http://en.wikipedia.org/wiki/7_World_...As the North Tower collapsed on September 11, 2001, debris hit 7 World Trade Center, causing heavy damage to the south face of the building. The bottom portion of the building's south face was heavily damaged from debris, including: damage to the southwest corner from the 8th to 18th floor, a large vertical gash on the center-bottom extending at least ten floors, and other damage as high as the 18th floor. 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 sprinkler floor level controls had just a single connection to the sprinkler water riser, and the sprinkler system required some power for the fire pump to deliver water. Loss of power to the fire pump or other damage to the structure would have meant no functioning sprinklers. Also, water pressure was low, with little or no water to feed sprinklers.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. A massive fire burned into the afternoon on the 11th and 12th floors of 7 World Trade Center, the flames visible on the east side of the building. During the afternoon, fire was also seen on floors 6--10, 13--14, 19--22, and 29--30. At approximately 2:00 p.m., firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors which was a sign that the building was unstable and might collapse. During the afternoon, firefighters also heard creaking sounds coming from the building. Around 3:30 pm, given that 7 World Trade Center was unstable and would possibly collapse, FDNY Chief Daniel 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. At 5:20 p.m. EDT on September 11, 2001, 7 World Trade Center collapsed. The building had been evacuated and there were no casualties associated with the collapse.In May 2002, the Federal Emergency Management Agency (FEMA) issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under leadership of Dr. W. Gene Corley, P.E. FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1 WTC and 2 WTC but by fires on multiple stories ignited by debris from the other two towers that continued unabated due to lack of water for sprinklers or manual firefighting. Structural elements were exposed to high temperatures for a sufficient period of time to reduce their strength to the point of collapse.The report did not reach final conclusions about the cause of the collapse, but listed several issues requiring further investigation. FEMA made these findings:Loss of structural integrity was likely a result of weakening caused by fires on the 5th to 7th floors. The specifics of the fires in WTC 7 and how they caused the building to collapse remain unknown at this time. Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue. [Ch. 5, p. 31.]