Federal investigationoftheevacuationoftheWorld ... · SEPTEMBER 11, 2001—EVACUATION 5.1. WTC 1 In...

Federal investigation of the evacuation of the World Trade Center onSeptember 11, 2001

Jason D. Averill1,*,†, D. Mileti2, R. Peacock1, E. Kuligowski1, N. Groner3, G. Proulx4,P. Reneke1 and H. Nelson5

1Natonal Institute of Standards and Technology, Gaithersburg, MD, USA2University of Colorado – Boulder, Boulder, CO, USA

3John Jay College, New York City, NY, USA4National Research Council Canada, Ottawa, ON, Canada

5Independent Consultant

Received 9 July 2012; Accepted 10 July 2012

KEY WORDS: evacuation; world trade center; human behavior

1. SCOPE OF THE US FEDERAL INVESTIGATION

The National Institute of Standards and Technology (NIST) announced its building and fire safetyinvestigation of the World Trade Center (WTC) disaster on August 21, 2002. The NIST WTCInvestigation was conducted under the authority of the National Construction Safety Team Act(Public Law [P.L.] 107 231).

The goals of the WTC Investigation were the following: (1) investigate the building construction,the materials used, and the technical conditions that contributed to the outcome of the WTC disasterand (2) serve as the basis for the following:

• Improvements in the way buildings are designed, constructed, maintained, and used,• Improved tools and guidance for industry and safety officials,• Recommended revisions to current codes, standards, and practices, and• Improved public safety.

The objectives of the NIST-led Investigation{ of the WTC disaster were as follows: (1) determinewhy and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft, (2)determine why the injuries and fatalities were so high or low depending on location, including alltechnical aspects of fire protection, occupant behavior, evacuation, and emergency response, (3)determine what procedures and practices were used in the design, construction, operation, andmaintenance of WTC 1 and 2, and (4) identify, as specifically as possible, areas in current buildingand fire codes, standards, and practices that warrant revision.

The investigation included eight interdependent projects that, in combination, met the objectives. Adetailed description of each of these eight projects is available at http://wtc.nist.gov.

{NIST is a nonregulatory agency of the US Department of Commerce. NIST investigations are focused on fact finding,not fault finding. No part of any report resulting from an NIST investigation into a structural failure or from aninvestigation under the National Construction Safety Team Act may be used in any suit or action for damages arisingout of any matter mentioned in such report (15 USC 281a, as amended by P.L. 107 231).

*Correspondence to: Jason D. Averill, Natonal Institute of Standards and Technology, Gaithersburg, MD, USA.†E-mail: [email protected]

Q1

Copyright © 2012 John Wiley & Sons, Ltd.

FIRE AND MATERIALSFire Mater. (2012)Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fam.2162

Journal Code Article ID Dispatch: 19.07.12 CE: Vilan, Sandy CheromylF A M 2 1 6 2 No. of Pages: 9 ME:

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http://wtc.nist.gov

2. BACKGROUND

Although most attention has properly focused on the nearly 3000 people who lost their lives at theWorld Trade Center site that day, five times that many people successfully evacuated from the WTCtowers because of heroic efforts of occupants, as well as emergency responders. Understanding whymany, yet not all, survived the World Trade Center attacks was one of the four objectives of thisinvestigation.

Success in evacuating a building in an emergency can be characterized by two quantities: the timepeople needed to evacuate and the time available for them to do so.

To the extent that the first time exceeded the second, it follows that there will be casualties. Whenthe second time exceeds the first, perhaps by some suitable margin, nearly all should be able toevacuate the building.

For the World Trade Center towers, the times available for escape were cataclysmically establishedby the collapses of the buildings. Those times were not known in advance by the building occupants orthe responders. The times were also considerably shorter, by a factor of three or four, than the timeneeded to clear the tenant spaces of WTC 1 following the 1993 bombing and an additional factor oftwo shorter than the time needed to clear the last person from the elevators in the building. Further,some occupants would have been unable to evacuate the buildings given any amount of timebecause of injuries, disabilities, entrapment, and/or toxic exposure.

