The Two Towers: A Challenge To Two Professions

April 1, 2002
John P. Flynn outlines why the collapse of the World Trade Center towers represents a need for the fire service to re-examine its very philosophy of response, as well as being a call to action for the engineering profession.History is marked - in fact, defined - by events which shape the course of future events due to the very magnitude of their occurrence.

John P. Flynn outlines why the collapse of the World Trade Center towers represents a need for the fire service to re-examine its very philosophy of response, as well as being a call to action for the engineering profession.History is marked - in fact, defined - by events which shape the course of future events due to the very magnitude of their occurrence. The attack and subsequent collapse of the World Trade Center towers represents the single most heinous and starkly unreasonable single incident in the history of mankind.

Photo from the Firehouse CollectionEach floor of the towers possessed an open unobstructed area of approximately 200 by 200 feet. The potential incidence of fire on each floor approached an acre in area, virtually impossible to extinguish under normal conditions with serviceable standpipes and elevators.

As with events of this nature in the past, mankind has sought to avoid recurrence through investigation resulting in an increased level of awareness and a resultant adaptation in policy and procedures. The fire service in particular bears the unfortunate legacy of repeatedly learning from its experiences - departmental guidelines are largely founded upon the loss of firefighters' lives.

The World Trade Center collapse, however, was unprecedented not only in scale, but in the depth of its potential impact on our existing policies and procedures. More profoundly, it represents a need for the fire service to re-examine its very philosophy of response. Furthermore, this catastrophe is a call to action for another noble profession - the profession of engineering, a vital yet all-too-often unrecognized silent partner in the quest for civilian and firefighter life safety.

In a manner no less unmistakable than the impact of the jetliners with the towers, this incident must be viewed as a convergence of two professions which have strayed apart from this common goal of life safety. Both professions must learn and both must act decisively to prevent similar future events in a world which has presented us with so many new challenges. This article shall enumerate lessons which must be learned on both sides of this issue - firefighting and engineering - lessons not necessarily based upon mistakes, but rather the result of ignorance and underestimation of the challenges which face us. Although the recommendations herein are somewhat specific to New York City, similar concerns exist at all major cities in the United States and in many smaller communities.

The Fire Service Mission

The philosophy of structural firefighting within the City of New York at virtually all operations is based upon life saving and is defined by an aggressive interior attack on two fronts: extinguishing efforts directed at the seat of the fire and a simultaneous coordinated life saving and evacuation operation in all locations affected by the fire. This philosophy has existed over the more than 200-year history of the department and has served the public well. The number of lives saved through this approach certainly far exceeds those lost through its implementation and the department as a whole has developed a level of competence and confidence exceeded by none.

Photo from the Firehouse CollectionWhen the south tower collapsed, it created a huge gash in a skyscraper located on the south side of Liberty Street across from Tower 2. The exterior walls of the falling tower lie in the street while a large section still hangs from the upper floors.

It is no surprise, then, that an aggressive interior operation serves as the "default" at incidents which defy past experience and at those for which no standard operating procedure exists. The alarm at the World Trade Center on Sept. 11 represented the ultimate challenge to the long history of the department in this regard and the command structure on the scene responded by deploying forces in predictable fashion: ascend to the fire for possible extinguishment while performing rescues and evacuating the structure. The time-tested response, however, was proven to be both ineffective and disastrous for this type of incident.

The Engineering Community Mission

The structural engineer does not have as his primary function the protection of building occupants from the hazards of fire. He is primarily concerned with the structural efficiency of the building - ensuring that it will maintain integrity under the anticipated loading conditions and at a reasonable cost.

In this pursuit the engineer is "client driven." The client is interested in the most usable floor area at the least cost, and the livelihood of the engineer is largely predicated on his success at performing well in this regard. Fire safety is far from the top of his list of priorities as it contributes to neither structural integrity nor a reduction in cost; in fact, it is likely to detract from the latter. Generally, the minimum code requirements are met for expediency. The concerns cited within this article relevant to the World Trade Center disaster are likely to have had their origins in this lack of emphasis on "fire considerate" design.

Historically, the engineering profession, similar to the profession of firefighting, is based upon a commitment to life safety. Mankind recognized long ago through unfortunate experience that materials behaved in a somewhat unpredictable fashion and that a methodology was required to ensure reliable, predictable performance of structural systems. Quite simply: people needed assurances that buildings and similar structures would not fall down. Engineers provided them with the means to this end.

