lose the connection between them, however, you lose the ability to carry the floor loads and allow the floors to slide back and forth under stress. If a damaged floor system were to fall, it would break the end connections in the lower floors and down and down the floors would go."
"In my theory, the hot fire weakened the supporting joint connection," Connor continued. "When it broke, one end of a floor fell, damaging the floor system underneath, while simultaneously tugging (pulling) the vertical members to which it was still attached toward the center of the building and down." This phenomenon started a parasitic process that accelerated until total failure and the structure fell in on itself, he said.
Eduardo Kausel proposed an alternative failure explanation that he acknowledged was independently developed by Zdenek Bazant, a professor at Northwestern University. "I believe that the intense heat softened or melted the structural elements—floor trusses and columns—so that they became like chewing gum, and that was enough to trigger the collapse," he said. "The floor trusses are likely to have been the first to sag and fail. As soon as the upper floors became unsupported, debris from the failed floor systems rained down onto the floors below, which eventually gave way, starting an unstoppable sequence. The dynamic forces are so large that the downward motion becomes unstoppable."
Via two simple models, Kausel was able to determine that the fall of the upper building portion down onto a single floor must have caused dynamic forces exceeding the buildings’ design loads by at least an order of magnitude. He also performed some computer simulations that indicate the building material fell almost unrestricted at nearly the speed of free-falling objects. "The towers' resistive systems played no role. Otherwise the elapsed time of the fall would have been extended," he noted. As it was, the debris took about nine seconds to reach the ground from the top.
"It's difficult to judge which of these failure mechanisms occurred first; probably all occurred and interacted," said panel member Oral Buyukozturk, professor of civil and environmental engineering at M.I.T.. "The prolonged effect of high heat is likely to have led to the buckling of the columns, collapse of the floors, as well as to the shearing of the floors upon the failure the joints." He noted that videotapes of the catastrophe showed some tilting of the top portion of the south tower before it collapsed. "This indicates the buckling of one building face while the adjacent face was bending [placed into tension]." After that, the upper portions of the tower are shown disintegrating, with "a dynamic effect and amplification process" following that led to a progressive collapse—"a kind of pancaking or deck of cards effect"—down to ground zero, Buyukozturk stated.Kausel addressed the oft-asked question of why the towers did not tip over like a falling tree. "A tree is solid, whereas building is mostly air or empty space; only about 10 percent is solid material. Since there is no solid stump underneath to force it to the side, the building cannot tip over. It could only collapse upon itself." Robert McNamara said his failure mechanism theory "focuses on the connections that hold the structure together," but he cautioned that "we really need to wait for a detailed investigation, before we decide if we have to up the code ratings for these connections in signature structures."
Protecting Skyscraper Occupants
The expert panel then turned its attention to changes for future tall structures in the wake of what has been learned. Though the recent "disaster couldn't be envisioned as a design scenario in the 1970s, it means we have to change the way we design and construct tall buildings in the future," Buyukozturk said.
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Existing skyscrapers should probably be retrofitted with some additional safety measures, but the professors say that it doesn't make sense economically—and aesthetically—to protect them all physically from similar catastrophes. "Retrofitting is very expensive and is therefore usually done only for monumental buildings," Connor said.
"There will never be a building that won't fall," Kausel noted. "The best we can do is to ensure that it will stand long enough for all the people to escape." Back when the WTC was built, no one seems to have anticipated the need to evacuate an entire large building at once. To do so successfully means boosting a building's structural redundancy—the provision of additional means to assist system function. Panel members discussed providing improved fire protection for the structural elements, alternative load paths to stand in for damaged structures and fixing diaphragm floor beams more strongly to vertical members. Also mentioned was the idea of installing blast-resistant, energy-absorbing materials such as concrete-encased steel exterior columns and/or cavities (reinforced concrete cores) in future large structures that could help them survive or at least promote failure in certain slower, less deleterious sequences.
One audience attendee, a West Coast-based structural engineer who did not give his name, created a provocative moment when he claimed that it would cost about 10 percent more than the original building cost to install floor joint reinforcements for greater redundancy. "According to our analysis, it could add several more hours to the evacuation period," he stated. "If each tower cost about $1 billion to build, then an extra hundred million dollars could have saved most of the occupants. Though it's horrible to contemplate," he continued, "a human life is valued for insurance purposes at about a million dollars apiece, so this helps put the extra investment into perspective. After all, the World Trade Center was retrofitted with 10,000 viscoelastic dampers to reduce its swaying, so safety improvements can't be ignored. Building clients have to become more demanding, even if the probabilities of a repeat disaster are very slim...."
The panel also considered the need to improve the effectiveness of building safety systems. Kausel pointed out problems with the twin towers' emergency communications systems ("just when coordination was most critical, the people didn't know what to do"), the emergency illumination system and protection against smoke ("the great killer in building fires is smoke inhalation"). He also suggested that more effort should be expended to create "alternative escape routes, so evacuees aren't faced with a wall of smoke. If two stairwells are close together," he noted, "one explosion could block them both." Other ideas floated included installing better fire-suppression systems, using the aqueous film-forming foams employed in aviation fires, and creating protected access ways for firefighters. There was also mention of the need to harden stairwells and egress pathways, and perhaps develop "deployable evacuation systems" for building occupants. Beyond robotic stairway evacuation devices, deployable systems might include escape tubes deployed out windows, exterior people-lowering machines, flying platforms or even parachutes."
