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Both collapsed within two hours of impact, prompting several investigations and spawning a variety of conspiracy theories. They were constructed from steel and concrete, using a design that was groundbreaking at the time. Most high-rise buildings since have used a similar structure. They conclude it was not caused by direct impact by the aircraft, or the use of explosives, but by fires that burned inside the buildings after impact. But the answer becomes clear once you consider the details.
Aircraft are made from lightweight materials, such as aluminium. If you compare the mass of an aircraft with that of a skyscraper more than metres tall and built from steel and concrete, it makes sense the building would not topple over.
That said, the aircraft did dislodge fireproofing material within the towers, which was coated on the steel columns and on the steel floor trusses underneath concrete slabs. The lack of fireproofing left the steel unprotected.
As such, the impact also structurally damaged the supporting steel columns. When a few columns become damaged, the load they carry is transferred to other columns. In response to comments from the building community, NIST conducted an additional computer analysis. The investigation team concluded that the column's failure under any circumstance would have initiated the same sequence of events. Why did NIST withhold from public release limited and specific input and results files for certain collapse models used in the WTC 7 study?
This information was exempt from public disclosure under Section 7d of the National Construction Safety Team Act because it was determined by the Director of NIST that release of the files might jeopardize public safety.
The withheld information contains detailed connection models that have been validated against actual events, and therefore, provide tools that could be used to predict the collapse of a building. The information contained in the withheld files is sufficiently detailed that it might be used to develop plans to destroy other, similarly constructed, buildings. In Michael Quick v. District Court of the District of Columbia, Apr.
Debris from the collapse of WTC 1, which was feet to the south, ignited fires on at least 10 floors in the building at its south and west faces. However, only the fires on some of the lower floors—7 through 9 and 11 through 13—burned out of control. These lower-floor fires—which spread and grew because the water supply to the automatic sprinkler system for these floors had failed—were similar to building fires experienced in other tall buildings.
The primary and backup water supply to the sprinkler systems for the lower floors relied on the city's water supply, whose lines were damaged by the collapse of WTC 1 and WTC 2. These uncontrolled lower-floor fires eventually spread to the northeast part of WTC 7, where the building's collapse began. The heat from the uncontrolled fires caused steel floor beams and girders to thermally expand, leading to a chain of events that caused a key structural column to fail.
The failure of this structural column then initiated a fire-induced progressive collapse of the entire building. 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 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, and Then, progressing from east to west across WTC 7, all of the columns failed in the core of the building 58 through The sprinkler systems did not fail.
The water main served as both the primary and backup source of water for the sprinkler system in the lower 20 floors. Therefore, the sprinkler system could not function. In contrast, the sprinklers and standpipes on the building's middle levels 21st floor through 39th floor and upper levels 40th floor through 47th floor received water from two large overhead storage tanks on the 46th floor, and used the city's water mains as a backup. Due to the effectiveness of the spray-applied fire-resistive material SFRM , or fireproofing, the highest steel column temperatures in WTC 7 only reached an estimated degrees Celsius degrees Fahrenheit , and only on the east side of the building did the steel floor beams exceed degrees Celsius 1, degrees Fahrenheit.
However, fire-induced buckling of floor beams and damage to connections—which caused buckling of a critical column initiating collapse—occurred at temperatures below approximately degrees Celsius where thermal expansion dominates.
Above degrees Celsius 1, degrees Fahrenheit , there is significant loss of steel strength and stiffness. In the WTC 7 collapse, the loss of steel strength or stiffness was not as important as the thermal expansion of steel structures caused by heat.
What are the major differences between "typical" major high-rise building fires that have occurred in the United States and the fire in the WTC 7 building on Sept.
The following factors describe the fire events that occurred in both WTC 7 and the referenced buildings:. There were some differences between the fires in WTC 7 and those in the referenced buildings, but these differences were secondary to the fire factors that led to the collapse of WTC The differences in the fires were not meaningful for the following reasons.
By the time WTC 7 collapsed, the fires in WTC 7 had advanced well beyond the likely points of origin on multiple floors i. Additionally, in each of the other referenced buildings, the fires burned out several floors, even with available water and firefighting activities except for WTC 5. 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.
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