Photo from the Firehouse CollectionHeavy, thick smoke rises near 7 World Trade Center. Smoke is visible from the upper floors of the 47-story building. Firefighters using transits to determine whether there was any movement in the structure were surprised to discover that is was moving. The area was evacuated and the building collapsed later in the afternoon of Sept. 11.

The engineering profession has also enjoyed a pattern of success which parallels that of the firefighting profession: technical competence and a progressively increasing knowledge of the behavior of materials has resulted in a higher degree of confidence than previously obtainable. It is therefore not surprising that the engineering community also experienced what is likely to be its greatest failure on Sept. 11.

Failure Of Engineering Community And Fire Service

An analysis of the sequence of events on the day of this catastrophe begs the question: how and why could such a dramatic all-encompassing structural failure occur with the commensurate failure of the fire department operations? The answer lies in the simple statement that the representatives of each profession (on the one hand, the designers of the World Trade Center; on the other, the command staff of the FDNY) were painfully ignorant of the characteristics, philosophies and limitations inherent in the philosophy of their counterparts. This ignorance resulted in the existence of mutually exclusive parameters: a building thoroughly unsafe to enter and a response policy which prescribed immediate entry.

A full investigation into the mechanism of collapse specific to this incident would likely produce volumes of technical information and in fact it appears that such an investigation is underway. A more useful approach for the avoidance of similar incidents in the future, however, may be to describe the primary underlying inconsistencies which exist and which result in designs which are incompatible with fire department response.

The engineering profession must fully understand the fire department mindset and the physical limitations which exist in the response, and the fire department must understand the mind of the engineer, the constraints of his profession and the limits of the materials he specifies within those constraints. Only then can effective changes be made by each to increase life safety.

Failure of the engineering community: Exits. Floor area is precious and costly, particularly in high-rent areas such as Manhattan. Stairwell enclosures occupy potential floor space. The lack of an adequate means of egress at the World Trade Center became apparent in the reports from survivors fleeing the towers: "We had to squeeze to the side of the stairwell to allow firefighters to pass." In certain instances the advancement of firefighters up the stairs may have actually hindered egress of building occupants. Specifically, only three staircases existed in each tower, and each possessed a width of approximately five feet.

A quick analysis of the exit time required results in the bracing realization that an exit time of 30 seconds per floor - not unreasonable in light of the number of people seeking egress, the length of the walk down, and the obstruction of the stairwells by the elderly and wheelchair-bound civilians - would result in a 55-minute travel time to reach the base of the stairs. The first tower fell in 53 minutes. It is noted that a mass exodus is not specific to this event, but would similarly be required for any significant fire, accident, power failure or biological/chemical attack.

Failure of the engineering community: Structural stability. The World Trade Center towers were unique in their construction in that a repetitive floor plan consisting of vast open floor areas without intermediate walls or partitions was repeated on each and every floor. This arrangement was accomplished through the use of as few apparent vertical supporting members as possible in an economically efficient manner. In essence, the configuration of the supporting members consisted of a column line at the perimeter, a column line at the core and a connecting truss which acted as a long slender bridge across almost one-half the width of the building.

Redundancy is the provision of "alternate load paths" and is a desirable construction feature in that it provides a secondary support system in the event one or several of the members in the support system fail. Due to their inherent design features, the World Trade Center towers possessed what amounted to a negative redundancy: the failure of one of its elements would not only result in an inability to sustain the load of the remaining members, but would likely increase the extent of failure due to interconnectivity of the few members which remained in a given spatial plane.

Failure of the engineering community: Large open floor space. Each floor of the towers possessed an open unobstructed area of roughly 200 by 200 feet. The pursuit of adaptable and open floor space by the client, and satisfied by the engineer, resulted in a problem for the fire service which was two-fold: the potential incidence of fire on each floor approached an acre in area, virtually impossible to extinguish under normal conditions (typical fire loading and serviceable standpipes and elevators), and the absence of intermediate walls and partitions which would have served as redundancies in the structural system of the frame. Interior walls would have both compartmentalized the floor area to resist the spread of fire and simultaneously provided an alternate load path in the event of floor collapse from above.

Failure of the engineering community: Lightweight construction. The floor structure at each level of the towers was characterized by a four-inch-thick lightweight concrete slab atop open web parallel chord steel bar joists spanning approximately 65 feet. The hazards associated with the use of such trusses in floor and roof construction have long been recognized by the fire service due to the rapid unpredictable collapse and loss of life associated with fires in buildings employing this construction.

These truss elements possess a very high surface-area-to-mass ratio (resulting in a rapid heat rise throughout the cross section of the steel). They are also highly interdependent upon each other and provide very little structural reserve if compromised, owing to their design efficiencies of 90% or more. Within the fire service they are perhaps the most widely studied and feared of all structural elements.