One audience member asked the assembled experts whether a reinforced concrete skyscraper such as the current height record-holder, the 452-meter Petronas Towers in Kuala Lumpur, Malaysia, would have better resisted a collision with a fuel-filled airliner. Their response indicated that a concrete structure would have probably lasted for a couple of more hours than did the steel World Trade Center towers. Robert McNamara stated that many of the more recent superskyscrapers were constructed of reinforced concrete mainly because of the high cost of steel in Asia. He also mentioned that the Petronas Towers contain "safe refuge floors" to allow building occupants to reach fresh air during fires. McNamara said that this concept was now somewhat discredited, as similar refuges in the WTC would not have ultimately saved anyone.A lively discussion then ensued about whether the terrorist pilots knew where to hit the buildings for maximum effect. McNamara opined that the position of impact seems significant. "They hit them at just the right place—about two thirds to three quarters of the way up. The earlier [truck bomb] attack showed that the explosion at bottom had little effect and that it's much easier to collapse a building from the top than the bottom. If they had hit the very top of the building, the fire damage wouldn't have had such a catastrophic effect. At the bottom, the columns are much heavier and stronger and so they would have taken a much larger load." Connor offered that one would "need graduate-level engineering training to choose the prime target location."
In the aftermath of the World Trade Center disaster, questions arose whether superskyscrapers should be built in the future. Clearly, these top engineers would reply in the definite affirmative. Inevitably, they said, new tall towers will rise.
Why the Twin Towers Fell
Why the Twin Towers Fell | ||||||||||
Engineers who studied the World Trade Center after the September 11 attacks tell why the Twin Towers stood as long as they did, and why they eventually collapsed. For the most current information on New York's World Trade Center, be sure to visit our WTC Index. | ||||||||||
New York's World Trade Center Twin Towers were designed to withstand fire and hurricane-force winds. Some engineers believed the Twin Towers could even survive impact from a Boeing 707. But no engineer or architect could have anticipated the terrorist attack that turned the Twin Towers to rubble, and experts often don't agree what steps might have been taken to make the buildings stronger. For the most recent findings and detailed technical information, be sure to visit the World Trade Center Coverage page at icivilengineer. How did the Twin Towers fall?1. Impact from the Terrorist PlanesWhen Boeing jets piloted by terrorists struck the Twin Towers, some 10,000 gallons (38 kiloliters) of jet fuel fed an enormous fireball. But, the impact of the planes and the burst of flames did not make the Towers collapse right away. Like most buildings, the Twin Towers had redundant design. The term redundant design means that when one system fails, another carries the load. Each of the Twin Towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. When some columns were damaged, others could still support the building. 2. Heat from the Fires 3. Collapsing Floors Why did the collapsed towers look so flat?Before the terrorist attack, the Twin Towers were 110 stories tall. Constructed of lightweight steel around a central core, they were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.Could the World Trade Center have been made stronger?In a report produced by the Federal Emergency Management Agency (FEMA), the American Society of Civil Engineers (ASCE), and other organizations, experts concluded that no skyscraper could have withstood the impact of the terrorist airplanes. Further, the experts warned that it would not be "technically feasible" to design a building that could survive this type of terrorist attack. Instead, engineers and architects are suggesting that we focus our efforts on designing better warning and evacuation systems so that we can save more people inside the buildings.Is it possible to design a truly indestructible building? Tell us what you think. See photos and read more about the construction of the Twin Towers > |
William F. Baker
First public hearing of the National Commission on Terrorist Attacks Upon the United States
Statement of William F. Baker to the
National Commission on Terrorist Attacks Upon the United States
April 1, 2003
I wish to extend my sincere appreciation for being asked to address the Commission on behalf of the ASCE/FEMA World Trade Center Building Performance Assessment Team.
My comments relate to my role on the ASCE/FEMA team as well as my experiences as a structural engineer assisting the fire fighters and contractors in the early days after the attack on the World Trade Center. In addition, I will offer key recommendations that may be valuable to New York and other urban areas in dealing with the aftermath of future terrorist attacks. These recommendations include:
- Immediate coordination of on-site structural engineers;
- An archived depository for construction drawings of all citywide buildings and infrastructure; and
- Federal "good Samaritan" legislation
When I arrived at the WTC site, I was awestruck at the extent of the devastation. While the media focused on the destruction of the twin towers, the damage went far beyond anything that was conveyed to the general public. The damage included the total collapse of four major buildings, the partial collapse and burnout of three major buildings and extensive damage to seven additional major buildings. Many other buildings suffered minor damage. The district's infrastructure including utilities and the subway system area was extensively damaged and parts of it were destroyed.
The New York City Department of Design and Construction (DDC) performed a brilliant job in organizing the efforts of the engineering and construction industries to support the search and rescue and, later, clean up the site. DDC divided the district into four sectors. Each sector was assigned to a team of contractors. This was a very successful approach.
Because the contractors were top-notch and adept at managing very large private and public sector projects, they knew how to organize the teams, deal with heavy equipment and marshal resources. However, the contractors needed the professional assistance of structural engineers.
