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Questions and Answers about the NIST WTC 7 Investigation (Updated 12/18/2008)What was WTC 7? When did WTC 7 collapse? What caused the fires in WTC 7? How did the fires cause WTC 7 to collapse? According to the report’s probable collapse sequence, heat from the uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, damaging the floor framing on multiple floors. Eventually, a girder on Floor 13 lost its connection to a critical column, Column 79, that provided support for the long floor spans on the east side of the building (see Diagram 1). The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the 5th floor. Many of these floors had already been at least partially weakened by the fires in the vicinity of Column 79. This collapse of floors left Column 79 insufficiently supported in the east-west direction over nine stories. The unsupported Column 79 then buckled and triggered an upward progression of floor system failures that reached the building’s east penthouse. What followed in rapid succession was a series of structural failures. Failure first occurred all the way to the roof line—involving all three interior columns on the easternmost side of the building (79, 80, 81). Then, progressing from east to west across WTC 7, all of the columns failed in the core of the building (58 through 78). Finally, the entire façade collapsed.
What is progressive collapse? Progressive collapse did NOT occur in the WTC towers, for two reasons. First, the collapse of each tower was not triggered by a local damage or a single initiating event. Second, the structures were able to redistribute loads from the impact and fire-damaged structural components and subsystems to undamaged components and to keep the building standing until a sudden, global collapse occurred. Had a hat truss that connected the core columns to the exterior frame not been installed to support a TV antenna atop each WTC tower after the structure had been fully designed, it is likely that the core of the WTC towers would have collapsed sooner, triggering a global collapse. Such a collapse would have some features similar to that of a progressive collapse. How did the collapse of WTC 7 differ from the collapses of WTC 1 and WTC 2? The fires in WTC 7 were quite different from the fires in the WTC towers. Since WTC 7 was not doused with thousands of gallons of jet fuel, large areas of any floor were not ignited simultaneously as they were in the WTC towers. Instead, separate fires in WTC 7 broke out on different floors, most notably on Floors 7 to 9 and 11 to 13. The WTC 7 fires were similar to building contents fires that have occurred in several tall buildings where the automatic sprinklers did not function or were not present. Why did WTC 7 collapse, while no other known building in history has collapsed due to fires alone? Factors contributing to WTC 7’s collapse included: the thermal expansion of building elements such as floor beams and girders, which occurred at temperatures hundreds of degrees below those typically considered in current practice for fire-resistance ratings; significant magnification of thermal expansion effects due to the long-span floors in the building; connections between structural elements that were designed to resist the vertical forces of gravity, not the thermally induced horizontal or lateral loads; and an overall structural system not designed to prevent fire-induced progressive collapse. Some people have said that a failure at one column should not have produced a symmetrical fall like this one. What’s your answer to those assertions? In a video, it appears that WTC 7 is descending in free fall, something that would not occur in the structural collapse that you describe. How can you ignore basic laws of physics? To further clarify the descent of the north face, NIST recorded the downward displacement of a point near the center of the roofline from first movement until the north face was no longer visible in the video. Numerical analyses were conducted to calculate the velocity and acceleration of the roofline point from the time-dependent displacement data. The instant at which vertical motion of the roofline first occurred was determined by tracking the numerical value of the brightness of a pixel (a single element in the video image) at the roofline. This pixel became brighter as the roofline began to descend because the color of the pixel started to change from that of the building façade to the lighter color of the sky. The approach taken by NIST is summarized in Section 3.6 of the final summary report, NCSTAR 1A (released Nov. 20, 2008; available at http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201A.pdf) and detailed in Section 12.5.3 of NIST NCSTAR 1-9 (available at http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201-9%20Vol%202.pdf). The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline) revealed three distinct stages characterizing the 5.4 seconds of collapse:
This analysis showed that the 40 percent longer descent time—compared to the 3.9 second free fall time—was due primarily to Stage 1, which corresponded to the buckling of the exterior columns in the lower stories of the north face. During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below. This is consistent with the structural analysis model which showed the exterior columns buckling and losing their capacity to support the loads from the structure above. In Stage 3, the acceleration decreased as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below. Does this mean there are hundreds or thousands of unsafe tall buildings with long span supports that must be retrofitted in some way? How would you retrofit a building to prevent this problem? Several existing, emerging, or even anticipated capabilities could have helped prevent the collapse of WTC 7. The degree to which these capabilities improve performance remains to be evaluated. Possible options for developing cost-effective fixes include:
NIST is recommending that building standards and codes be strengthened beyond their current intent to achieve life safety to prevent structural collapse even during infrequent building fires like those in WTC 7 when sprinklers do not function, do not exist, or are overwhelmed by fire. Did investigators consider the possibility that an explosion caused or contributed to the collapse of WTC 7? In addition, no blast sounds were heard on the audio tracks of video recordings during the collapse of WTC 7 or reported by witnesses. According to calculations by the investigation team, the smallest blast capable of failing the building’s critical column would have resulted in a sound level of 130 decibels (dB) to 140 dB at a distance of at least half a mile, if unobstructed by surrounding buildings. This sound level is consistent with a gunshot blast, standing next to a jet plane engine, and more than 10 times louder than being in front of the speakers at a rock concert. For the building to have been prepared for intentional demolition, walls and/or column enclosures and fireproofing would have to be removed and replaced without being detected. Preparing a column includes steps such as cutting sections with torches, which produces noxious and odorous fumes. Intentional demolition usually requires applying explosive charges to most, if not all, interior columns, not just one or a limited set of columns in a building. Is it possible that thermite or thermate contributed to the collapse of WTC 7? Thermite is a combination of aluminum powder and a metal oxide that releases a tremendous amount of heat when ignited. It is typically used to weld railroad rails together by melting a small quantity of steel and pouring the melted steel into a form between the two rails. To apply thermite to a large steel column, approximately 0.13 lb of thermite would be needed to heat and melt each pound of steel. For a steel column that weighs approximately 1,000 lbs. per foot, at least 100 lbs. of thermite would need to be placed around the column, ignited, and remain in contact with the vertical steel surface as the thermite reaction took place. This is for one column … presumably, more than one column would have been prepared with thermite, if this approach were to be used. It is unlikely that 100 lbs. of thermite, or more, could have been carried into WTC 7 and placed around columns without being detected, either prior to Sept. 11 or during that day. Given the fires that were observed that day, and the demonstrated structural response to the fires, NIST does not believe that thermite was used to fail any columns in WTC 7. Analysis of the WTC steel for the elements in thermite/thermate would not necessarily have been conclusive. The metal compounds also would have been present in the construction materials making up the WTC buildings, and sulfur is present in the gypsum wallboard used for interior partitions. An emergency responder caught in the building between the 6th and 8th floors says he heard two loud booms. Isn’t that evidence that there was an explosion? Did fuel oil systems in WTC 7 contribute to its collapse? As background information, the three systems contained two 12,000 gallon fuel tanks, and two 6,000 gallon tanks beneath the building’s loading docks, and a single 6,000 gallon tank on the 1st floor. In addition one system used a 275 gallon tank on the 5th floor, a 275 gallon tank on the 8th floor, and a 50 gallon tank on the 9th floor. Another system used a 275 gallon day tank on the 7th floor. Several months after the WTC 7 collapse, a contractor recovered an estimated 23,000 gallons of fuel from these tanks. NIST estimated that the unaccounted fuel totaled 1,000 ±1,000 gallons of fuel (in other words, somewhere between 0 and 2,000 gallons, with 1,000 gallons the most likely figure). The fate of the fuel in the day tanks was unknown, so NIST assumed the worst-case scenario, namely that they were full on Sept. 11, 2001. The fate of the fuel of two 6,000 gallon tanks was also unknown. Therefore, NIST also assumed the worst-case scenario for these tanks, namely that all of the fuel would have been available to feed fires either at ground level or on the 5th floor. Why did the investigation take so long to complete? The WTC 7 investigation was an extensive, state-of-the-art reconstruction of the events that affected WTC 7 and eventually led to its collapse. Numerous facts and data were obtained, then combined with validated computer modeling that is believed to be close to what actually occurred. A single computer simulation of the structural response to fires took about eight months to complete on powerful computing workstations and clusters. Did debris from the collapse of WTC 1 cause damage to WTC 7’s structure in a way that contributed to the building’s collapse? Would WTC 7 have collapsed even if there had been no structural damage induced by the collapse of the WTC towers? Why did WTC 7’s sprinkler systems fail during the fires? How hot did WTC 7’s steel columns and floor beams get? Did the electrical substation next to WTC 7 play a role in the fires or collapse? Special elements of the building’s construction—namely trusses, girders, and cantilever overhangs, which were used to transfer loads from the building superstructure to the columns of the electric substation (over which WTC 7 was constructed) and foundation below—also did not play a significant role in the collapse. Why were there no fatalities from the collapse of WTC 7? Why didn't the investigators look at actual steel samples from WTC 7? Your entire investigation included no physical evidence. How can you be so sure you know what happened? Nonetheless, the NIST investigation of WTC 7 is based on a huge amount of data. These data come from extensive research, interviews, and studies of the building, including audio and video recordings of the collapse. Rigorous, state-of-the-art computer methods were designed to study and model the