The investigation team examined the design of the building, the behavior of the people, and theevacuation process in detail to ascertain the parameters that factored prominently in the time neededfor evacuation. In order to accomplish this objective, numerous sources of data were collected andanalyzed, including over 1000 new interviews with survivors; a collection of over 700 publishedinterviews with WTC survivors; 9-1-1 emergency calls; transcripts of emergency communicationamong building personnel and emergency responders; historical building design drawings,memoranda, and calculations; building modifications and upgrades; formal complaints filed withOccupational Safety and Health Administration (OSHA); and other relevant material.

3. INTERVIEW METHODOLOGY

There were three forms of interviews with survivors: 803 telephone interviews, 225 face-to-faceinterviews, and six focus groups.

3.1. Telephone interviews

The telephone interviewees were randomly selected using independent proportionate stratificationfrom a list of occupants who had badges to enter WTC 1 or WTC 2 on September 11, 2001. Inother words, each occupant of a particular tower had an equal probability of being selected. Roughly400 occupants in each tower were interviewed in order to achieve a high level of statistical precisionwithin each tower. Reported percentages from tower-specific survey data (n = 400) exhibitedsampling errors no greater than 2.5% age points and 95% confidence intervals of percentages are nogreater than� 5 percentage points. This level of precision was more than adequate for examiningcharacteristics of occupants and egress attributes. With telephone interview results, primarystatistical analyses were in the form of tabulations and linear statistics (e.g., reporting of percentagesand average/means). The telephone interview results enabled a scientific projection of the populationand distribution of occupants in WTC 1 and WTC 2, as well as causal modeling and multivariateregression analysis to explore fundamental egress issues such as sources of evacuation delay.

3.2. Face-to-face interviews

The objective of the face-to-face interviews was to gather first-hand accounts and observations of theactivities and events inside the buildings on the morning of September 11. This approach identifiedunknown information, aided in the evaluation of technical hypotheses, and explored motivations foroccupant behaviors, while allowing for comparisons to the telephone interview data. There was no

2 J. D. AVERILL ET AL.

Copyright © 2012 John Wiley & Sons, Ltd. Fire Mater. (2012)DOI: 10.1002/fam

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recording of the face-to-face interviews, other than random selections, with consent of respondents, forquality control purposes. A typical face-to-face interview averaged approximately 2 h. Themethodology for the face-to-face interviews was a synthesis of two established methodologies,designed to assist survivors in providing comprehensive and accurate accounts of their evacuation,given the latency between experience and interview. Some groups of occupants were specificallysought in order to explore targeted unknowns. These included occupants near the floors of impact,witnesses to fireballs, mobility challenged occupants, floor wardens, building personnel withemergency response responsibilities, family members who spoke to an occupant after 8:46:30 AM

and occupants from regions of the building not addressed by other groups in order to ensureadequate interview coverage for all areas of both towers.

3.3. Focus group interviews

Six focus groups were conducted in order to elicit accurate group representations of specific events orthemes and complement the findings of the telephone and face-to-face interviews. The focus groupswere the following: (1) occupants located near the floors of impact, (2) floor wardens, (3) mobilitychallenged occupants, (4) persons with building responsibilities, (5) randomly selected evacuees inWTC 1, and (6) randomly selected evacuees in WTC 2.

4. OCCUPANT CHARACTERISTICS

Using the statistical sampling methods of the telephone interviews, NIST was able to estimate thatthere were 8900� 750 people in WTC 1 at 8:46:30 AM on September 11, 2001. Similarly, NISTestimated that there were 8540� 920 people inside WTC 2 at 8:46:30 AM. New York City officiallyannounced 2749 fatalities at the World Trade Center, including emergency responders, airplanepassengers and crew (not including the 10 hijackers), and bystanders. NIST estimated that of the17,400� 1180 occupants inside WTC 1 and WTC 2 at 8:46:30 AM, 2163–2180 perished. (Noinformation could be found for 17 persons. The remaining individuals were emergency responders,airline passengers, or bystanders.) More than twice as many occupants were killed in WTC 1 asWTC 2, largely because of the fact that occupants in WTC 2 used the 16min between the attacks onWTC 1 and WTC 2 to begin evacuating, including the use of elevators by some occupants in WTC 2.