The Structural Engineers Association of New York (SEAoNY) stepped forward to organize the services of structural engineers from across the city, state and country. SEAoNY assembled teams of structural engineers to assist each of the (4) contractor teams. I urge the Commission to use this approach developed by the DDC and SEAoNY as a model for dealing with possible large urban disasters. In many ways, New York City was fortunate to have the DDC and major players from the engineering/construction industries available to assist at the WTC site.
In the first, critical hours and days following the attack, what was not available or well organized were the drawings of the buildings, plazas, subway tunnels, freight tunnels, etc. As engineers and contractors were investigating the extent and severity of damage, drawings of the original structure were sorely needed. When it was necessary to bring very large cranes and other equipment across tunnels, vaults and plazas, drawings were an absolute necessity. I strongly urge all major cities to develop archive depositories for construction drawings and other critical information to be available to the authorities on short notice. A duplicate copy should be housed in a redundant location.
The structural engineers who assisted at the WTC site were often in uncharted territory with respect to professional liability. There should be appropriate national "Good Samaritan" legislation to promote the assistance of structural engineers in such situations. While such legislation exists in high seismic states such as California, all states are vulnerable to terrorist attacks and natural disasters and should have appropriate legislation. Federal "good Samaritan" legislation may be appropriate.
At this point, I would like to focus my comments on the efforts of the ASCE/FEMA Building Assessment. The Structural Engineering Institute (SEI), a division of the American Society of Civil Engineers (ASCE), was responsible for organizing this effort and bringing together the relevant professional societies with support from FEMA, for an assessment of the WTC site. Under the leadership of ASCE, a team of structural engineering and fire engineering experts from around the country were brought together. Because of my expertise in tall buildings, I was asked to join the effort and be on the five-member core group that directed the assessment.
This type of effort is important because advancement in the construction of buildings has often come from the analysis of failures. These tragic events of 9-11 provided an opportunity to see how building emergency systems and structural systems behave in extraordinary events. Although the media has focused on the twin towers, there is more to be learned from the behavior of the more "ordinary" buildings that were damaged by the events. We saw and documented the performance of structures that resisted extraordinary forces and maintained their overall integrity. We also saw and documented collapses that, based on previous experiences, were unexpected. It is through the study of these behaviors that the art of building design is advanced.
Unfortunately, the ASCE/FEMA team faced many obstacles while studying the WTC. The team was not able to assemble on the site until October 6th. We could only request and cajole to get drawings and other information. And, in fact, we did not receive access to the twin tower drawings until January. Nonetheless, the team was able to perform an invaluable service in our initial, overall evaluation of the buildings in order to focus and prioritize future investigations and research.
Fortunately, the National Construction Safety Act that was signed into law on October 1, 2002 addresses the difficulties faced by the ASCE/FEMA team at the WTC site. This act authorizes the National Institute of Standards and Technology to investigate building failures. This is similar to the National Transportation Safety Board investigations of airline and other transportation accidents.
This act allows NIST teams to access building failure sites; provides the power to subpoena evidence; provides access drawings, records and other documents; and allows for the removal and storage of evidence.
This legislation is a significant step forward in creating a vehicle by which the design and construction industry can learn from failures. This will help to advance building technology and improve the safety and reliability of future construction.
As a structural engineer, the WTC collapses represent the largest structural failure in the history of mankind. From this tragedy, I am confident that we can learn how to approach catastrophic building failures in the future and through the National Construction Safety Act we will continue to learn how to improve building construction.
Thank you for your attention.
Mr. Baker is the partner in charge of Structural and Civil Engineering for Skidmore, Owings & Merrill LLP (SOM). Mr. Baker has been involved in a variety of innovations in building systems, including low-rise and high-rise buildings, long span roofs and special structures. Current projects include Tower Palace III, a 69-story, 2-million sf residential tower for the Samsung Group, which is nearing completion in Seoul, Korea, as well as Trump Tower Chicago, a 2.6 million sf tower that will be completed in Chicago, Illinois in 2005.
In the weeks just after the WTC disaster, Mr. Baker provided structural engineering assistance to search and rescue teams in addition to evaluating damaged buildings and advising contractors on debris removal. He has recently been appointed as one of five members of the Core Group of the ASCE/SEI Building Assessment Team that is evaluating the WTC disaster.
Mr. Baker was educated at the University of Illinois, where he received a Master of Science in Civil Engineering in 1980. He received his Bachelor of Science in Civil Engineering at the University of Missouri, in 1975.
Building Operating Management
The Future of High-Rise Building Design
How will the collapse of the World Trade Center towers change tall buildings?
By BOM Editorial Staff
Email the BOM editors
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But even as measures to thwart future attacks were put in place, one issue remained shrouded: Why did the towers collapse?
Now, after nearly four and a half years of intensive study, there is consensus about how the towers collapsed. But even today, there is no agreement on the lessons that can be drawn from that tragedy. The most ambitious attempt to define those lessons came in the final report on the collapse of WTC 1 and WTC 2 by the National Institute of Standards and Technology (NIST). That report offered 30 recommendations on ways that tall buildings can be made safer.
Some recommendations are concerned with structural stability. Others discuss how emergency responders should react to situations involving high-rises. Others lay out improved procedures for evacuating buildings.
The most far reaching of the recommendations have provoked a debate among facility executives, code officials and others involved in building, designing and operating high-rise structures.
