BY GERALD TRACY
Today’s fires burn with such intensity that the structural integrity of high-rise buildings and other structures constructed worldwide are more susceptible to compromise and failure. This was evident on March 30, 2017, with the failure and collapse of a portion of the Interstate 85 highway overpass in Atlanta, Georgia.1 You would think that the massive concrete elements supporting a section of highway would survive exposure to fire, but they did not, and the highway collapsed. Would a concrete high-rise building fail under similar circumstances? Historically, high-rise structures are built of steel, reinforced concrete, or a combination of both. Now, wood with cross-laminated timber elements is being introduced to support high-rise “ply-scrapers” to grace our skylines and further challenge the fire service.
多多在线观看免费视频The public and the fire service trust that every edifice is designed and erected so that it will effectively maintain its structural stability if exposed to fire and that the passive and active fire protection systems the building codes require will operate and perform efficiently. Yet, in reality, this tacit covenant of trust has been compromised. The stakeholders who own, design, and build these monoliths have failed to account for the reality and potential of today’s fuel loads, the potential heat release rates, and the time to full fire involvement of open spaces allowed without sprinklers.
Everyone involved in the construction of a high-rise building from the planning to the issuance of a certificate of occupancy is accountable for the life safety of the public and the firefighters who will respond and remain in the building to extinguish a fire. Any oversights, imperfections, or modifications during the construction phase could introduce a plane of weakness to any portion of the structure.
Using computers and other technology, structural engineers can calculate the maximum strengths of steel, concrete, and wood materials using minimal material. However, the equations used to calculate compliance to fire resistance standards are flawed.2多多在线观看免费视频 Unfortunately, the fire resistance requirements as written are based on the fire loading (fuel package) standards of the early 1900s! Fires involving those types of fuels attained their peak temperatures at a slower rate than fires involving today’s fuels.
2003 Cook County (IL) Administration Building Fire
After the tragic Cook County (IL) Administration Building (CCAB) fire on October 17, 2003, the National Institute of Standards and Technology (NIST) investigated to determine the progression of the fire once it ignited and the energy it would have released.3 NIST acquired the identical fuel loading to perform a test burn in its research lab. The fuel load for the research was a four-cubicle work station with a suspended ceiling representing the height and configuration of the office occupancy (fire location) of the 12th floor of the CCAB.
Once ignited, the fuel (work stations) flashed over in less than five minutes and produced an energy output of more than 20 megawatts. The temperature measured above the suspended ceiling was close to 1,700°F (950°C), well above the temperature that would cause steel to lose its strength. Although the cubicle partitions that office supply manufacturers and distributors offer are treated with fire retardant chemicals, when exposed to flame, they will burn and allow fire to communicate. As such, fires of today are growing at alarming rates of speed.
The fire involved approximately 14 percent of the total floor area when the Chicago (IL) Fire Department arrived to launch its attack. The conditions were so intense and overwhelmed the attempt to suppress the fire from the interior with hoselines supplied by the standpipe system. It was possible to reach the fire with high-flow water streams (master streams) directed into the 12th floor from ladder apparatus from the exterior. Once these high-flow streams were activated, the fire was under control within 18 minutes. Luckily, the CCAB did maintain its structural integrity. The building was designated as Type I (fire resistive) construction, built entirely of noncombustible construction. The structural columns and floors were reinforced concrete, and the walls of the central core were concrete block. The exterior walls consisted of reinforced concrete columns with full-height windows in between.
A fire with similar fire conditions on arrival occurred in January 1993 in the Bankers Trust Building on Park Avenue in New York City.4, 5 This building was not sprinkler protected when it was built in 1961, and it contained more than 1.2 million square feet of space (111,500 square meters). Its construction was typical of the high-rises of that era, featuring steel columns, girders, and beams protected by sprayed-on asbestos fire protective insulation. The floors were constructed of corrugated metal decking covered with lightweight concrete (Q-deck). The central core was constructed primarily of steel encased in cement and gypsum board construction enclosing the vertical shafts. The exterior curtain walls were nonload bearing and included large windows separated by metal spandrel panels. The fire building was one of two interconnected towers on Park Avenue between 48th and 49th streets.