Failure of the fire service: Outdated tactics. The fire service is socially and professionally traditional in nature. Responses today are not much different than responses of the late 19th century. Firefighters exit the same firehouses with approximately the same number of individuals carrying very similar tools as their forebears. Certain items of new equipment, such as self-contained breathing apparatus (SCBA), assist firefighters in making more rapid and deeper entry into the fire building but essentially the tactics and procedures remain the same as in yesteryear.

This is acceptable and proper when a fire of typical magnitude occurs in a building for which these tactics and procedures were intended: old-law tenements, private dwellings and non-fireproof low-rise projects. The concern today, however, is the advent of taller buildings and buildings which utilize new methods and materials of construction. The fire service in general is aggressive in its attempts to identify and understand such new methods and materials. It is slow, however, to adapt changes in existing protocol; old habits die hard. The terrible wounds suffered by the service at the World Trade Center cause reflection on the following factors which may be viewed as pre-existing contributing factors.

Failure of the fire service: Lack of information. There are many buildings within the City of New York and cataloging each and every one as to details of construction is a monumental task. A working system presently exists whereby general hazards and notable characteristics of the building which would face the operating forces on their arrival are recorded and provided in writing to the units as they respond. Examples of such data include areas containing medical waste, floors which cross into a separate wing and the location of staircases which are obscured from view.

This system is far from exhaustive, however, and is generally not specific to the details of construction which might prove useful to the fire commander. Such was the instance of the World Trade Center response: very few if any of the responding officers and firefighters were aware of the construction characteristics of the building. Had they been cognizant of the lack of redundancy and the truss construction, the response may have been modified (prior to arrival or during the response).

The post-collapse reaction of most fire department members to the realization as to exactly how the buildings were constructed was one of surprised amazement. Furthermore, the manner in which the information in the existing system is collected is random, unregulated and haphazard; the company officer performing fire duty on a given shift voluntarily enters any information he feels pertinent into the system. He is limited by his training and experience relative to building systems and by the many distractions faced in his daily routine as a fire officer (including responding to emergencies).

Failure of the fire service: Radio communications. The simple fact of the matter is that communications on the scene of most fire emergencies are at very best marginal and at worst non-existent. The operating forces are severely limited by the capabilities of the radios and high-rise buildings have proven to be among the most challenging environments for transmission. Communications from floor to floor are severely diminished by loss of power of the radio signal and a company officer only a few floors away from a chief at the command post is likely to be incommunicado. The FDNY has been fully aware of this concern for years and firefighters have grown to function within this constraint insofar as possible. The World Trade Center incident consisted of dozens of companies in a multitude of locations, many splintered by the havoc in the staircases, and represented a communications nightmare.

Photo from the Firehouse CollectionPieces of the exterior wall pierced the street. The weight of the collapsing south tower nearly leveled building 4 at the corner of Liberty and Church streets.

Failure of the fire service: Training and education. The training of firefighters has become more and more important as the incidence of fire has decreased in our society. Structural firefighters today do not benefit from a continuous diet of "on-the-job" training as experienced in the 1960s and 1970s, when fire was epidemic. The fire service has responded in kind and today the number and variety of training schools, seminars and publications is at an all-time high. Confined space, collapse, rope rescue, and water rescue are typical subjects covered in great depth and with great frequency.

Lacking, however, is training which focuses on incident command and which emphasizes large incident response. The Federal Emergency Management Agency (FEMA) has funded the development of courses in advanced structural remediation and command operations for catastrophes of great magnitude. These courses are provided to the urban search and rescue teams which FEMA sponsors; most "local" fire commanders, however, do not benefit from this training. Additionally, the training relative to structural concerns is generally carried out not by professional educators or engineers, but by firefighters limited in their breadth of knowledge and teaching experience. The result is a command structure which is often not thoroughly prepared to fully realize the "big picture" at a scene, such as inherent weaknesses in a modern structure, and is therefore unable to respond accordingly.

Conclusions & Needed Improvements

The concerns cited herein are the result of philosophies which developed over time in both the fire service and the structural engineering community. They may be described as "natural" in that they evolved in response to needs which have existed in the past. They exist largely because to this date they have remained unchallenged. The history of human experience seems to possess the common threads of underestimation and growth through misfortune. With this in mind, fire and engineering professionals must learn from this most tragic event and anticipate future structural and firefighting challenges. Though the wake-up call of Sept. 11 is exceptional in its magnitude and impact, it represents concerns which exist at a lesser "day-to-day" level in the worlds of firefighting and engineering.