This series of articles explores some of the issues that will determine how high-rises are constructed in the years to come. The first article looks at the NIST effort as a starting point for the discussion on how high-rises can be made safer. The second article looks at the issues surrounding a single recommendation — full building evacuation — that would change how building occupants and emergency workers respond to emergencies in high-rises. A third article looks at an element of building operations that NIST was not charged with investigating: building security.
As industry bodies grapple with possible changes to codes and standards, they will have to decide what sorts of disasters high-rises should be designed to survive. Although NIST officials say the recommendations were not developed solely on the basis of the terrorist attacks, others question whether they go too far in protecting against other kinds of threats. Those critics say that the World Trade Center attacks are unique, and therefore should not be used to justify taking very costly steps to implement some of the NIST recommendations.
Some industry observers might also use the contents of a forthcoming NIST report investigating the collapse of a third World Trade Center tower — WTC 7. Because that tower was spared the impact of an airliner, many are viewing its collapse as more typical of the type of emergency high-rises around the nation might face. That tower was spared the impact of an airliner. Although the NIST report due out in spring will detail the cause of the tower’s collapse, the agency has already reported fires resulting from the initial attack on the other two towers played a role.
Decisions of whether to require building codes that call for thicker fire-resistive coatings on high-rise structural members, use of redundant sprinkler systems, for example, will be influenced by whether such improvements are worth the expense. Many of the NIST recommendations will cost building owners money. In the case of some recommendations, perhaps a lot of money.
The codes and standards groups reviewing the recommendations face an arduous task. On one side, they’ll hear from a group of people who say nothing in the NIST report is worth implementing. On the other side, they’ll hear that everything in the report is not only valid, but that some specific recommendations don’t go far enough toward protecting buildings.
Code groups will be charged with plotting the middle ground without knowing exactly what future terrorist acts or natural disasters will test the limits of high-rise design. Those groups are charged with finding the elusive answer to the question, “How safe is safe enough?”
forming codes and standards
Speaking In Code
National organizations consider fundamental changes to how
high-rises are designed, built and operated
By Mike Lobash, Executive Editor
The collapse of the World Trade Center towers triggered an investigation unprecedented in cost and scope. Nearly 200 investigators spent more than $40 million and three years to determine why the towers collapsed and to issue recommendations on how to make high-rises safer.
Now, with 43 reports constituting more than 10,000 pages filed, the question has changed. No longer is the question, “Can high-rises be safer?” Rather, it is, “How safe should high-rises be?”
“The bottom line is no one can tell us how safe is safe,” says Gerald Jones, co-chairman of a National Institute of Building Sciences (NIBS) committee reviewing recommendations in the federal government’s final report on the collapse of two of the three World Trade Center towers that were destroyed in the attacks. “Everything is a compromise between safety and economics.”
Code organizations will likely spend the next several years discussing 30 recommendations contained in a National Institute of Standards and Technology (NIST) report aimed at improving the safety of buildings taller than 120 feet. At issue in the discussion will be whether the recommendations — taken individually — should be made part of the model building codes and national standards that govern how buildings are designed and constructed.
The recommendations are broken into eight subject areas. They are:
- Increased structural integrity.
- Enhanced fire resistance of structures.
- New methods for fire resistant design of structures.
- Improved active fire protection.
- Improved building evacuation.
- Improved emergency response.
- Improved procedures and practices.
- Continuing education and training.
As the recommendations are reviewed, some are likely to become model building codes with little debate. And a few recommendations, particularly those regarding education and training of emergency responders and of architects and engineers, require little or no action by code and standards organizations. One recommendation suggesting that academic, professional short courses and Web-based training materials be offered in the use of computational fire dynamics and thermodynamics, for example, affects professional associations and doesn’t require code changes.
Fundamental Shift
Other recommendations will prove to be more controversial, with the amount of debate surrounding the adoption of each being proportionate to the cost involved with implementing it. Perhaps none of the recommendations are more controversial than the ones that aim to change the way high-rises are evacuated. One recommendation calls for full building evacuations during building-specific and large-scale emergencies, such as widespread power outages, fires, explosions, terrorist attacks and unforeseen natural disasters. Another would require high-rises to contain more remote exits and two sets of structurally hardened elevators. During emergencies, one set of elevators would be used by occupants and the second set by emergency personnel. The exact number of exits and elevators required would depend upon building size and the number of occupants.
Those recommendations constitute a sea change in the approach to the way high-rises are designed to accommodate evacuations. By and large, high-rises are designed to handle the evacuation of a single floor. If necessary, multiple floors are evacuated one at a time in what’s known as a staged evacuation.
That was the design approach at the World Trade Center towers, says Shyam Sunder, deputy director of NIST’s Fire Research Laboratory and lead investigator on the collapse of the towers. Stairwells were designed to be wide enough and positioned so that 390 occupants — the maximum on any one floor — could be evacuated at one time.
On Sept. 11, 99 percent of the people located below the floors of impact were able to escape the World Trade Center towers before they collapsed, even though the building wasn’t designed to accommodate full building evacuations, according to the final NIST report on the collapse of the towers. The story would have been different if the buildings were fully occupied.