多多在线观看免费视频The fire originated on the sixth floor of its west tower and was determined to have started in the plenum space because of an electrical fault. The first-arriving fire teams encountered fire conditions beyond the capabilities of two hoselines flowing a combined 500 gallons per minute (1,900 liters per minute) of water. The fire ultimately gained control of the south half of the sixth floor, which was separated from the north half by a fire-rated wall at the core area. The fire’s location on the building’s lower floors allowed the use of master streams to effectively knock down and control the main body of fire from the exterior, after the fire teams were evacuated to a lower floor. The fire’s heat damaged secondary support elements (steel beams were warped, floors buckled, and one floor beam was displaced from its connection). The major elements of columns and girders did not fail. The United States Fire Administration report stated that if this fire had occurred on a floor above the reach of exterior master streams, it would not have been possible to control. (5) This raises the question, would the structure have incurred major compromise and collapse if the fire had burned without intervention?
The New York City building codes for unsprinklered buildings at that time required floor areas not to exceed 7,500 square feet (697 square meters) and to be separated by walls with a one-hour fire rating, or 15,000 square feet (1,393 square meters) separated by walls with a two-hour fire rating. This report considered the premise and assumption that the combination of passive structural fire protection and manual fire suppression will successfully confine a fire to compartments of this size to be overly optimistic! The fuel load was typical of most business occupancies with office furniture and cubicles with low-height partitions. Each work space included some combination of desktop computers, monitors, printers, fax machines, telephones, and other equipment, along with a generous supply of paper. The report said that this fuel loading was relatively high, with numerous electrical devices and an open area, which allowed unrestricted rapid fire growth over a sizeable area. When the fire began to involve the contents of the tenant space, there was rapid fire growth. It was concluded that the fire area flashed over in less than 15 minutes from the activation of the smoke detector.
多多在线观看免费视频Decades later, building codes have changed, but many “grandfathered” unsprinklered structures still exist. The public and the fire service must depend on the implicit trust we have of the architects and design engineers that these legacy buildings will maintain their structural integrity in a fire. We also have faith and trust that the contractors that built these buildings have not modified or adjusted from the original plans to introduce a defect that under fire conditions could precipitate localized or global structural failure.
601 Lexington (originally Citicorp Center) in New York City is a 59-story skyscraper built in 1977 that was the seventh tallest building in the world at the time.6多多在线观看免费视频 The structure was also unique in that it was built to accommodate an existing building below. The structure was built on stilts for its first nine floors to cantilever above a church on the site below. The upper portion of the structure was designed with cross bracing (chevron design) to provide lateral strength and support for gravity loads and for the lateral wind forces. Originally, the steel cross braces’ connections were to be welded, but the steel contractor fabricating the building requested and received permission to use structural bolts instead. The chief structural engineer, William LeMessurier, was not informed of this change.
In 1978, a student of architecture studying the building’s design informed LeMessurier of the consequence of the on-site change. The student had discovered a deficiency in the building’s ability to resist quartering winds (winds that strike the building at its corners). Normally, buildings are strongest at their corners, and the perpendicular winds that strike the building at its faces are what cause the greatest strain. Because of the structure’s unique design, the Citicorp Building behaved differently. Although the bolted connections would have been sufficient for the perpendicular winds, they were not for the quartering winds. This defect was corrected by having all of the splice connections welded. How common are such changes that may influence the structural integrity of the building made during construction?
Back to the Fire (Literally)
According to ASTM 119, the minimum time and temperatures that the structural elements (columns, beams, floors, and load-bearing walls) must endure are not consistent with today’s fuel loading. (2) To provide thermal protection to steel, the industry relies on sprayed-on fire-resistant materials (commonly referred to as “fireproofing”) to provide insulation from direct exposure of fire. The fire service’s experience with sprayed-on fire protection is that it has proved to be deficient in high-rise fires.
