Latest Findings from NIST World Trade Center Investigation Released

Key findings not previously reported by the WTC investigation team include the following:

• Leading hypotheses for the collapse of WTC 1 and WTC 2 (the towers) developed
• Time delay between the collapses of the WTC towers explained
• Post-impact capabilities of the WTC towers assessed
• Fire-induced core column shortening detected
• Role of fireproofing determined
• Majority of steel found stronger than minimum requirements
• Full-building evacuation presented challenges for occupants
• Movement in WTC 1 stairwells perceived as a problem
• Evacuees did not receive coordinated or informative communications
• Mobility impaired occupants faced special evacuation challenges
• First responder command and control was hampered

Details below.

Leading hypotheses for the collapse of WTC 1 and WTC 2 (the towers) developed.
These hypotheses identify the chronological sequence of major collapse events for each tower and identify specific load redistribution paths and damage scenarios. Previously, a single working hypothesis was defined for both towers without identification of the load redistribution paths and damage scenarios resulting from aircraft impact and the subsequent fires. The two hypotheses are detailed in the attached sheet.

Time delay between the collapses of the WTC towers explained.
Although the WTC towers were attacked by virtually identical aircraft, WTC 1 stood for 103 minutes before it collapsed—nearly twice as long as WTC 2, which survived for 56 minutes. The buildings themselves, although not identical, had many similarities. The time delay between the collapses was due primarily to: (1) the asymmetrical structural damage of the aircraft impact to WTC 2 compared to the aircraft damage to WTC 1; (2) the time it took for heat to soften, buckle and shorten core columns that had fireproofing dislodged by debris impact; (3) the structure’s ability to redistribute loads as the core columns shortened; (4) the time it took for fires to traverse from their initial location to the face of the towers where perimeter columns were bowing inward (as seen only minutes before the collapse of each tower); and (5) the time it took for heat to soften and buckle those columns.

Post-impact capabilities of the WTC towers assessed.
Demand to capacity ratios—the calculations indicating whether or not structures can support the loads put on them—showed that for the floors affected by the aircraft impacts, the majority of the core and perimeter columns in both towers continued to carry their loads after the impact. The loads from damaged or severed columns were carried by nearby undamaged columns. Although the additional loads strained the load-bearing capabilities of the affected columns, the results show that the columns could have carried them. This shows that the towers withstood the initial aircraft impacts and that they would have remained standing indefinitely if not for another significant event such as the subsequent fires. NIST previously reported that the towers had significant reserve capacity after aircraft impact based on analysis of post-impact vibration data obtained from video evidence on WTC 2, the more severely damaged tower.

Fire-induced core column shortening detected.
Due to heating from fires following the aircraft impacts and subsequent buckling, there was a shortening of core columns seen in both towers on floors at or near the fire-affected impact sites. Shortening of the core columns caused the floor system to pull the perimeter columns inward—the observed inward bowing that was seen minutes prior to the collapse of each tower. Significant thermal sagging of the floor system exacerbated the inward pull on the perimeter columns in WTC 2. Vertical loads carried by shortened columns were redistributed to perimeter columns, putting additional strain on their load-bearing capabilities.

Role of fireproofing determined.
The structural components that became weakened due to the fires and eventually caused the towers to collapse had their fireproofing dislodged by debris from the aircraft impact. The region of dislodged fireproofing was determined from the predicted path of the debris. Had the fireproofing not been dislodged, the temperature rise of the structural components would likely have been insufficient to cause the global collapse of the towers. Fireproofing dislodged by debris left the components more sensitive to heat than any areas where there was missing or thin fireproofing before the aircraft impacts.

Majority of steel found stronger than minimum requirements.
Approximately 87 percent of the recovered WTC steel specimens tested exceeded the required minimum yield strengths specified in the building design criteria; some 13 percent did not. However, the safety of the towers was most likely not affected by the small percentage of steel below the minimum. Building designs routinely allow structures to withstand greater loads than are expected by including significant factors of safety. Moreover, the structural loads on Sept. 11, 2001, were well below this design level.

Full-building evacuation presented challenges for occupants.
Based on first-person interview data, an assessment of WTC 1 and 2 occupant preparedness concluded that in both towers:

• Occupants often were unprepared for the physical challenge of full building evacuation;
• Occupants often were unprepared to encounter transfer hallways during the stairwell descent; and
• Mobility challenged occupants were not universally identified or prepared for full building evacuation.

Movement in WTC 1 stairwells perceived as a problem.
Although a number of persons who evacuated WTC 1 reported that they perceived a problem with counterflow (the movement of firefighters in the opposite direction) on the stairwells, it was determined not to be a significant factor in the total evacuation time of WTC 1 occupants when compared to other factors including delays in evacuation initiation, evacuation interruption and encountering obstacles in the evacuation path (such as smoke, water and debris).

Based on first-person interviews, NIST estimates the average surviving occupant spent 48 seconds per floor descending the stairwell, which is about half as fast as previously reported for non-emergency evacuations. NIST also estimates that each stairwell door exited about 37 people per minute, which is comparable to the slowest rate previously reported for non-emergency evacuations. In other words, the average surviving occupants moved slower down stairs and through stairwell exits than previously reported for non-emergency evacuations.

Firefighters and other first responders reported difficulty in climbing the stairs due to crowding by evacuating occupants. Based on first-responder interviews, NIST estimates that they took an average of 1.4 to 2 minutes per floor to climb up to their maximum height (mostly to floors in the 20s and 30s). Therefore, it would have taken more than two hours for a first responder wearing personal protection equipment and carrying gear to reach the 60th floor using the stairwell, while it would have taken about 1-1/2 hours to do so without equipment and gear.

Evacuees did not receive coordinated or informative communications. During interviews, survivors said that they felt emergency communications could have been more helpful during the evacuation of the towers. Specific knowledge about the location of fires and aircraft impact damage was only occasionally communicated to occupants who requested the information. Those communications were apparently uncoordinated.

Additionally, some contradictory announcements—first to return to offices and then to start an orderly evacuation—were heard by occupants in WTC 2 immediately prior to the aircraft impact on that tower.

Mobility impaired occupants faced special evacuation challenges. About 6 percent of the surviving occupants reported a pre-existing limitation to their mobility. Examples of these limitations include obesity, heart conditions, pregnancy, advanced age and recent surgery.

Firefighters and police officers found 40 to 60 mobility impaired occupants on the 12th floor of WTC 1 as they attempted to clear each floor on their way out. The impaired individuals had been placed on this floor to await rescue in an attempt to clear the stairway. Emergency responders were assisting approximately 20 of these persons down the staircase just prior to the tower’s collapse.

First responder command and control was hampered.
While a significant amount of evidence showed that the different first responder agencies were, for the most part, working together, they were hampered by inadequate information, dispatch and unit assignment records.