Beyond Terrorism
Even so, Sunder says the recommendations in the NIST report were not based on the ability of a high-rise to withstand the impact of an airliner or to evacuate occupants after such a terrorist attack. Rather, they were developed to aid the evacuation of buildings for any emergency situation, including earthquakes, widespread power outages, tornadoes and other events.
“The need to evacuate a high-rise might need to happen even without a terrorist attack,” Sunder says. “If you look at the nominal life of a building as 100 years, there are likely emergencies that require full evacuation of buildings.”
If the recommendations will make high-rises safer in case of events other than terrorist attacks, that would help justify their being adopted in model codes. Some perceive the NIST report as being narrowly focused on preventing the catastrophic results of Sept. 11.
Samir Mokashi of the engineering firm IDC says immediately following the terrorist attacks, everybody thought that hardening buildings to withstand the impact of airliners was a potential design goal. But now that the event hasn’t been repeated in more than four years, support for that idea has waned.
“Is a plane going into a building something we need to protect against?” he asks.
Phil DiNenno, one of eight members of the National Construction Safety Team Advisory Committee, which advised NIST in its investigation, says of the 30 recommendations issued by NIST, one requiring additional remote exits might have changed the outcome of the attacks on Sept. 11. The addition of those exits — stairwells on the corners and on the perimeter of the building — might have allowed more occupants located above the floor of impact to evacuate.
“But that is singularly an expensive change,” says DiNenno, who is also president of Hughes Associates, a fire protection engineering firm.
The expense comes not only in the design and construction of additional exits, but in the loss of rentable real estate, most notably, the amount of premier daylit space landlords could lease to tenants along a building’s perimeter.
As the recommendations work their way through the code adoption process, expect debate among code-making bodies, building owners — who will ultimately fund the cost of the new codes through increased high-rise design and construction expenses — and building component and material manufacturers whose businesses stand to benefit or suffer from the adoption of certain recommendations as codes.
Seeking Help
NIST has already contracted with NIBS to work on turning the recommendations in the final WTC report into code language that can be adopted by model code groups, including the International Code Council and the National Fire Protection Association (NFPA). Jones’ committee, aptly called the Committee for Translating the NIST WTC Investigation into Model Building Codes, is moving quickly to get at least some changes incorporated into the 2007 supplement to the International Building Code
. The deadline for submitting code change proposals for consideration in the next code cycle is March 24, with final action scheduled for May 2007.
“We’re going to do our best to get at least some of the recommendations adopted in the current code process,” Jones says.
Some of the measures recommended by NIST began working their way into codes and standards even before the agency completed its investigation. For example, NFPA included a provision in its 2003 edition to require wider stairwells for buildings with more than 2,000 occupants. In the 2006 code cycle, there were no dramatic code changes based on the NIST report, says Gary Keith, vice president of building and life safety for NFPA.
For the 2009 code cycle, which will be available for public comment in 2007, Keith says the association’s High-Rise Safety Advisory Committee will review the recommendations and monitor proposals for code changes.
“The questions will be whether anyone looking at the report sees recommendations that are worthy of code changes,” he says.
Shortly after NIST’s final report was issued, BOMA International, representing building owners and managers, issued a statement calling for further study, including a risk-benefit analysis, into some of the recommendations. One comment took issue with a NIST recommendation calling for the development of national standards and codes to prevent the progressive collapse of buildings.
“The big issue here would be to what extent it would be required to prevent progressive collapse?” wrote BOMA’s David Johnston, a member of the organization’s advocacy staff, in submitting written comments to NIST. “How many column failures (specific number, a percentage of a floor, etc…) would need to be able to be withstood? ‘Progressive collapse should be prevented’ is a pretty vague goal without a better explanation of what factors must be considered.”
Another recommendation that has drawn early criticism from BOMA is one requiring the fire resistance performance of structures. NIST recommends that uncontrolled building fires result in burnout without a structure suffering either partial or total collapse.
“The effort is a bit unrealistic and unwarranted,” BOMA wrote. “NIST assumes the complete loss of the sprinkler system, and no intervention by fire departments.”
As the process of reviewing the recommendations moves forward, code-making bodies will likely hear those types of comments as well. While the arguments might not be geared at questioning whether a certain code will make buildings safer, they will be pointing out that the cost of implementing a code might not be worth the expense.
“The real question as the process moves forward is how much will a change cost,” DiNenno says. “That’s the real world where code is written.”
elements of future high-rises
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new evacuation procedures proposed
Evacuation 911
Full building evacuations and use of elevators during emergencies are among sweeping proposals for egress procedures
By Brandon Lorenz, Senior Editor
After more than four years, it’s possible to forget not only how tragic the events of Sept. 11, 2001, actually were, but also how much more tragic they could have been. Had the buildings been fully occupied, the National Institute of Standards and Technology (NIST) has estimated that it would have taken more than three hours to fully evacuate the buildings. In the process, 14,000 people — 28 percent of the occupants — would have died because of insufficient stairwell capacity.
Those stark numbers prompted NIST to call for changes in high-rise building design to improve evacuation time in an emergency. If the recommendations are adopted, facility executives could one day oversee high-rise buildings that have timely full building evacuation plans for non-fire emergencies. The buildings would have hardened elevators and stairwells. Stairwells would be wider and spaced farther apart, and upgraded elevators would be used for evacuation and for emergency responders.
