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World Trade Center Case Study

In September 11, 2001 terrorist attacks on the World Trade Center horrified the civilized world. An estimated 2,830 human lives were extinguished by these attacks-the second greatest number of lives ever lost in a single event in the United States. These attacks also resulted in extensive damage to a large number of buildings at the World Trade Center complex and within the vicinity of the complex. In the aftermath of the events, the Federal Emergency Management Agency (FEMA) and the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE)-in association with several other federal agencies and professional organizations-dispatched a team of civil, structural, and fire protection engineers to study the performance of the buildings at the site.

The BPAT was to review the damage caused by these events, collect available data, develop understanding of the performance of each affected building, determine the causes of observed behavior, and reveal the need for any further studies that should be performed. In particular, the team studied the immediate effects of the aircraft impact on each tower, the spread of fire following the crashes, the reduction in structural strength caused by the fire, and the mechanism that led to the collapse of each tower.

The first aircraft struck WTC 1 roughly between the 94th and 98th floors, inflicting extensive damage to the north face of the tower in this area. At least five of the prefabricated, three-column sections that formed the exterior walls broke off in the area where the airplane fuselage and engines impacted, and parts of these sections were thrust inside the building envelope by the impact. Floors locally supported by these exterior wall sections appeared to partially collapse, losing their support along the exterior wall. In areas struck by the outer wing structures, the exterior columns fractured upon impact. Subsequent interpretation of photographic evidence will suggest that between 31 and 36 columns on the north facade were destroyed over portions of a four-story area and that a partial collapse of floors in this area appears to have occurred over a horizontal length of wall of approximating 65 ft; floors in other portions of the building will appear to have remained intact.

The second aircraft struck the eastern half of the south face of WTC 2 approximately between the 78th and 84th floors. Massive damage was inflicted on the south face of the building in the zone of impact. Within the central zone of impact, where the airplane fuselage and engines struck, six of the prefabricated, three-column sections that formed the exterior walls were broken loose of the structure, and some of the building elements were apparently thrust inside the building envelope. As in the impact of the plane on WTC 1, the floors supported by these exterior wall sections appear to have partially collapsed, losing their support along the exterior wall. In the areas impacted by the outer wing structures, the exterior steel columns were fractured on impact. Photographic evidence will subsequently suggest that between 27 and 32 columns along the south building face were destroyed over portions of a five-story range. Partial collapse of floors in this zone appears to have occurred over a horizontal length of wall of approximately 70 ft; floors in other portions of the building appear to have remained intact. It is probable that the columns in the southwest corner of the core also experienced some damage because they would have been in the direct path of the fuselage and port engine.

Each of the fireballs expanded to its maximum diameter within about two seconds as the expelled fuel dispersed and flames traveled through the resulting fuel/air mixture. If an explosion or detonation had occurred, this expansion would have taken place in microseconds. Preliminary calculations suggest that the resulting overpressures were less than 1 lb per square inch (PSI). Although this pressure was sufficient to extensively break windows on the affected floors, it should not have resulted in significant structural damage.

Collapse of the twin towers astonished most observers, including many knowledgeable structural engineers. These were structures notable for their robust, redundant framing systems. Many believed that their structural anatomy would have enabled them to withstand the attacks. The twin towers of the World Trade Center were the primary components of the seven-building World Trade Center complex, and although they were similar, they were not identical. Each of the towers encompassed 110 stories above grade and 6 levels below. WTC 1 had a roof height of 1,368 ft; WTC 2 was nearly as tall, with a roof height of 1,362 ft. WTC 1 also supported a 360 ft tall television and radio transmission tower. Each building had a square floor plat 207 ft 2 in. long on a side. Corners were chamfered 6 feet 11 in. Nearly 1 acre of floor space was provided at each level. A rectangular service core with overall dimensions of approximately 87 by 137 ft was present at the center of each building, housing three exit stairways, 99 elevators, and 16 escalators.
The service core in WTC 1 was oriented east to west; the core in WTC 2 was oriented north to south. In addition to these basic differences in configuration, the presence of each building affected the wind loading on the other structure, resulting in a somewhat different distribution of design wind pressures and, therefore, somewhat different structural design of the lateral-force-resisting system. In addition, tenant improvements over the years resulted in the removal of portions of floors and the placement of new, private stairways between floors, in a somewhat random pattern.

The features in question include the following:
1.Steel floor trusses used as the primary horizontal framing elements for floor systems outside the structural core and their structural robustness and redundancy compared with other construction;
2.Gypsum-board-sheathed walls in stairwells and shafts for impact resistance and as vertical fire separation between building floors;
3.Spray-applied fire protection materials on steel framing and the adequacy of these materials to provide protection for the steel frame;
4.Emergency egress stairways grouped in the central building core, as opposed to being dispersed throughout the structure.

The most important factors which effected to the WTC debris are as following :-
1. The towers' ability to withstand additional loading and have increased the susceptibility of the structures to fire-induced failure. It is likely that the force of the impact and the speed with which debris traveled through the structures compromised the sprayed
2. The advancing fires elevate the temperature within the tower. Future estimates will place it between 1,700? and 2,000?F-further stressing the structure.
3. Damage inflicted by the impacts and the rapid movement of debris through the impacted floors has most likely impaired the sprinkler and fire standpipe systems
4. As the fires rage throughout the upper reaches of both towers, the evacuation of the structures progresses as successfully as thought possible given the nature of these incidents. Several building features will be identified as key to the towers' ability to remain standing for as long as they did, enabling most of the buildings' occupants to escape. These include the robustness and redundancy of the steel framing system, the presence of adequate egress stairways that are well marked and lighted, and the rigorous emergency egress training programs provided for building tenants. It will ultimately be determined that 99 percent of the occupants of the floors below the areas of impact survived, largely because the stairways at these levels remained intact and passable. Tragically, those occupying floors within and above the impact areas cannot escape because the stairways in the impact areas have been destroyed. The high percentage of those able to escape may also be attributed-at least in part-to the fact that some physical changes and training programs were put in place following the 1993 bombing of WTC 1. These physical modifications included the placement of photoluminescent paint on the egress paths to assist occupants during egress and the installation of emergency lighting in the stairways. Additionally, some of the occupants of WTC 2 vacate that tower upon learning of the crash of the first plane into WTC 1. The world would later learn the devastating news that a number of survivors who had occupied WTC 2 reported that a message was broadcast over the building's loudspeaker system indicating that WTC 2 was secure and that occupants should return to their offices. While many people did not heed this announcement, there were those who did. The decision would cost them their lives.
Preliminary analyses of the damaged structures, together with the fact that the structures remained standing for an extended period of time, suggest that absent severe loading events, such as a windstorm or earthquake, the buildings could have remained standing in their damaged states indefinitely. However, the structures were subjected to a severe loading event in the form of the fires caused by the aircraft impacts.