Fire, Life Safety

World Trade Center building emergency lighting disasters

While overshadowed by the 9/11 attack, an earlier WTC terrorist bombing on Feb. 26, 1993, was an incredibly significant event in its own right.
By John Yoon, PE, LEED AP ID+C; McGuire Engineers Inc., Chicago March 28, 2019
Figure 2: Exit signage using a pictograph compliant with international ISO standard and NFPA 170-2018. All graphics courtesy: McGuire Engineers Inc.

The 1993 terrorist bombing and the 9/11 attack on the World Trade Center (WTC) buildings in New York City gave us a tragically unfortunate before-and-after comparison of the impact that improvements to the egress path can have on the ability to quickly evacuate a building. While overshadowed by the 9/11 attack, an earlier WTC terrorist bombing on Feb. 26, 1993, was an incredibly significant event in its own right. The chaos in the immediate aftermath of that bombing highlighted serious deficiencies in the design of the facility’s life safety systems.

During this event, a truck bomb parked in the underground B2 level of the complex exploded shortly after noon. This occurred when the buildings were nearly fully occupied. It was estimated that 50,000 people were evacuated—the largest building evacuation ever recorded. Due to the prevailing “defend-in-place” doctrine where only select floors are evacuated, the number and sizes of egress paths were never designed to accommodate a mass evacuation.

One thousand and forty-two injuries were reported, with the most occurring during the evacuation of the facility. Of those injured, only 15 received injuries from the blast itself. The estimated evacuation times from the upper floors of the first and second WTC buildings ranged anywhere from 1.5 to 3 hours. Aside from congested stairwells, a number of factors contributed to this extended evacuation time frame—the failure of the fire command center on level B1, loss of power to the elevators, the inability of numerous occupants with physical conditions (heart issues, pregnant women, older people, etc.) to navigate the stairs, etc. However, let’s focus on two additional major causes that impaired evacuation efforts:

  • Smoke had migrated from the basement through the elevator shafts and stairs to midrise levels, which limited visibility. Reports stated that the smoke on the 44th floor of the first WTC building reduced visibility to less than 1 ft. The amount of smoke present on the mezzanine level, where many of the stairwells discharged, also confused people, making it difficult to identify where to exit the building.
  • The lighting in the stairwells had failed. The cooling systems supporting the emergency generators on level B6 were damaged in the blast and the generators overheated after 20 minutes, plunging the stairwells into darkness.

In the aftermath, significant life safety upgrades were made to the WTC complex. These upgrades included;

  • Installation of multiple sources of power for the stair lighting including unit battery backup for every other light fixture in the stairwells.
  • New secondary backup generators.
  • Photoluminescent paint on handrails, stair treads, and stair centerlines.
  • LED exit signs for extra brightness and visibility through smoke conditions.

Less than 10 years later, the 9/11 WTC attack tested the effectiveness of those life safety upgrades. The National Institute of Standards and Technology (NIST) commissioned numerous technical reports exhaustively evaluating almost every imaginable aspect of the 9/11 disaster. One particular report, titled “Occupant Behavior, Egress, and Emergency Communications” provided egress-performance data that can be directly compared with the earlier Federal Emergency Management Agency (FEMA) report on the 1993 terrorist bombing.  According to that NIST report, crowded stairwells were the overwhelming impediment to evacuation. However, overall evacuation times were faster during 9/11 by a factor of 3 to 4 than those in 1993. The average evacuation time in the first WTC building was 41.9 minutes, with floors above the 77th floor taking 70.3 minutes (the airplane impacted the building at the 91st floor). For the second WTC building, the average evacuation time was 25 minutes. This significant difference was generally attributed to the use of elevators in the second WTC building prior to the second impact. Over 90% of the second WTC building’s survivors started to evacuate in the 16-minute gap between the first impact on the first WTC building and the second impact on the second WTC building.

Based on survivor interviews, constraints to evacuation caused by poor lighting was limited to 11% of the first WTC building’s respondents and 4% of the second WTC building’s respondents. In addition, 33% of survivors from the first WTC building and 17% of those in the second WTC building reported that they were helped by the photoluminescent markings in the stairwell. This discrepancy between the two towers was attributed in the report to two facts: (1) Lights were lost in the first WTC building after the second WTC building collapsed; (2) Those who used the elevators in the second WTC building would not have observed the photoluminescent markings in the stairwells.

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John Yoon, PE, LEED AP ID+C; McGuire Engineers Inc., Chicago
Author Bio: John Yoon is lead electrical engineer at McGuire Engineers. He is a member of the Consulting-Specifying Engineer editorial advisory board.