But the NIST recommendations are both controversial and broadly defined, which means industry code bodies will have a significant role to play shaping the recommendations into code — if they are adopted at all.
New Approach to Evacuation
High-rises have been built assuming that the entire building wouldn’t need to be evacuated at once. Instead, during a fire on one floor, occupants evacuate to adjacent floors until it is safe to return.
What does timely full building evacuation entail? NIST’s recommendation says that a building’s size, population, function and iconic status should be taken into account in designing an egress system. “The key question is how much time should be allowed or specified for total safe evacuation,” says Milosh Puchovsky, principal fire protection engineer for the National Fire Protection Association (NFPA).
There is little agreement about how much time should be allowed for a full building evacuation because the topic is relatively new, says Ron Klemencic, chairman of the Council on Tall Buildings and Urban Habitat. “Clearly, if you put in 10 stairwells, you could get people out of the building much faster,” he says. “But how fast is fast enough?”
A full building evacuation during a prolonged power outage, for example, would not necessarily need to be rapid. An explosion, however, might require faster evacuation.
Even if there is disagreement regarding how fast a full building evacuation should be, elevators, stairwells and evacuation plans will all need major revisions to make full evacuation practical.
Structural Changes Required
The final NIST report on the collapse of the World Trade Center towers indicated that the overall evacuation rate in WTC 2 was 108 survivors per minute, about 50 percent faster than WTC 1 at 73 survivors per minute. That’s because occupants in WTC 2 used the elevators in the 16 minutes before the second tower was struck.
For more than 30 years, building occupants have been told not to use an elevator during a fire. With new recommendations from NIST for full building evacuation, the American Society of Mechanical Engineers (ASME) is examining whether elevators could safely be used to evacuate high-rises during an emergency.
Designing elevators that could safely be used during a fire or other emergency is a serious challenge, says Edward Donoghue, administrator for the National Elevator Industry Inc. (NEII) and a safety and codes consultant. For example, one hazard that needs to be overcome is keeping water used during firefighting out of the hoistway, Donoghue says. ASME is conducting separate hazard analyses to see whether elevators can safely be used by occupants for evacuation and whether they can also be used for emergency responders during a fire or other emergency. The NIST report also recommends emergency responders have access to a dedicated elevator.
“It’s a very extensive analysis,” Donoghue says. “It’s probably going to take another couple of years to complete.”
Upsides and Downsides
While elevators factor into the timely evacuation of high-rise buildings, NIST has not ignored stairwells. NIST recommends designing stairwells wide enough to accommodate both descending occupants and ascending emergency responders. That means wider stairwells. “For certain size buildings, provisions have already been implemented in NFPA 101 and NFPA 5000 that require additional stair width to accommodate the counterflow of emergency responders,” says Puchovsky. NFPA 101 is a life safety code. NFPA 5000 is a model building code.
The 28-story Cira Centre in Philadelphia, for example, has stairwells that are 50 inches wide, as opposed to the 44 inches required by code. Construction on the building began in 2003, and the stairwells were voluntarily widened, says Stephen Rush, leasing agent for Brandywine Realty Trust.
NIST has also recommended that stairwells be marked with consistent signage, be located farther apart without increasing average travel distance and maintain their integrity under foreseeable building-specific or large-scale emergencies. Each World Trade Center tower had three emergency stairwells, but only one stairwell in WTC 2 remained passable after the towers were struck, according to NIST. Increasing stairwell distance means more stairwells could move from the building core to the perimeter.
However, maximizing the distance between stairwells could slow evacuation as it takes longer for occupants to reach a stairwell, Klemencic says.
New York City’s World Trade Center Building Code Task Force issued a variety of recommendations in 2003 that are now required by the city’s Local Law 26. Three requirements regarding stairwells apply to buildings 75 feet and higher and are retroactive. Photoluminescent markings are required in all exit doors and exit stairs. Additional signage is required when the exit path is not clear and such signs must have battery or generator power.
Disaster Planning
Disaster planning in a post-9/11 world means evacuation plans should move beyond fires. Power outages, earthquakes, tornadoes, fires, explosions and terrorist attacks should all be taken into account when evacuation plans are formulated, according to NIST.
“When we are talking about full building evacuation, we are talking about evacuation for potentially any type of situation,” says Jim Carrigan, supervising engineer for fire and life safety for the Syska Hennessy Group. Prior to 9/11, for example, facility executives really didn’t talk about hazardous material issues when formulating evacuation plans.
Unlike NIST’s recommendation for full building evacuation, the recommendation for better evacuation planning appears to be gaining traction.
In one case, a tenant occupying multiple floors in a high-rise paid to have its evacuation plan reviewed and upgraded after the developer declined, says Carrigan. The original plan only contained provisions for a fire emergency, while the revised evacuation plan was broadened to take other potential emergencies into account. It was later adopted by all 40 building tenants, Carrigan says.
In New York City, Local Law 26 requires that emergency evacuation plans include non-fire-related events. “The idea was that while the codes have had sufficient fire protection and fire safety plans, the conditions of other emergencies that are not directly related to a fire have not been considered,” says Ronny Livian, chairman of the World Trade Center Building Code Task Force.
In earthquake-prone Seattle, a full building evacuation plan doesn’t necessarily make sense, says Ben Barron, vice president of development for Clise Properties, which manages 1700 Seventh Avenue, a 23-story office building. “We feel that the tenants are safer in our building than out on the street, at least until we analyze what is going on.”
