To: Alex MG who wrote (253167 ) 6/17/2014 4:13:47 PM From: Alex MG Respond to of 541957 geez, mystery solved As the North Tower collapsed on September 11, 2001, heavy debris hit 7 World Trade Center, damaging the south face of the building [37] and starting fires that continued to burn throughout the afternoon. [7] The collapse also caused damage to the southwest corner between Floors 7 and 17 and on the south face between Floor 44 and the roof; other possible structural damage included a large vertical gash near the center of the south face between Floors 24 and 41. [7] The building was equipped with a sprinkler system , but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps, rather than being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser; and the sprinkler system required some power for the fire pump to deliver water. Also, water pressure was low, with little or no water to feed sprinklers. [38] [39] After the North Tower collapsed, some firefighters entered 7 World Trade Center to search the building. They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts. [40] Over the course of the day, fires burned out of control on several floors of 7 World Trade Center; the flames visible on the east side of the building. [41] [42] During the afternoon, fire was also seen on floors 6–10, 13–14, 19–22, and 29–30. [37] In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon. [8] At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might collapse. [43] During the afternoon, firefighters also heard creaking sounds coming from the building. [44] Around 3:30 pm, FDNY Chief Daniel A. Nigro decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7 World Trade Center and evacuate the area due to concerns for the safety of personnel . [45] At 5:20:33 pm EDT (according to FEMA), the building started to collapse, with the crumble of the east mechanical penthouse, and at 5:21:10 pm EDT, according to FEMA, and 5:20:52 pm EDT according to NIST, it collapsed completely. [6] [7] [46] There were no casualties associated with the collapse.