Additionally, in each of the other referenced buildings, the fires burned out several floors, even with available water and fire fighting activities except for WTC 5. Thus, whether the fire fighters fought the WTC 7 fires or not is not a meaningful point of dissimilarity from the other cited fires.
WTC 7's collapse, viewed from the exterior most videos were taken from the north , did appear to fall almost uniformly as a single unit. This occurred because the interior failures that took place did not cause the exterior framing to fail until the final stages of the building collapse. The interior floor framing and columns collapsed downward and pulled away from the exterior frame.
There were clues that internal damage was taking place, prior to the downward movement of the exterior frame, such as when the east penthouse fell downward into the building and windows broke out on the north face at the ends of the building core. The symmetric appearance of the downward fall of the WTC 7 was primarily due to the greater stiffness and strength of its exterior frame relative to the interior framing.
In the draft WTC 7 report released Aug. This time period is 40 percent longer than the 3. During the public comment period on the draft report, NIST was asked to confirm this time difference and define the reasons for it in greater detail.
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.
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. This analysis showed that the 40 percent longer descent time—compared to the 3. 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. While the partial or total collapse of a tall building due to fires is a rare event, NIST strongly urges building owners, operators, and designers to evaluate buildings to ensure the adequate fire performance of structural systems.
Of particular concern are the effects of thermal expansion in buildings with one or more of the following characteristics: long-span floor systems, connections that cannot accommodate thermal effects, floor framing that induces asymmetric forces on girders, and composite floor systems, whose shear studs could fail due to differential thermal expansion i. Engineers should be able to design cost-effective fixes to address any areas of concern identified by such evaluations.
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.
Yes, this possibility was investigated carefully. NIST concluded that blast events inside the building did not occur and found no evidence supporting the existence of a blast event. 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 decibels dB to 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.
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.
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. For a steel column that weighs approximately 1, lbs. This is for one column. It is unlikely that lbs.
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. 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.
The sound levels reported by all witnesses do not match the sound level of an explosion that would have been required to cause the collapse of the building. If the two loud booms were due to explosions that were responsible for the collapse of WTC 7, the emergency responder-located somewhere between the 6th and 8th floors in WTC 7-would not have been able to survive the near immediate collapse and provide this witness account.
The primary and backup water supply to the sprinkler systems for the lower floors relied on the city's water supply. Those water lines were damaged by the collapse of WTC 1 and 2.
These uncontrolled fires in WTC 7 eventually spread to the northeast part of the building, where the collapse began. After 7 hours of uncontrolled fires, a steel girder on Floor 13 lost its connection to one of the 81 columns supporting the building. Floor 13 collapsed, beginning a cascade of floor failures to Floor 5. Column 79, no longer supported by a girder, buckled, triggering a rapid succession of structural failures that moved from east to west.
All 23 central columns, followed by the exterior columns, failed in what's known as a "progressive collapse"--that is, local damage that spreads from one structural element to another, eventually resulting in the collapse of the entire structure. The report clarifies a number of widely debated issues concerning the collapse, particularly the role of the building's many diesel fuel tanks and the importance of structural damage from falling WTC 1 debris.
The secondary conclusion of our study is that the collapse of WTC 7 was a global failure involving the near-simultaneous failure of every column in the building. Project Info Lead Researcher s J. Project Summary This is a study of the collapse of World Trade Center Building 7 WTC 7 — a story building that suffered a total collapse at PM on September 11, , following the horrible events of that morning. Thus, whether the firefighters fought the WTC 7 fires or not is not a meaningful point of dissimilarity from the other cited fires.
Progressive collapse is defined as the spread of local damage from a single initiating event, from structural element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it.
The failure of WTC 7 was an example of a fire-induced progressive collapse. First, the collapse of each tower was not triggered by 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.
WTC 7 was a more typical tall building in the design of its structural system. It was not struck by an aircraft. The collapse of WTC 7 was caused by a single initiating event—the failure of a northeast building column brought on by fire-induced damage to the adjacent flooring system and connections—which stands in contrast to the WTC 1 and WTC 2 failures, which were brought on by multiple factors, including structural damage caused by the aircraft impact, extensive dislodgement of the sprayed fire-resistive materials or fireproofing in the impacted region, and a weakening of the steel structures created by the fires.