Barron says that a man threatened to blow himself up while standing in the lobby of a nearby courthouse. Rather than completely evacuate the 1700 Building, tenants were moved away from the side of the building nearest the courthouse. The reason? Tenants who left the building likely would have gathered in a courtyard near the courthouse, placing them in danger.
Beyond the Exits
Facility executives can’t forget about occupants once they leave a building. Proper planning means making sure occupants remain safe once they are evacuated, says Livian.
“In a larger building, you may have an occupant load of 10,000 or 20,000,” he says. “Where do you put them and still be safe? You can’t just leave them out in the street.”
To be successful, full building evacuation plans must be tailored for specific buildings and the types of occupants in the buildings, says Carrigan. In addition, the plans require training so that facility executives know when it is appropriate to call for a full evacuation and when to call for a phased evacuation. What’s more, facility executives should not call for a full-building evacuation by paging the entire building and asking occupants to leave, as was the case on 9/11, Carrigan says.
“The purpose of a full building evacuation is to do a staged evacuation in a controlled manner,” he says. “With the current buildings, if the stairwells haven’t been widened, you have to do a staged evacuation or there will be a bottleneck.”
Counting Occupants
Comprehensive disaster planning also means accounting for mobility-impaired occupants. According to NIST, 6 percent of the occupants in the World Trade Center were mobility-impaired. In a traditional phased evacuation, mobility-impaired occupants can be moved away from a fire or other emergency with relatively little difficulty. A full building evacuation is more troublesome. Knowing which occupants are impaired and where they are located is key. Ideally, an evacuation plan also takes into account visitors and occupants who are temporarily impaired because of illness. In the case of non-fire emergencies, Carrigan recommends using elevators to evacuate mobility-impaired occupants.
In Seattle, Clise Properties has identified mobility-impaired tenants. In case of an emergency, a “buddy system” has been arranged to make sure the mobility-impaired tenants can get the assistance needed to evacuate, if necessary. The company has ordered an evacuation chair that will allow occupants to be evacuated down a stairwell, Barron says.
Even if the NIST recommendations governing full building evacuation and elevators are approved by code bodies, implementation will likely take years. While not all cities require high-rises to develop separate emergency action plans for fire and non-fire events, a proper plan that considers fire and non-fire-related emergencies can ensure that a building disaster doesn’t become a greater tragedy.
“In a post-9/11 world, everybody needs to have an emergency evacuation plan for fire and non-fire events,” says Carrigan. “We have different threats today.”
security guidelines to keep high-rises safe
De Facto Standards
Although codes don’t govern building security systems,
these high-rise systems have been upgraded across the board
By Greg Zimmerman, Managing Editor
Designing and maintaining high-rise security following 9/11 represents a multifaceted challenge to building owners. Absent a report like the one the National Institute of Standards and Technology produced with its list of recommendations for fire safety, evacuation procedures and technologies, high-rise building owners are largely on their own to determine how best to protect people and property at their buildings.
The need for particular high-rise security measures depends on a building’s location, proximity to other types of buildings, tenants, purpose and visibility, among other things. However, access control is one area of security that building owners have ramped up in high-rises nearly across the board since 9/11.
In the past, standard operating procedure had been to control access in many high-rises only after business hours, says Carlos Villarreal, vice president, national security and life safety for Trizek. Many high-rises had a single card reader in the elevator vestibule that wasn’t turned on until the close of the business day. While some access control was usually in place at the building’s main entrance, once a visitor or occupant made it through the first point, the person was free to wander.
“Traditional high-rise buildings were much like schools before Columbine,” says Sean Ahrens, senior security consultant at Schirmer Engineering and a member of the ASIS commercial real estate council. “They were open and people could walk around at will.”
Controlling Entry
Since 9/11, that has changed. Access is controlled 24/7 at elevators, including freight and service elevators. Visitors and occupants may need specific permissions tied to their credentials to gain access to certain floors. Also, gone are the days of flashing a badge to a security guard at a building’s main entrance.
“Optical turnstiles are being used more in large buildings,” says Geoff Craighead, vice president of high-rise and real estate services at Securitas and chair of ASIS commercial real estate council. “It’s all very well to have card readers at elevators, but if you can’t control piggy-backing at the main access points, then you’re not controlling access at all.”
Visitor management policies have also become more sophisticated. Gone is the simple sign-in sheet. In its place are automated systems that allow occupants to pre-register visitors, who are then given a badge, which must be returned when they leave. In many federal high-rises, or other buildings with many visitors, a dedicated visitor management station exists so as not to slow the entry of regular building occupants.
And it’s not just visitor management that’s getting its own lobby space. In general, experts say high-rise building lobbies are becoming more segmented and compartmentalized.
“It’s important to separate people that haven’t been screened yet,” says Lauris Freidenfelds, vice president at Sako & Associates. “Lobbies are getting bigger, too. The lobby is the area where there is most likely to be an incident. If there is an incident, it’s essential that the lobby is designed so the incident has the least impact on the rest of the building.”
Security by Design
To make controlling access easier, many high-rise buildings, including newly designed ones, are cutting back on the number of places people are able to get in and out. In addition, many high-rises have designated a single entry for mail, packages and large deliveries. New high-rises are being designed so that the mailroom is situated at the exterior of the building and can be cordoned off at a moment’s notice in case of a threat, says Ahrens.