The total failure and collapse of a high-rise building had not occurred until 9/11. The Twin Towers may have been designed to withstand the physical impact of an aircraft, but the architect had not considered the consequence of fire ignited with jet fuel. The resulting fire would cause the failure of bolts that connected the floor sections that spanned from the core to the load-bearing columns at the perimeter of the building. The core and exterior columns were load bearing (tube type construction), and the floor sections provided lateral strength and support to these vertical columns and the core. The failure and collapse of the towers should not gain our attention as much as the collapse of Building 7, which occurred later in the day. It was not built with the same characteristics as that of the towers.
多多在线观看免费视频Building 7 was unique in that it had to be designed and constructed above an existing electric distribution substation, subway tunnels, and vehicle ramps that gave access below the Trade Center complex. The framed steel structure included two-story-high belt trusses at the seventh and 22nd stories. This structure was designed as a perimeter moment-resisting frame structure. It was designed to endure greater force and stress than typical frame buildings with “pinned” connections, because of the model results of a wind tunnel test. That test revealed that the building would be subject to wind forces greater than the local codes would dictate.
Seven World Trade Center was built and occupied by early 1987 and was expected to stand for centuries. Although it did withstand the assault of fire for many hours, the fire, left to burn uncontrolled, caused horizontal beams to expand and fail at their connections. These beams provided added support to the vertical columns to which they were connected, ultimately supporting the entire building. The NIST fire report7多多在线观看免费视频 indicated that “factors contributing to the building failure were thermal expansion occurring at temperatures hundreds of degrees below those typically considered in design practice for establishing structural fire resistance ratings; significant magnification of the thermal expansion effects due to the long-span floors, which are common in office buildings in widespread use; connections that were designed to resist gravity loads, but not thermally induced lateral loads; and a structural system that was not designed to prevent fire-induced progressive collapse.” This collapse mechanism highlights the disconnect between the historical fire resistance tests and modern fuel loads and construction means and methods.
The Fire Service’s Responsibility
多多在线观看免费视频The fire service, which will enter the building during a fire, must understand the imperfect standards and the human factors of change and compromise that occur in the construction of a high-rise. The fire service and the public who occupy these buildings suffer the consequences of design and construction deficiencies.
Understanding this, the fire service should gain the attention and collaboration with the authorities who influence and establish building codes. The fire service as a whole must be in accord and speak out with a loud unified voice. The laws of physics are the same everywhere on the planet. We should all agree and fully understand the challenges we face when attacking high-rise structure fires because that’s where the fire service differs in concept.
Many U.S. departments that have little experience with high-rise firefighting operations use a tactical approach that is successful at nonhigh-rise structures. Much of the American fire service uses small-diameter hose for structural firefighting and is quite successful when the volume of water being applied to the burning fuels is appropriate. This is only appropriate for confined and less advanced fires; however, for advanced fires or those with significant fuel loads, it will not be effective!
These departments fail to realize and evidently ignore the fact that the typical fire loading of today generates tremendous amounts of heat (British thermal units and radiant flux). If our suppression operations do not effectively absorb the heat, then the structure will. The structural elements of these high-rise structures can and have failed. Suppression teams approaching fires with the limited flows of small-diameter hose will not get close enough to the source of burning fuels to accomplish extinguishment. If rapid cooling and extinguishment are not possible, the structure’s integrity will be compromised; and if it is occupied, especially above the floor of fire operations, then the public will suffer the consequence of our inefficiency. Confronted with a well-advanced fire, we should direct our hose stream up into the space above the ceiling (the plenum) to cool the smoke (the fuel-fire gases) below their ignition point, cooling the structural support elements, and the water will rain down on the fire below. The strength and pressure of a solid stream from the flows of a smooth bore tip and large-diameter hose will penetrate dropped ceiling tiles well ahead of the nozzle team. Our equipment selection [hose, nozzle, and pressure (flow through) gauges] should support high flow at low pressures. Your department should research what size hoseline will provide the greatest flows at pressures that are available and reasonable to handle. Handling high-flow hoselines requires technique, coordination, and sufficient personnel.
