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Natural Hazards Observer

January 2006
Volume XXX | Number 3

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Invited Comment

Mr. Hazards looking at picture of WTC towers

Reconstructing the Collapse:
The Final Report on the
World Trade Center Towers

The final report from the most detailed examination of a building failure ever conducted was released October 26, 2005, at a hearing of the U.S. House of Representatives Science Committee on the National Institute of Standards and Technology’s (NIST) investigation of the fires and collapses of New York City’s World Trade Center (WTC) towers following the terrorist attacks of September 11, 2001. Featured in the report are 30 recommendations designed to improve the safety of tall buildings, their occupants, and emergency responders.

The recommendations are based on the findings and conclusions contained within 43 separate reports (totaling some 10,000 pages) that cover:

  • Specific improvements to building standards, codes, and practices;
  • Changes to, or the establishment of, evacuation and emergency response procedures; and
  • Research and other appropriate actions needed to help prevent future building failures.

Based on nearly 500 comments received during the six-week public review period following the release of the draft WTC towers report on June 23, 2005, the reports were amended and clarified.

The specific objectives of the investigation were:

  1. Determine why and how WTC 1, 2, and 7 collapsed;
  2. Determine why the injuries and fatalities were so high or low depending on location, including technical aspects of fire protection, occupant behavior, evacuation, and emergency response;
  3. Determine what procedures and practices were used in the design, construction, operation, and maintenance of WTC 1, 2, and 7; and
  4. Identify areas in current building and fire codes, standards, and practices that warrant revision.

Summary of Findings

Objective 1: Determine why and how WTC 1, 2, and 7 collapsed.

  • The two aircraft hit the towers at high speed and did considerable damage to principal structural components. However, the towers withstood the impacts and would have remained standing if not for the dislodged insulation (fireproofing) and the subsequent multifloor fires. In each tower, a different combination of impact damage and heat-weakened structural components contributed to the abrupt structural collapse.
  • In WTC 1, fires weakened the core columns and caused the floors on the south side of the building to sag. The floors pulled the heated south perimeter columns inward, reducing their capacity to support the building above. As columns on the south wall buckled, neighboring columns quickly became overloaded. The top section of the building tilted to the south and began its descent.
  • In WTC 2, the core was damaged severely at the southeast corner and was restrained by the east and south walls. The steady burning fires on the east side of the building caused the floors there to sag. The floors pulled the heated east perimeter columns inward, reducing their capacity to support the building above. As columns on the east wall buckled, neighboring columns quickly became overloaded. The top section of the building tilted to the east and to the south and began its descent. WTC 2 collapsed more quickly than WTC 1 because there was more aircraft damage to the core and there were early and persistent fires on the east side of the building, where the aircraft had extensively dislodged insulation from the structural steel.
  • The WTC towers likely would not have collapsed under the combined effects of aircraft impact damage and the extensive, multifloor fires if the thermal insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.
  • In the absence of structural and insulation damage, a conventional fire similar to or less intense than the fires of September 11 likely would not have led to collapse.

Objective 2: Determine why injuries and fatalities were so high or low depending on location, including technical aspects of fire protection, occupant behavior, evacuation, and emergency response.

  • Approximately 87 percent of the estimated 17,400 occupants, and 99 percent of those located below the impact floors, evacuated successfully. In WTC 1, where the aircraft destroyed all escape routes, 1,355 people were trapped on upper floors when it collapsed. Of the people below the impact floors, 107 did not survive.
  • In WTC 2, before impact, about 3,000 people got low enough to escape by a combination of self-evacuation and use of elevators. After impact, the elevators and two of the three stairways were unusable. Of the people above the impact zone, 18 found a passage through the damaged third stairway and escaped. The other 619 people in or above the impact zone and 11 people below it perished.
  • About 6 percent of the survivors described themselves as mobility impaired; few, however, required a wheelchair. Among the 118 decedents below the impact floors, 7 were identified as mobility challenged. The mobility of the other 111 could not be determined.
  • A principal factor limiting loss of life was that the buildings were one-third to one-half occupied at the time of the attacks. Since the flow of people from both buildings had slowed considerably before collapse, the stairwell capacity was adequate to evacuate the occupants on that morning. However, if the towers had been fully occupied with 20,000 occupants each, evacuation would have taken just over three hours. About 14,000 people might have perished because of insufficient stairwell capacity. Egress capacity required by current building codes is determined by single floor calculations that are independent of building height and does not consider the time for full building evacuation.
  • Due to the assembly use spaces at the top of each tower, designed to accommodate over 1,000 occupants per floor, the New York City Building Code would have required a minimum of four independent means of egress (stairs), one more than were available. Given the low occupancy level on September 11, NIST found that egress capacity from these places of assembly, and from elsewhere in the buildings, was not a significant factor. If the buildings had been fully occupied, the required fourth stairway would likely have mitigated the insufficient egress capacity.
  • Evacuation was assisted by participation in fire drills within the previous year by two-thirds of survivors and perhaps hindered by a local law that prevented employers from requiring occupants to practice using the stairways. The stairways were not easily navigated in some locations due to their design, which included “transfer hallways” that evacuees had to traverse to get from one stairway to another. Additionally, many occupants were unprepared for the physical challenge of full building evacuation.
  • The functional integrity and survivability of the stairwells was affected by their separation and the structural integrity of their enclosures. In WTC 1’s impact region, the stairwell separation was the smallest over the building height, and all stairwells were destroyed by the impact. By contrast, the separation of stairwells in the impact region of WTC 2 was the largest over the building height, and one of three stairwells remained marginally passable. The shaft enclosures were fire rated but were not required to have structural integrity under typical accidental loads: there were numerous reports of stairwells obstructed by fallen debris from damaged enclosures.
  • The fire safety systems (sprinklers, smoke purge, and fire alarms) were designed to meet or exceed current practice. However, they played no role in life safety on September 11. The water supplies to the sprinklers were fed by a single pipe that was damaged by the impact. The smoke purge systems, designed for use by the fire department after fires, were not turned on but would also have been ineffective due to aircraft damage. The violence of the aircraft impact served as its own alarm. In WTC 2, contradictory public address announcements contributed to confusion and evacuation delay.
  • For the approximately 1,000 emergency responders on the scene, this was the largest disaster they had ever seen. Despite attempts by the responding agencies to work together and perform their own tasks, the extent of the incident was well beyond their capabilities. Communications were erratic due to the high number of calls and the inadequate performance of some of the gear. Even so, there was no way to digest, test for accuracy, and disseminate the vast amount of information being received. Their jobs were complicated by the loss of command centers in WTC 7 and then in the towers after WTC 2 collapsed. With nearly all elevator service disrupted and progress up the stairs taking about two minutes per floor, it would have taken hours for the responders to reach their destination, assist survivors, and escape had the towers not collapsed.