The demographic characteristics of the evacuees were explored where the characteristics wererelevant to the evacuation on September 11, 2001. Few differences in the characteristics of WTC 1or WTC 2 were observed. Men outnumbered women roughly two to one. The average age was mid40s. The mean length of employment at the WTC site was almost 6 years, whereas the median was2 and 3 years for WTC 1 and 2, respectively. A total of 16% of 2001 WTC evacuees were alsopresent during the 1993 bombing, although many other occupants were also knowledgeable aboutthe 1993 evacuation. Two-thirds of the occupants had participated in at least one fire drill during the12months immediately prior to September 11, 2001. A total of 18% did not recall whether they hadparticipated in a fire drill during that period and 18% reported that they did not participate in a firedrill during that period.

5. SEPTEMBER 11, 2001—EVACUATION

5.1. WTC 1

In WTC 1, all three stairwells and the elevators were destroyed in the impact region, extending as lowas floor 92. No occupant evacuated from above the 91st floor, although some survived until thebuilding collapsed after 102min. Helicopter rescue from the roof was considered by an NYPDaviation unit but deemed not possible because of the heat and smoke from the building fire.Occupants of both towers delayed initiating their evacuation after WTC 1 was hit. In WTC 1, themedian time to initiate evacuation was 3min for occupants from the ground floor to floor 76 and5min for occupants near the impact region (floors 77–91). Occupants observed various types of

FEDERAL INVESTIGATION OF THE EVACUATION OF THE WORLD TRADE CENTER 3


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impact indicators throughout the building, including wall, partition, and ceiling damage and fire andsmoke conditions. The most severe damage was observed near the impact region, fatally trappingsome occupants. Announcements in WTC 1 were not heard by the occupants, despite repeatedattempts from the lobby fire command station to order an evacuation. Damage to criticalcommunications hardware likely prevented announcement transmission. Evacuation rates reached apeak, steady-state in approximately 5min and remained roughly constant until the collapse ofWTC 2, when the rate in WTC 1 slowed to about one-fifth of the peak, steady-state. WTC 1collapsed at 10:28:22 AM, resulting in approximately 1500 occupant deaths, 111 of which wereestimated to be below the 92nd floor. A rest station for mobility challenged occupants wasestablished in WTC 1 somewhere between floors 12 and 20. Less than 10min prior to the collapseof WTC 1, the occupants and helpers on the floor were ordered to evacuate, although it remainsunclear whether all rest station residents survived. Figure F11 shows the rate of evacuation for WTC 1and WTC 2 during the time from the attack on WTC 1 until the collapse of WTC 1.

5.2. WTC 2

The evacuation of WTC 2 was markedly different from the evacuation of WTC 1. There was a 16-minperiod after WTC 1 was attacked but before WTC 2 was attacked. During this period, occupants wereforced to decide whether to remain inside WTC 2, and if they decided to leave, they had to choosebetween using one of the three stairwells or using an elevator. Further complicating this decision processwere multiple, conflicting announcements around 9:00AM, first instructing occupants to return to theiroffices, and then within 1min of impact, instructing them to begin an evacuation if conditions on theirfloor warranted that decision. Over 90% of WTC 2 survivors started to evacuate the building prior to itsbeing attacked. A total of 16% of the survivors used elevators to evacuate. Approximately, 75% of theoccupants who were above the 78th floor (the lowest floor of impact) descended to at least below theimpact region prior to the attack on WTC 2. Over 40% of the survivors had left WTC 2 prior to9:02:59AM. After WTC 2 was attacked, at least 18 individuals used stairwell A, located in the northwestcorner and furthest from the impact damage, to descend below the 78th floor to evacuate the building.Additional public address announcements were made after the airplane strike on WTC 2, althoughoccupants who survived generally did not hear those announcements. After the initial peak in evacuationrate due to concurrent elevator and stairwell usage, the rate reached a steady-state similar to the rateobserved in WTC 1 until approximately 20min prior to the collapse of WTC 2. The evacuation rateduring the final 20min dropped significantly, likely because of a decreased number of occupantsremaining in the egress system below the 78th floor. NIST analysis indicated that only eight occupantsinitially below the 78th floor were killed when WTC 2 collapsed at 9:58:59AM. Overall, NIST estimatedthat 626 occupants of WTC 2 perished.