Consideration must be given to the following potential remedies:

  1. Education. Firefighters must become familiar with the behavior of modern materials and building systems as well as their propensity for sudden failure. An increasingly technical world mandates that the firefighting community become more technically informed. Likewise, the engineering community must become more considerate of the challenges which firefighters and civilians face in major emergencies: egress requirements, inadequate communications and access capabilities, and the implications of large undivided floor areas.
  2. Proactivity. True to their calling of "life safety," engineers must become more proactive in the design process by requiring owners to provide such critical elements as truly adequate means of egress and fire protection measures such as sprinklers. Proactivity must also take the form of recognition and avoidance of the use of lightweight minimum redundancy structural systems such as bar joists and wooden I-beams when they cannot be adequately protected from fire, or maintained in satisfactory condition.
    The engineer is responsible for instituting practical solutions to the potential real-life problems associated with major emergencies and must look beyond the minimum code requirements and performance tests sponsored by manufacturers. Client needs and manufacturer claims must take a back seat to a reasoned approach to firefighter and civilian safety, even if at some financial cost.
  3. Preparation. Buildings must be inspected, classified and recorded in a database with consideration of structural features which might prove significant in the event of a fire or explosion. Haphazard unregulated data collection is inadequate. A dedicated effort must be made to establish a working system of inspection, particularly for buildings which pose increased concern due to their construction, size, occupancy or symbolic nature. This may be accomplished through teams of firefighters and engineers intended specifically for this purpose. Ignorance of our structural weaknesses combined with an aggressive interior response is a recipe for future disaster.
  4. Growth. Fire commanders must be trained to respond to large-scale emergencies henceforth unanticipated. Extreme events including terrorist activity and earthquakes mandate greater command capabilities than presently exist. Incident command capabilities must be extended beyond that posed by a three-room fire in a private dwelling or in a six-story building. Such in-creased capabilities can be accomplished only through a professionally administered course of training provided to command chiefs by professionals in the required disciplines.
  5. Communications. It is apparent that the need exists for a more reliable means of communications than presently exists, particularly in high-rise buildings and below-grade installations such as train tunnels and stations. Complete investigation must be made to ascertain if communication capabilities can be improved. If not, existing protocol must be modified to reflect this constant potentially disastrous element of our daily response. No fire company or individual should be alone in a stairwell or basement without a means of communicating with those in command.
  6. A broader view. By expert accounts, an earthquake of significant magnitude has the potential to produce 100 times the debris and loss of life as seen at the World Trade Center. Concurrently, the damage inflicted by terrorists has become progressively greater with each incident of occurrence over a (very short) recent past.
    An efficient organized emergency response to a terrorist or earthquake calamity at this time does not appear to be possible; to remain unprepared is to guarantee mayhem and confusion when order is most needed. Firefighters must see a larger picture and engineers must recognize the risks faced by those who are likely to enter the damaged structure to save lives.
    Furthermore, the fire service, police departments, federal agencies, the structural engineering community, utility companies, political leaders and the public must all take a lesson from the World Trade Center disaster in this regard: "Be Prepared." Preparation will require a massive pre-planning effort amongst all of the above referenced parties, and more. The task is daunting - but the end will certainly justify the means.

The Terms "Engineer" And "Architect" Defined

The terms "engineer" and "architect" are often seen, but seldom understood by the public. Both professions are regulated by the state, and often by an entity such as the State Education Department (as in New York).

An individual who is determined to have satisfied specific educational requirements must then meet state requirements relative to experience and must also pass a series of extensive examinations. Upon successful accomplishment, an engineer receives the designation P.E. (Professional Engineer) and the architect is assigned as an R.A. (Resident Architect). Each possess similar powers under the law - however, an engineer is trained to determine or ensure structural adequacy of an engineered system while an architect is primarily concerned with aesthetics and overall functionality.

Fire safety design falls within the jurisdiction of both and represents a "gray" area. For purposes of this article and expediency the term engineer is used in a general sense to encompass the roles which each professional may play in the design process. Architects may possess an equal or even greater potential impact on fire safety design for a structure.

—John P. Flynn

John P. Flynn is a lieutenant with the New York City Fire Department presently working at Haz-Mat 1. He is also a licensed Professional Engineer in New York State. Flynn has extensive experience in forensic failure analysis of structural systems, and has functioned as a Structural Specialist on the FEMA-sponsored Urban Search and Rescue (USAR) team since its inception in 1991.

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