High-Rise Egress Problems
The egress travel distance and exit locations are also of concern. Some propose requiring additional exits remote from the high-rise core areas to offer a choice in direction if the fire is near the core area and the smoke and heat are deterrents. Additional means of egress reduce evacuation time for occupants when warranted. But owners and designers tend to reject this because it means losing revenue-producing floor space to accommodate a remote or an additional stair shaft. Jake Pauls is among the proponents for code change. For more than 35 years, he lent his experience in research, codes/standards development, and public health and safety in the promotion of egress issues.8 He has advocated for stairwell design with increased width not only to accommodate total evacuation of buildings but also to reduce the time needed to account for counter flow of those exiting and emergency responders ascending the stairs to suppress fires or mitigate emergencies.
In 1993, I was a lieutenant assigned to the Fire Department of New York’s Truck 4 when it responded to the World Trade Center Bombing and encountered counter flow issues while ascending Tower One. Firefighters had to request those exiting down the stairs to move to the right (the outer wall) while the firefighters stayed left (the inner wall) to ascend. The members of Truck 4, in their search of the upper floors, gathered a group of persons with disabilities (in wheelchairs) and placed them in a room that was vented with fresh air while they awaited removal to the street level. The fires raging in the basement blast area had to be extinguished and the smoke had to be dissipated for the stair shafts to be clear of smoke for these individuals to be evacuated. Evacuation by elevators was not possible because of the loss of electric and emergency backup power. Pauls, a member of the National Fire Protection Association (NFPA) Technical Committee on Means of Egress, introduced his data for increased widths of stairs, which was accepted by committees and by the NFPA membership for the new NFPA 5000™, Building Construction and Safety Code™; but, when appealed by the U.S. General Services Administration, with support from the Building Owners and Managers Association, the NFPA Standards Council reversed the acceptance and rejected the change.
多多在线观看免费视频The fire service will continue to practice “shelter in place” instead of total evacuation when appropriate to expedite suppression operations, relying on the passive and active fire protection features of the building. If fire suppression must be postponed, then the fire may grow in size and intensity to a point that it may be beyond the extinguishment capabilities of the arriving forces. This ultimately weakens the structural stability during that time.
Over the years, the building design community has compromised exit designs and reduced the safety aspects once used in high-rise buildings. “Having more than one means of egress” has been bastardized by designing exits (scissor stairs) into a core design that meets code requirements. However, the stairways are not truly remote from each other. The exits meet the travel distance requirements, but a person must negotiate his way back to the structure’s core to reach safety, and there is no exit in the opposite direction away from the core.
In high-rise residential occupancies, the two means of egress offered are exterior of the living spaces, meaning people must be able to exit their apartment to reach an exit. Apartments are designed with the kitchen facilities at or near the entrance door. Statistics reflect that many fires originate in the kitchens. Hence, egress may be blocked because of fire, negating access to the only means of egress, let alone a secondary means of egress!
多多在线观看免费视频Smoke has been the most significant issue in civilian fatalities in high-rise building fires, and the fire service is adopting methods to reduce smoke contamination to exit stairways. The Fire Department of New York has included in its matrix of response to high-rise fires a unit dedicated to introduce stairwell pressurization to reduce contamination to these vital arteries of egress.
A common stairwell design in high-rises (pre-1960s) was the fire tower (smokeproof stairwells). Also called the Philadelphia Tower, these stair system designs have been phased out because they require added space for the intermediate vestibule and smoke shaft. This space has been calculated instead to generate increased revenue rather than insurance for life safety. A 1975 study of smokeproof stair tower performance concluded that if the occupancy door and the second door leading into the stairwell were open simultaneously, the stairwell was susceptible to contamination if the draft of the intermediate vestibule (smoke shaft) was inadequate.9, 10
The fire service has requested that increased safety be designed into exits and the emergency operation of elevators and elevator shafts be enhanced. Pressurizing stairwells and elevator shafts and incorporating elevator vestibules at each landing would enhance the safety of high-rise occupants and would also add to firefighters’ safety and efficiency. When we are required to respond in the upper portions of high-rise structures, it requires extra time to place our members in position to operate. If elevators are unavailable for safety reasons, ascending the stairs takes time and effort, especially when transporting the tools and equipment needed. We must also take into account a recovery period or we overtax our bodies, undermining our ability to function effectively and safely.
