#Freefall worksheet answers series#

Finally, the entire façade collapsed.ĭiagram 1-Typical WTC 7 floor showing locations of columns (numbered). Then, progressing from east to west across WTC 7, all of the columns failed in the core of the building (58 through 78). 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).

#Freefall worksheet answers series#

What followed in rapid succession was a series of structural failures. The unsupported Column 79 then buckled and triggered an upward progression of floor system failures that reached the building's east penthouse. This collapse of floors left Column 79 insufficiently supported in the east-west direction over nine stories. Many of these floors had already been at least partially weakened by the fires in the vicinity of Column 79. 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.

The failure of this structural column then initiated a fire-induced progressive collapse of the entire building.Īccording 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.Įventually, 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 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. How did the fires cause WTC 7 to collapse? These uncontrolled lower-floor fires eventually spread to the northeast part of WTC 7, where the building's collapse began. 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 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. However, only the fires on some of the lower floors-7 through 9 and 11 through 13-burned out of control. What caused the fires in WTC 7?ĭebris from the collapse of WTC 1, which was 370 feet to the south, ignited fires on at least 10 floors in the building at its south and west faces. until 5:20:52 p.m., when WTC 7 collapsed. 11, 2001, WTC 7 endured fires for almost seven hours, from the time of the collapse of the north WTC tower (WTC 1) at 10:28:22 a.m.

Completed in 1987, it was built on top of an existing Con Edison substation and located on land owned by The Port Authority of New York and New Jersey. The original World Trade Center Building 7 (WTC 7) was a 47-story office building located immediately to the north of the main World Trade Center (WTC) complex. Questions and Answers about the NIST WTC 7 Investigation (, ARCHIVE, incorporated into update) What was WTC 7?