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Figure 1. Percentage of occupants remaining inside WTC 1 and WTC 2 as a function of time.

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6. CAUSAL MODELING

Using the statistical power of the telephone interview results, two models were constructed to explorethe primary components of total evacuation time: evacuation initiation delay and stairwell traversaltime, as shown in Figure F22. Each model explained between 49% and 56% of the variance in theultimate dependent variable, which are high levels for human behavior studies.

6.1. Evacuation initiation delay

The first component of total evacuation time is the time delay prior to starting evacuation. This wasdefined as the time from the attack on WTC 1 until the occupant left the floor using a stairwell orelevator to leave the building. The factors that best predicted evacuation initiation delay in WTC 1were (1) which floor the respondent was on when WTC 1 was attacked, (2) whether occupantsencountered environmental cues (smoke, fire, debris, etc.), and (3) seeking additional information(or milling) about the nature of the event. In WTC 2, the same process occurred as in WTC 1,except that perceived risk (sense of immediate danger) was a predictor of seeking additionalinformation (along with floor and environmental cues).

6.2. Normalized stairwell evacuation time

The second component of the total evacuation time was the time spent in the stairwells. This analysisdetermined the factors and social processes that influenced the normalized stairwell evacuation timeper flight of stairs for the people who evacuated WTC 1 on September 11, 2001. WTC 2 wasexcluded from this analysis because evacuees used stairs, elevators, and/or a combination of both fortheir evacuation and could not be separated for the analysis. Evacuation time was defined as theaverage number of seconds per flight of stairs that it took people from the time they entered astairwell until they completed their evacuation out of the building. The model used to predictimportant factors in stairwell evacuation time again used variables that preliminary analyses andgeneral evacuation theory suggested as salient.

The main process that led to increased normalized stairwell evacuation time in the evacuation ofWTC 1 on September 11 was straightforward and clear. Floor (increased distance to safety)substantially increased the odds that people would encounter environmental cues. Floor alsoincreased delay in starting evacuation (this relationship was elaborated upon in the first model),which, in turn, also increased the chances that people would encounter environmental cues. But itwas encountering environmental cues that had a large and direct effect on increasing the amount oftime that people spent, on average, to traverse their evacuation stairwell. In addition to this multistep

Figure 2. Causal models for (left) evacuation initiation delay (WTC 1 and WTC 2) and (right) normalizedstairwell evacuation time (WTC 1).

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process with environmental cues as the key predicting variable, interrupting the process of evacuationfor any reason also increased the amount of time, on average, that people used to descend theirevacuation stairwell.

6.3. Occupant speeds in the stairs

The reported time spent in the stairwells for occupants from floor 10 to floor 91 in WTC 1, on the basisof the NIST telephone interviews (n = 368), yielded a mean normalized travel speed of 1.3 floors perminute. Normalized travel speed is defined as the total time from entering the stairwell until leavingthe building, divided by the number of floor that had to be descended. According to Figure F33, 25%of the occupants traveled faster than 1.5 floors per minute, whereas 25% of the occupants traveledmore slowly than 0.9 floors per minute. The median normalized travel speed was 1.2 floors perminute. The data include all interruptions to the evacuation process, including crowding, transferfloors, smoke, water, switching stairwells, and resting. Assuming an average floor height of 3.7m(12 ft), the distance along the stair slope (including landings) would have been approximately 10m(33 ft), yielding a movement speed of approximately 0.2m/s (0.65 ft/s) for the median occupant inWTC 1 while in the stairwells, which is on the slow end of published scientific literature values forstairwell travel speeds. This is understandable given the frequency of crowding and the significantnumber of obstacles to evacuation reported by many occupants.

A ‘rule-of-thumb’ for calculating evacuation flow rate is to assume that a standard size door at thebottom of the stairwell is capable of discharging approximately one person per second. [1] By thatlogic, with three stairwells, the WTC system would have been capable of moving approximatelythree people per second from the occupied floors to the Mezzanine or Concourse. In WTC 1, whereelevators were not operational for the duration of the evacuation, approximately 7500 occupantsused the stairwells over a 100-min period, yielding a flow rate of 1.3 people per second from theoccupied floors. Even discounting the final 27min (after the collapse of WTC 2 when the flow ratedropped significantly), the flow rate was 1.5 people per second (or 0.5 people per second per door),or about one-half the ‘rule-of-thumb’ flow rate. This is consistent with the previous observation thatthe stairwell movement speed was slower than the published literature values would have predictedand that the slower rate was to be expected given the number of obstacles to egress encountered bythe evacuees and the total travel distance required.

