Why wtc 7

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 C degrees F , and only on the east side of the building did the steel floor beams exceed degrees C 1, degrees F.

However, fire-induced buckling of floor beams and damage to connections-that caused buckling of a critical column initiating collapse-occurred at temperatures below approximately degrees C where thermal expansion dominates.

Above degrees C 1, degrees F , 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. 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.

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-with approximately 4, occupants-at the times the airplanes struck the towers.

Occupants had recently participated in fire drills. Evacuation of the building took just over an hour, and the process was complete before the collapse of the first WTC tower WTC 2.

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. 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. 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.

The team found that the design of WTC 7 in the s was generally consistent with the New York City building code in effect at that time. WTC 7's designers intended its stairwells to evacuate nearly 14, 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,, and more than adequate to evacuate the approximately 4, occupants who were in the building on Sept.

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. This range is considered to represent long span floor systems. Thermal effects e. 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.

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. These expanded tools and derived, validated, and simplified analysis approaches can guide practitioners and prevent future disasters. The act gives NIST the responsibility for conducting fact-finding investigations of building failures that resulted in substantial loss of life or that posed significant potential of substantial loss of life.

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 destructive sequence of events. The first comprehensive set of eight model building code changes based on recommendations from NIST's investigation of the WTC towers were adopted by the International Building Code in 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 edition of the International Building Code.

Increased bond strength for fireproofing nearly three times greater than currently required for buildings feet in height and seven times greater for buildings more than feet in height. 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. Increasing by one hour the fire-resistance rating of structural components and assemblies in buildings feet and higher. It is physically impossible," she said. Today's report confirms that a fire was, indeed, the cause. The final report describes how debris from the collapse of WTC 1 ignited fires on at least 10 floors of WTC 7 at the western half of the south face.

Fires on Floors 7 through 9 and 11 through 13 burned out of control, because the water supply to the automatic sprinkler system had failed.

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 al Qaeda terrorists hijacked jetliners and smashed them into New York's iconic World Trade Center, killing 2, people. A third plane hit the Pentagon, killing The fourth hijacked plane was crashed into a field in Shanksville, Pennsylvania that killed 40 more. A plane flying from Boston to Los Angeles with 92 people on board, including five hijackers, was crashed into the North Tower of the World Trade Center at am local time.

An hour and 42 minutes after the impact, the North Tower collapsed, releasing a huge cloud of dust and smoke. At am local time , another plane with 65 people on board — including five hijackers — hit the South Tower, sparking a massive explosion and collapsed 56 minutes later.