What was held in wtc 7

The total weight of the structure was roughly , t, but wind load, rather than the gravity load, dominated the design. This permitted windows more than one-half meter wide. It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story. Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction that is about 95 percent air, explaining why the rubble after the collapse was only a few stories high.

The egg-crate construction made a redundant structure i. Prior to the World Trade Center with its lightweight perimeter tube design, most tall buildings contained huge columns on 5 m centers and contained massive amounts of masonry carrying some of the structural load.

The early news reports noted how well the towers withstood the initial impact of the aircraft; however, when one recognizes that the buildings had more than 1, times the mass of the aircraft and had been designed to resist steady wind loads of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising.

The only individual metal component of the aircraft that is comparable in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraft fuselage.

While the aircraft impact undoubtedly destroyed several columns in the WTC perimeter wall, the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure. The ensuing fire was clearly the principal cause of the collapse Figure 4.

The fire is the most misunderstood part of the WTC collapse. Even today, the media report and many scientists believe that the steel melted. It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true. Part of the problem is that people including engineers often confuse temperature and heat. While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density or mass.

Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material.

One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles. Thus, the fact that there were 90, L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.

In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame. A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber.

Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet. This is what occurs in a jet engine, and this is the flame type that generates the most intense heat. In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions. It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame.

A fireplace flame is a diffuse flame burning in air, as was the WTC fire. WTC 7 collapsed at p. Several factors contributed to the outcome of no loss of life—or serious injuries—in WTC 7. The building had only half the number of occupants on a typical day—approximately 4, persons—at the times the airplanes struck the WTC towers.

Occupants had recently participated in fire drills. Emergency responders provided evacuation assistance to occupants. No emergency responders were harmed in the collapse of WTC 7 because the decision to abandon all efforts to save WTC 7 was made nearly three hours before the building fell. The National Construction Safety Team Act was passed in to give NIST the responsibility for conducting fact-finding technical investigations of building failures that resulted in substantial loss of life or that posed significant potential of substantial loss of life.

How does the final report on WTC 7 issued on Nov. The final report is strengthened by clarifications and supplemental text suggested by organizations and individuals worldwide in response to the draft WTC 7 report, but the revisions did not alter the investigation team's major findings and recommendations, which include identification of fire as the primary cause for the building's failure.

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 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.

Where's The Pod? The Pentagon. Flight Type keyword s to search. Today's Top Stories. World Trade Center 7 stands amid the rubble of the recently collapsed Twin Towers. This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses.

You may be able to find more information about this and similar content at piano. 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. 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.

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.

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 7: 1 Fires in high rise buildings typically have a single point of origin on a single floor, whereas the fires in WTC 7 likely had a single point of origin on multiple 10 floors; 2 ; fires in other high rise buildings were due to isolated events, whereas the fires in WTC 7 followed the collapse of WTC 1; 3 water was available to fight fires in the other high rise buildings, but the water supply to fight fires in WTC 7 was impaired; and 4 while the fires in the other buildings were actively fought by fire fighters to the extent possible, in WTC 7, no efforts were made to fight the fires.

The differences in the fires were not meaningful for the following reasons. By the time that 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 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.