building’s collapse. These validated computer models produced a collapse sequence that was confirmed by observations of what actually occurred. In addition to using its in-house expertise, NIST relied upon private sector technical experts; accumulated copious documents, photographs and videos of this disaster; conducted first-person interviews of building occupants and emergency responders; analyzed the evacuation and emergency response operations in and around WTC 7; performed computer simulations of the behavior of WTC 7 on Sept. 11, 2001, and combined the knowledge gained into a probable collapse sequence. Did WTC 7 conform to building and fire codes? WTC 7's designers intended its stairwells to evacuate nearly 14,000 occupants, anticipated at the time to be the maximum occupancy of the building. Though the stairwell’s capacity was overestimated, it was adequate for evacuating the building’s actual maximum occupancy of 8,000, and more than adequate to evacuate the approximately 4,000 occupants who were in the building on Sept. 11. What improvements to building safety have been recommended as a result of the WTC 7 investigation? The new recommendation involves explicitly evaluating buildings to ensure the adequate fire safety performance of the structural system. Of particular concern are the effects of thermal expansion in buildings with one or more of the following characteristics:
Typical floor span lengths in tall office buildings are in the range of 40 ft. to 50 ft. This range is considered to represent long span floor systems. Thermal effects (e.g., thermal expansion) that may be significant in long-span buildings may also be present in buildings with shorter span lengths depending on the design of the structural system. The earlier recommendations encompass increasing structural integrity of buildings, enhancing structures’ endurance when exposed to fire, creating new methods for increasing fire resistance in structures, improving active fire protection, improving some aspects of emergency response, and increasing education and training. What are some of the firsts in this investigation? Why did NIST study the collapse of WTC 7? How does the final report on WTC 7 issued on Nov. 23, 2008, differ from the draft report that was released for public comment on Aug. 21, 2008? The extensive three-year scientific and technical building and fire safety investigation found that the fires on multiple floors in WTC 7, which were uncontrolled but otherwise similar to fires experienced in other tall buildings, caused an extraordinary event. Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down. In response to comments from the building community, NIST conducted an additional computer analysis. The goal was to see if the loss of WTC 7’s Column 79—the structural component identified as the one whose failure on 9/11 started the progressive collapse—would still have led to a complete loss of the building if fire or damage from the falling debris of the nearby WTC 1 tower were not factors. The investigation team concluded that the column’s failure under any circumstance would have initiated the destructive sequence of events. Other revisions to the final WTC 7 report included:
Have the recommendations from NIST’s investigation of the WTC towers led to any changes in building codes, standards, and practices? A second set of eight model building code changes based on NIST’s recommendations from its investigation of the WTC towers were approved by technical committees and are awaiting approval, along with potential appeals on several other code changes, at the Final Action Hearing for the 2009 edition of the International Building Code. NIST’s recommendations from its investigation of the WTC towers also have spurred actions to develop new provisions/guidelines within other standards, codes, and industry organizations, such as: the National Fire Protection Association, the American Society of Mechanical Engineers, ASTM International, the American Society of Civil Engineers, and the Council on Tall Buildings and Urban Habitat. What specific code changes based on recommendations from NIST’s investigation of the WTC towers have been approved for inclusion in the International Building Code? 1. An additional exit stairway for buildings more than 420 feet in height. 2. A minimum of one fire service access elevator for buildings more than 120 feet in height. 3. Increased bond strength for fireproofing (nearly three times greater than currently required for buildings 75-420 feet in height and seven times greater for buildings more than 420 feet in height). 4. Field installation requirements for fireproofing to ensure that:
5. Special field inspections of fireproofing to ensure that its as-installed thickness, density and bond strength meet specified requirements, and that a bonding agent is applied when the bond strength is less than required due to the effect of a primed, painted or encapsulated steel surface. The inspections are to be performed after the rough installation of mechanical, electrical, plumbing, sprinkler and ceiling systems. 6. Increasing by one hour the fire-resistance rating of structural components and assemblies in buildings 420 feet and higher. (This change was approved in a prior edition of the code.) 7. Explicit adoption of the “structural frame” approach to fire resistance ratings that requires all members of the primary structural frame to have the higher fire resistance rating commonly required for columns. The primary structural frame includes the columns, other structural members including the girders, beams, trusses, and spandrels having direct connections to the columns, and bracing members designed to carry gravity loads. 8. Luminous markings delineating the exit path (including vertical exit enclosures and passageways) in buildings more than 75 feet in height to facilitate rapid egress and full building evacuation.
Date
created: 08/21/08
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