This problem continues to worsen as our skylines are further changing with super- (984 feet/300 meters) and mega-tall (1,968 feet/600 meters) structures.11, 12多多在线观看免费视频 Should the elevator system fail in such a structure, it would place an extreme burden on the fire service to perform its fire suppression, search, rescue, and emergency medical services tasks.
多多在线观看免费视频The fire service has extended itself beyond maximum capabilities for years; it is unreasonable to expect us continue to do so. We must not ignore these limitations! We must now ensure that this is reflected in building codes governing design, fire resistance, and safety requirements in high-rise structures. We must re-educate those who determine how many firefighters are sufficient to perform our tasks about the current challenges we face at every type of structure fire, not just high-rises; and the public must support us. As the fire service performs its fire safety and prevention outreach, we must educate the public about the need for enhanced fire safety design in high-rise structures. We must succinctly and effectively explain the systems’ benefit and the significant expense needs to the occupants, investors, and owners of high-rise buildings. Investing in enhancing our firefighting and rescue capabilities may be costly, but it is essential, like insurance. The results of a failure of any consequence could be fatal. Furthermore, owners, architects, engineers, and building contractors need to recognize the antiquated building codes.
This compromise in trust is not fully understood or acknowledged, and the fire service is more at risk than the public. We will remain in a burning building long after the occupants have been evacuated. If our strategy, tactics, and equipment are not appropriate, we will not be effective in protecting life and extinguishing fire successfully.
1. “Bridge on I-85 in Northeast Atlanta Collapses.” thisischela.com..
2. ASTM International. ASTM 119, Standard Test Methods for Fire Tests of Building Construction and Materials多多在线观看免费视频. March 30, 2013. .
3. National Institute of Standards and Technology (NIST). Special Publication SP-1021, “Cook County Administration Building Fire.” October 17, 2003. .
4. Hughes, Ellsworth K. “Bankers Trust Fire, New York City.” Fire Engineering, June 1994, 89-97.
多多在线观看免费视频5. United States Fire Administration. “New York City Bank Building Fire: Compartmentation vs. Sprinklers.” USFA-TR-071, January 1993. .
多多在线观看免费视频6. www.slate.com. “The Design Flaw That Almost Wiped Out an NYC Skyscraper,” April 17, 2014. .
7. National Institute of Standards and Technology (NIST). “Final Report on the Collapse of World Trade Center Building 7, Federal Building and Fire Safety Investigation of the World Trade Center Disaster (NIST NCSTAR 1A).”
8. Pauls, Jake. “Have We Learned the Evacuation Lessons? A Commentary.” Fire Engineering多多在线观看免费视频, October 2002, 113-122.
9. National Fire Protection Association. “A Reporter’s Guide to Fire and the NFPA: The consequences of fire.”
10. Fung, FCW, and Zile, RH. “Evaluation of Smokeproof Stair Towers and Smoke Detector Performance.” National Bureau of Standards, September 1975.
11. Murphy, Jack. “The Changing Skyline: Super- and Mega-Tall Buildings.” Fire Engineering多多在线观看免费视频; January 2017, 67-75.
12. Lapolla, Thomas. “Response Considerations: Super- and Mega-Tall Buildings Under Construction.” Fire Engineering; January 2017, 70-71.
GERALD TRACY, a retired battalion chief, served 31 years with the Fire Department of New York (FDNY). He developed numerous training programs for FDNY for all ranks, including chief officers. His articles have appeared in FDNY’s WNYF and in Fire Engineering. With the National Institute of Standards and Technology and New York University Polytechnic Institute, he conducted live fire research on smoke management in high-rise buildings and at wind-driven fires. He was a member of the National Fire Protection Association Project Technical Panel reviewing “Firefighting Tactics Under Wind-Driven Conditions.”
Originally ran in Issue 3 of Volume 171.