Reconstruction of the collapses of the New York World Trade Center buildings (2000s)
- After the 9/11 disaster, Congress created the National Construction Safety Team, authorizing NIST to investigate disastrous building failures.
- Based on its investigation, NIST created a series of recommendations for ways to make buildings safer.
- Those recommendations led to many upgrades to the nation’s building and fire codes that will reduce the risk of future tall building collapses and improve the safety of occupants and emergency responders.
The attacks on the United States on Sept. 11, 2001, shocked the world. Hijacked airliners were flown into the World Trade Center (WTC) in downtown Manhattan. The resulting fires led to the collapse of the two main towers, WTC 1 and WTC 2, killing more than 2,700 building occupants, emergency responders, bystanders, and aircraft passengers and crew. The neighboring 47-story building, WTC 7, collapsed later that day, also because of uncontrolled fires.
It was important to understand how the collapse happened and whether other tall buildings could be protected. So the U.S. Congress passed the National Construction Safety Team Act (NCSTA), which expanded NIST’s authority to investigate “events causing the failure of a building or buildings that has resulted in substantial loss of life or that posed significant potential for substantial loss of life.”
NIST quickly started the work to determine how and why the three buildings collapsed; why the injuries and fatalities happened; how the buildings were designed, constructed and operated; and whether current fire and building codes and standards should be revised based on this new knowledge.
This analysis was unprecedented in its technical demands and societal importance. Using photographs and video of the three buildings that day, NIST staff created visual renderings of the spread of the fires, the distortion of the buildings, and their eventual collapse.
NIST and its contractors modeled the aircraft impact on each of the two towers, as well as the damage to each building structure and the thermal fire insulation on the structural members. NIST staff greatly enhanced the Fire Dynamics Simulator (FDS) to recreate the fires. The FDS output was fed into the innovative structural models of the buildings in order to understand how the heat weakened the steel and concrete structures.
NIST found that the airplanes considerably damaged the columns and floors of both towers where they hit. However, the impact alone was not enough to cause the collapse. Each tower would have remained standing were it not for the dislodged structural insulation and the rapidly spreading, multifloor fires ignited by the aircraft’s burning jet fuel. The automatic fire sprinklers in the buildings might have controlled the flames, but the impact had damaged their water supply lines.
In WTC 1, the fire-weakened floors on the side opposite the aircraft impact sagged, pulling inward on the heat-weakened perimeter columns. As more columns weakened, the structure could no longer support its mass, and the building collapsed in 102 minutes. In WTC 2, the structural damage was more severe than that of WTC 1, and the building collapsed in just 56 minutes, also as a result of fire-weakened structural elements. WTC 7 was struck by the debris from the collapse of WTC 1. The structural damage was relatively minor, but the debris ignited fires on at least 10 stories. Over the next seven hours, the fires spread around the building, weakening the floors and their connections to a critical interior column. The unsupported column buckled, leading to other structural weaknesses and finally the collapse of the entire building.
The two towers were less than half occupied at the time of the attack. From computer modeling, NIST estimated that an evacuation of each fully occupied building would have taken three hours, and 14,000 people would have perished. Instead, all but 107 of the estimated 7,500 WTC 1 occupants who were below the impact floors survived; none of the 1,355 occupants in or above the impact zone survived since the aircraft had destroyed all escape routes. In WTC 2, only 11 of the estimated 6,000 occupants who were initially below the impact floors perished. Of the 637 occupants of the higher floors, 18 found an escape route through a damaged stairwell. Over 400 emergency responders in or near the towers perished when the towers collapsed. The occupants of WTC 7 had evacuated well before that building collapsed.
Following their investigation, NIST recommended 31 improvements to address the fire safety of tall buildings, their occupants, and emergency responders. NIST staff partnered with other professionals to effect changes in the nation’s building and fire codes that address at least 16 of these. These changes for tall buildings include:
- The ratings of structural components and assemblies must be increased to resist fire for one additional hour.
- All members of the structural frame of a building must have the same (or higher) fire resistance rating as the rating commonly required for columns.
- The bond strength of fireproof coatings to structural components must be increased by a factor of three for medium-height buildings and a factor of seven for taller buildings.
- There must be at least two water supply pipe systems for each sprinkler zone. These water sources must be located in stair enclosures that are far away from each other.
- Exit stairways must be enclosed and be a minimum distance apart.
- Tall buildings must have an additional stairway exit, and the width of stairway exits must be increased by 50% in all new buildings with large floor areas.
- Tall buildings must have specially protected elevators for emergency evacuation.
- Medium and tall buildings must have luminous markings along the exit paths.
- Radio coverage for emergency responders within the building must be as effective as public safety communication systems at the exterior of the building.
- Stair-descent devices must be installed for mobility-impaired occupants.
- Structural design must meet a new standard provision designed to keep modest damage in one area from collapsing the entire building.
Many of these responses to the NIST recommendations have been incorporated into the Freedom Tower and the rebuilt WTC 7 on the World Trade Center site. The lessons learned from the 9/11 tragedy will help ensure that future buildings are safer from collapse.
In 2021, Dick Gann wrote a first-person account about his experiences as part of the WTC investigation team, and specifically his role in reconstructing the fires that brought down WTC 1, 2 and 7.
Additional Reading:
NIST report: Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report on the Collapse of World Trade Center Building 7, 2008.
NIST report: Best Practice Guidelines for Structural Fire Resistance Design of Concrete and Steel Buildings, 2010.
NIST news: New Building Standard Paves the Way for Collapse-Resistant Structures, 2023.