6.4. Impact of wider stairwell on stairwell evacuation times

An analysis of the hypothesis that wider stairwells correspond to faster overall evacuation times is mostreliable when all other variables are held constant. In WTC 1 on September 11, 2001, however, manyother variables were not constant, complicating the comparison: (1) Stairwell B (56 in. wide) exitedinto the Concourse, whereas Stairwells A and C (44 in.) exited to the Mezzanine where occupants

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typically traversed to the escalator in order to descend to the Concourse; (2) Stairwell B only requiredone (relatively short) horizontal transfer section, whereas Stairwells A and C required multiple,sometimes lengthy (over 100 ft) horizontal transfers; (3) emergency response personnel preferentiallyused Stairwell B to climb to higher floors; and (4) an occupant may have switched stairwells duringegress, introducing a significant uncertainty. Therefore, these four factors confound any conclusionsregarding stairwell width that may be drawn from the evacuation of the WTC towers. Respondentsreported three pieces of information critical to this analysis: number of floors they had to climbdown, the total time spent in the stairwells, and which stairwell they used. Each reported time wasnormalized (divided by) with the number of floors descended in order to compare the resultsindependently of starting location. The normalized times were then averaged over all occupants whoreported using that stairwell.

In Stairwell B in WTC 1, the average occupant spent approximately 61� 38 s per floor, whereas inStairwells A and C (the narrower stairwells), the average occupant spent 53� 34 s per floor. Theuncertainty was calculated using a standard deviation. Unfortunately, the large uncertainty, relativeto the average, in the data collected from WTC 1, combined with the confounding conditionsreferred to previously, do not allow differentiation of stairwell movement speeds for the widerstairwell compared with the narrower stairwells.

6.5. Aids and constraints to evacuation

Constraints or aids to the evacuation progress were documented in Table T1I. Building announcementswere cited by many in WTC 2 as a constraint to their evacuation, principally because of the 9:00 AM

announcement instructing occupants to return to their work spaces. Crowdedness in the stairwells,firefighter counterflow, lack of instructions and information, as well as injured or disabled evacueesin the stairwells were the most frequently reported obstacles to evacuation. The most commonlymentioned forms of aid were assistance from from coworkers and emergency responders and thephotoluminescent markings in stairwells. About 6% of the survivors in WTC 1 and WTC 2 reporteda mobility challenge that slowed their evacuation. Sometimes, the evacuation speed of others in theimmediate area slowed down occupant evacuation speed. Recent pre-existing injuries, medications,or medical treatments were the most commonly reported mobility challenges, whereas a smallnumber used wheelchairs, were pregnant, or were elderly.

7. EGRESS MODELING

Multiple evacuation models were used to simulate different WTC tower evacuations, subject to anumber of assumptions. The goal of the modeling was to frame an understanding of actualevacuation findings on September 11, 2001. Simulations demonstrated that a phased evacuation(also known as defend-in-place, whereupon occupants on the fire floor and the immediatelysurrounding floors descend to three floors below the fire floor) would have taken between 4min tocomplete without delays in evacuation initiation and 11min to complete with evacuation initiationdelays between 0 and 10min. Total evacuation of a tower assuming a full occupant load withoutvisitors (19,800) would have required approximately 92–112min. With visitors (total population25,500 people), total evacuation would have required approximately 114–142min. The ranges

Table I. Reported aids and constraints to evacuation in WTC 1 and WTC 2.

WTC 1 (%) Evacuation aids WTC 2 (%) WTC 1 (%) Evacuation constraints WTC 2 (%)

66 Support from others 60 73 Crowded stairwells 6944 Firefighter/Police 30 51 Firefighter/Police 2732 Exit signs 35 24 Lack of clear instructions 2918 Photoluminescent paint 11 16 Locked doors 710 Floor warden 13 11 Poor lighting 4

WTC, World Trade Center.