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 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? The collapse of WTC 7 is the first known instance of a tall building brought down primarily by uncontrolled fires. The fires in WTC 7 were similar to those that have occurred in several tall buildings where the automatic sprinklers did not function or were not present.
These other buildings, including Philadelphia's One Meridian Plaza, a story skyscraper that burned for 18 hours in , did not collapse due to differences in the design of the structural system see the answer to Question 9. 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.
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? The debris from WTC 1 caused structural damage to the southwest region of WTC 7—severing seven exterior columns—but this structural damage did not initiate the collapse.
The fires initiated by the debris, rather than the structural damage that resulted from the impacts, initiated the building's collapse after the fires grew and spread to the northeast region after several hours. The debris impact caused no damage to the spray-applied fire-resistive material that was applied to the steel columns, girders and beams except in the immediate vicinity of the severed columns.
A separate analysis showed that even without the structural damage due to debris impact, WTC 7 would have collapsed in fires similar to those that occurred on Sept. Even without the structural damage, WTC 7 would have collapsed from the fires that the debris initiated. The growth and spread of the lower-floor fires due to the loss of water supply to the sprinklers from the city mains was enough to initiate the collapse of the entire building due to buckling of a critical column in the northeast region of the building.
Did the electrical substation beneath WTC 7 play a role in the fires or collapse? There is no evidence that the electric substation contributed to the fires in WTC 7. The electrical substation continued working until p. Alarms at the substation were monitored, and there were no signals except for one event early in the day.
No smoke was observed emanating from the substation. 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.
The building had three separate emergency power systems, all of which ran on diesel fuel. As background information, the three systems contained two 12,gallon fuel tanks, and two 6,gallon tanks beneath the building's loading docks, and a single 6,gallon tank on the 1st floor. In addition, one system used a gallon tank on the 5th floor, a gallon tank on the 8th floor, and a gallon tank on the 9th floor.
Another system used a gallon day tank on the 7th floor. Several months after the WTC 7 collapse, a contractor recovered an estimated 23, gallons of fuel from these tanks. The fate of the fuel in the day tanks and the two 6,gallon tanks was unknown, so NIST assumed they were full on Sept. NFPA acknowledges that each investigation is unique, and that some investigations will require broader procedures than it can accommodate.
The investigation was carefully planned, sources of information were identified and contacted, the building fire and collapse event and the investigation were documented, available evidence was obtained including documents about the design and construction of the structure , and the origin of the fire was determined based on images, laboratory testing conducted for the towers, but applicable to WTC 7 , and mathematical analyses. Additionally, in its study of WTC 7, NIST considered all available data and evaluated a range of possible collapse mechanisms: uncontrolled fires on the tenant floors, fuel oil fires, hypothetical blast events, and fires within the Con Ed substation.
NIST developed a working hypothesis, modeled the fires and the building, and then used the models to test the hypothesis against the observed behavior of the building. This approach is fully consistent with the principles of scientific inquiry.
Steel samples were removed from the site before the NIST investigation began. In the immediate aftermath of Sept. Once it was removed from the scene, the steel from WTC 7 could not be clearly identified. Unlike the pieces of steel from WTC 1 and WTC 2, which were painted red and contained distinguishing markings, WTC 7 steel did not contain such identifying characteristics. NIST's entire investigation included no physical evidence. How can the investigators be so sure they know what happened?
The steel for WTC 1 and WTC 2 contained distinguishing characteristics that enabled it to be identified once removed from the site during recovery efforts. However, the same was not true for the WTC 7 steel. Certainly, there is a lot less visual and audio evidence of the WTC 7 collapse compared to the collapses of the WTC 1 and WTC 2 towers, which were much more widely photographed. 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. This investigation is the first to show how fire can cause progressive collapse in a building.
It is also the first to show that under certain conditions thermal expansion effects—rather than loss of strength and stiffness due to fire—can lead to structural collapse. This was an analysis of unprecedented complexity—an end-to-end computer run for the WTC towers on some powerful computers took about two months while a similar run for WTC 7 took about eight months, or about four times as long. NIST expects that the tools developed from this investigation, as well as the knowledge obtained from it, will aid in the development of more robust building design practice and in studies of future building collapse processes.
0コメント