Objective 3: Determine what procedures and practices were used in the design, construction, operation, and maintenance of WTC 1, 2, and 7.

  • Because of the Port Authority’s establishment under a clause of the U.S. Constitution, its buildings were not subject to state or local building regulations. The buildings were unlike any others, both in height and structural innovation. Nevertheless, the design and approval process produced two buildings that were generally consistent with nearly all the provisions of the New York City Building Code and other building codes of the time that were reviewed by NIST. The loads for which the buildings were designed exceeded the New York City code requirements. The quality of the structural steels was consistent with building specifications. The departures from the building codes and standards did not have a significant effect on the outcome of September 11.
  • For the floor systems, the fire rating and insulation thickness used on the floor trusses, which together with the concrete slab served as the main source of floor support, were of concern since the initial construction. The minimum specified thickness of the insulation was adequate to delay heating of the trusses; the amount of insulation dislodged by the aircraft impact, however, was sufficient to cause the structural steel to be heated to critical levels.
  • Based on four standard fire resistance tests that were conducted under a range of insulation and test conditions, NIST found the fire rating of the floor system to vary between forty-five minutes and two hours. In all cases, the floors continued to support the full design load without collapse for over two hours.
  • The wind loads, which governed the structural design of the external columns and provided the baseline capacity of the structures to withstand abnormal events, such as major fires or impact damage, significantly exceeded the requirements of the New York City Building Code and other building codes of the day.

Recommendations

The tragic consequences of the attacks on September 11 were directly attributable to the fact that terrorists flew large jet-fuel laden commercial airliners into the WTC towers. Buildings for use by the general population are not designed to withstand attacks of such severity. Building codes do not require building designs to consider aircraft impact. In our cities, there has been no experience with a disaster of such magnitude, nor has there been any in which the total collapse of a high-rise building occurred so rapidly and with so little warning.

While there were unique aspects to the design of the WTC towers and the terrorist attacks, NIST has compiled a list of recommendations to improve the safety of tall buildings, their occupants, and emergency responders based on its investigation of the procedures and practices that were used for the WTC towers. These procedures and practices are commonly used in the design, construction, operation, and maintenance of buildings under normal all-hazards conditions. Public officials and building owners will need to determine appropriate performance requirements for those buildings that are at higher risk due to their iconic status, critical function, or design.

The report features eight major groups of recommendations:

  • Increased Structural Integrity: The standards for estimating the load effects of potential hazards and the design of structural systems to mitigate their effects should be improved to enhance structural integrity.
  • Enhanced Fire Resistance of Structures: The procedures and practices used to ensure the fire resistance of structures should be enhanced by improving the technical basis for construction classifications, fire resistance ratings, and standard fire resistance testing methods using the “structural frame” approach to fire resistance ratings and developing in-service performance requirements and conformance criteria for sprayed fire-resistive materials.
  • New Methods for Fire Resistance Design of Structures: The procedures and practices used in designing fire resistant structures should require an objective that uncontrolled fires result in burnout without local or global collapse. This effort should include the development and evaluation of new fire resistive coating materials and technologies and evaluation of the fire performance of conventional and high-performance structural materials.
  • Improved Active Fire Protection: Active fire protection systems (i.e., sprinklers, standpipes/hoses, fire alarms, and smoke management systems) should be enhanced through improvements to design, performance, reliability, and redundancy.
  • Improved Building Evacuation: Building evacuation should include system designs that facilitate safe and rapid egress, methods for ensuring clear and timely emergency communications to occupants, better occupant preparedness for evacuation during emergencies, and incorporation of appropriate egress technologies.
  • Improved Emergency Response: Technologies and procedures should enable better access to buildings, response operations, emergency communications, and command and control.
  • Improved Procedures and Practices: Procedures and practices used in the design, construction, maintenance, and operation of buildings should include encouraging code compliance by nongovernmental and quasi-governmental entities, adoption and application of egress and sprinkler requirements in codes for existing buildings, and retention and availability of building documents.
  • Education and Training: The professional skills of building and fire safety professionals should be upgraded through a national education and training effort for fire protection engineers, structural engineers, architects, and building regulatory and fire-service personnel.

NIST strongly urges that immediate and serious consideration be given to these recommendations, as they relate to both new and existing structures, by the building and fire safety communities to make buildings, their occupants, and emergency responders safer in future emergencies. The recommendations call for action by specific entities regarding standards, codes and regulations (their adoption and enforcement), professional practices, education and training, and research and development. Only when each of the entities carries out its role will the implementation of a recommendation be effective.

S. Shyam Sunder (sunder@nist.gov)
National Institute of Standards and Technology

Download the full report for free from http://wtc.nist.gov/.

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