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reflect two different model outputs, each assuming two different delay times (no delay and a 10-mindistribution of delay times). An evacuation simulation for 8800 people (approximately the numberpresent in each tower on September 11, 2001) in the absence of any damage to the building, wouldhave required 52–71min, depending on the model or the delay times. Finally, the model output was‘calibrated’ to approximate the gross evacuation rates observed in WTC 1 and WTC 2 on September11, 2001. Once the model input necessary to approximate the observables was determined,additional occupants were added in order to estimate how many occupants might have been unableto evacuate on September 11, 2001 (given the damage to the building and observed delay times) ifthe buildings had had larger occupant loads. NIST estimated that approximately 14,000 occupantswould have been unable to evacuate from WTC 1 and WTC 2 on September 11, 2001 had thestarting building population been 19,800 in each building, as shown in Table T2II.

8. RECOMMENDATIONS

The NIST investigation resulted in 30 recommendations for changes to building codes, procedures, andpractices. There were five recommendations specific to building evacuation and occupant safety. Tosummarize, building evacuation systems should be improved to include system designs that facilitatesafe and rapid egress, methods for ensuring clear and timely emergency communications tooccupants, better occupant preparedness for evacuation during emergencies, and incorporation ofappropriate egress technology.

Recommendation 16. NIST recommends that public agencies, non-profit organizations concernedwith building and fire safety, and building owners and managers should develop and carry outpublic education campaigns, jointly and on a nationwide scale, to improve building occupants’preparedness for evacuation in case of building emergencies.

Recommendation 17. NIST recommends that tall buildings should be designed to accommodatetimely full building evacuation of occupants because of building-specific or large-scale emergenciessuch as widespread power outages, major earthquakes, tornadoes, hurricanes without sufficientadvanced warning, fires, accidental explosions, and terrorist attack. Building size, population,function, and iconic status should be taken into account in designing the egress system. Stairwelland exit capacity should be adequate to accommodate counterflow because of emergency access byresponders.

Recommendation 18. NIST recommends that egress systems should be designed (1) to maximizeremoteness of egress components (i.e., stairs, elevators, and exits) without negatively impacting theaverage travel distance; (2) to maintain their functional integrity and survivability under foreseeablebuilding-specific or large-scale emergencies; and (3) with consistent layouts, standard signage, andguidance so that systems become intuitive and obvious to building occupants during evacuations.

Recommendation 19: NIST recommends that building owners, managers, and emergencyresponders develop a joint plan and take steps to ensure that accurate emergency information iscommunicated in a timely manner to enhance the situational awareness of building occupants andemergency responders affected by an event. This should be accomplished through bettercoordination of information among different emergency responder groups, efficient sharing of that

Table II. Simulated evacuation results for fully occupied WTC 1 and WTC 2 on September 11, 2001.

Building

Totalnumber ofoccupantsat t= 0.0

Potentialnumber

ofevacuees

Occupantsremainingin buildingat collapse

Occupantstrappedabove

floors ofimpact

Percentage of occupants whosuccessfully evacuated relative

to where they started

Total(%)

Belowimpact (%)

Aboveimpact (%)

WTC 1 19,800 16,000 6200 3800 69 85 0WTC 2 19,800 17,260 8377 3900 58 75 44Total 39,600 33,260 14,577 7700 63

WTC, World Trade Center.



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information among building occupants and emergency responders, more robust design of emergencypublic address systems, improved emergency responder communication systems, and use of theEmergency Broadcast System (now known as the Integrated Public Alert and Warning System) andCommunity Emergency Alert Networks.

Recommendation 20. NIST recommends that the full range of current and next generationevacuation technologies should be evaluated for future use, including protected/hardened elevators,exterior escape devices, and stairwell navigation devices, which may allow all occupants an equalopportunity for evacuation and facilitate emergency response access.

A complete history of the NIST World Trade Center Investigation, including all reports, findings,recommendations, and presentations can be found at http://wtc.nist.gov.

REFERENCE

1. Fruin JJ. Pedestrian Planning and Design Revised Edition. Elevator World, Incorporated: Mobile, AL, 1987.



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