Policies that govern vehicle access are receiving new scrutiny as well. It’s become common to design new structures with lower deck parking — structures with smaller floor to ceiling distances — near the building so that only small vehicles can fit. Higher deck parking for large vehicles that could potentially carry large amounts of explosives is farther away from the building. Also, experts suggest locating occupant parking near the building, with visitor or public parking located farther away. If the high-rise is serviced by underground parking, Ahrens warns that building owners should make sure measures are taken so that the building above is properly protected. “There needs to be increased vehicle screening,” he says. “That means putting mirrors under the cars as they come in and opening trunks.”
Other exterior measures intended to mitigate the effects of bombs include bollards, concrete barriers and even sculptures or artwork. Tiered landscaping or ponds, when the site allows, can also make approaching the building by vehicle more difficult.
However, if the high-rise’s site limits stand-off distances or physical measures, window film and blast-resistant glass can help reduce the impact of a bomb. New high-rises are sometimes being designed with fewer windows near grade. For instance, the new Freedom Tower, which will be built on the World Trade Center site, is being designed with no windows on the first 20 floors of the building, says Craighead.
Preventing CBR
Concerns about a possible chemical, biological or radiological (CBR) attack have led many building owners to monitor access to a once-ignored part of the building: grade-level exterior and interior air intakes. The danger is that a terrorist could dump into the air intakes an agent that is circulating through the building’s HVAC system within minutes. To combat that threat, some high-rise building owners are installing CCTV cameras with intelligent video algorithms on the air intakes. If a video frame is broken by motion, a security staff member is alerted automatically and can determine whether there is danger and, if so, begin evacuation procedures. This strategy probably won’t prevent an attack, but it can help speed the response and evacuation.
However, if the vulnerability is deemed severe enough, Craighead says some high-rise building owners have undertaken the significant cost of relocating the air intakes out of pedestrian reach.
Also, many new high-rise buildings are being designed so that exterior air intakes are at least 75 feet above ground.
In the case of an outside or nearby CBR attack, many building owners have instituted defend-in-place procedures, which were nearly nonexistent before 9/11. “Traditionally, tenants had been trained to evacuate,” says Villarreal. “But now we’re seeing seminars on how to shelter in place, and how to completely seal off a building so no one can get in or out.”
Freidenfelds says that practicing a procedure is just as important as instituting a procedure in the first place. “Make sure that response plans are exercised,” he says. “We don’t want to be dusting off these plans when there is an emergency.”
Guidelines for Securing Facilities
Several documents produced since 9/11 aim to provide direction to building owners on improving security. While these documents are all applicable to high-rise buildings, and security personnel responsible for those structures, none was developed specifically for tall buildings. They apply to a range of occupancies.
The newest documents that can help building owners cope with these challenges were developed by the National Fire Protection Association (NFPA). The project to develop the documents began in 1994 on a request from the insurance industry. After several starts and stops, constant goading by the insurance industry, and the terrorist attacks of 9/11, NFPA redoubled its efforts and released the documents in July 2005.
“NFPA was already going down the path to security guidelines,” says Rich Bielen, chief systems and applications engineer at NFPA. “But 9/11 pushed everything.”
NFPA 730, Guide for Premises Security, covers vulnerability assessments, designing a security plan and suggested security measures for different occupancies. NFPA 731, Installation of Premises Security Equipment, covers the application and installation of physical security technologies. Basically, NFPA 730 is the “what”; NFPA 731 is the “how.”
Bielen says 730 is considered a guide while 731 is considered a standard, which is more stringent. Originally, 730 was going to be a standard as well.
“Many of the users on the committee were not comfortable with the mandatory requirements format, so the committee debated whether to make it a standard, a recommended practice or a guide,” says Bielen. “The guide format was the most agreeable format as a mix of recommendations and information.” Bielen says the ultimate goal is for both 730 and 731 to become regulation.
Instead of Code
Currently, no regulations or codes exist that govern how building security should be planned and executed. That’s because creating security codes would be like creating separate fire codes for each building, says William Sako, executive vice president of RJA Group and chairman of the board for Sako & Associates. A central challenge for a high-rise building owner is determining what risks or threats exist for a specific property. Identifying where dollars should be spent to address the most urgent vulnerabilities and selecting systems that best mitigate threats are the other critical challenges.
ASIS has also developed several guidelines, which are oft-consulted tools for building owners to help assure that their security plans are on par with those of similar facilities in similar security situations.
“We create guidelines that are broad enough so that they’re applicable to Fortune 500 companies, as well as companies with 50 employees,” says Regis Becker, chairman of ASIS’s guidelines commission and global director of security and compliance at PPG Industries. “Security is more an art than a science.”
Topics such as business continuity, risk assessment, responsibilities and accountabilities of a chief security officer, private security officer selection and training, and workplace violence prevention and response are covered.
A guideline on selecting physical security elements, which Becker says will be most applicable to high-rise buildings, is under development.
As is the case with the NFPA and its documents, ASIS is an American National Standards Institute (ANSI)-accredited national standards developer, and all its documents were developed according to ANSI protocol.
—Greg Zimmerman, managing editor
