BY ELMER F. CHAPMAN AND DR. EDWARD KUSH
The collapse of the World Trade Center’s (WTC) Twin Towers killed almost all of the 343 members of the Fire Department of New York (FDNY) who died and a large percentage of the other 2,477 casualties. This massive overall loss of life was not the result of the airplane impacts or battling the flames. All the firefighter deaths resulted from failure of the building structures and resultant collapses. It is vital to the fire service to understand the specific reasons for the buildings’ failure. This can play a critical role in three areas:
1 Preventing construction that does not conform to code, including the replacement complex for the WTC;
2 Establishing procedures that minimize firefighter hazards in high-rise buildings; and
3 Promoting an atmosphere and philosophy of structural design that incorporates firefighter and occupant safety as an integral design component and accepts input from the fire service in the design process.
There are a number of detailed engineering reasons for the collapse of the towers. They have been already analyzed in some depth and will be studied for some time by experts. Differing opinions have evolved regarding the main cause and the relevance and hierarchy of secondary factors. But the definitive fact that should be acknowledged is this: The Twin Towers were not built in accordance with any fire safety or building code.
The WTC was constructed under the auspices of the Port Authority of New York and New Jersey, a bi-state organization, and it was able to be built without conforming to the New York City Fire and Building Codes or applicable New York State codes. For this reason, the buildings were of a very innovative design that allowed for extremely lightweight construction with maximized open space for offices (more than an acre per floor) at minimum cost. Moreover, the situation allowed for less rigid elevator and stairwell shaft configurations (also reduced safety of egress), inadequate sprinkler protection, reduced lateral (shear) strength, and insufficient thermal protection of structural members (particularly the radial bar trusses).
If a structure is not subject to New York City Building Code, then it is not afforded the benefit of inspection and oversight by New York City Building Department inspectors and engineers. Thus, there is no input from an independent source that would provide objective review of safety characteristics and structural integrity during construction. This situation also creates the potential for the involvement of inferior or unscrupulous contractors. The lack of fire safety in the Twin Tower configuration was pointed out prior to and during construction by FDNY, led by then Chief of Department John T. O’Hagan. Reports delineating the problems and the code violations were prepared, but they were ignored by the Port Authority. O’Hagan firmly opposed the large open spaces, the insufficient fire-resistive encasing of the steel, and the method of enclosing the stairs and elevators, among other fire and safety hazards-critical elements in the great loss of life on 9/11.
WTC CONSTRUCTION
Low building mass. The structural strategy to use the core and outer shell (acting essentially as a tube) for load bearing of floor trusses provided the technology for the very large square footage of office space on every floor and provided an extremely lightweight structure. The resulting estimated weight density figure of 7 pounds per cubic foot at the WTC was only half of that prescribed by the New York City 1968 revised code and less than one-third of the 23 pounds per cubic foot weight density estimated for the Empire State Building. Weight density is the total empty weight of the building above-ground divided by the total building volume aboveground and thus represents an average for the structure. The New York City code in effect at the time construction began (the 1945-1968 version) stipulated weight densities of between 15 and 18 pounds per cubic foot (decreased from 16 to 19 pounds per cubic foot in the 1938-1945 code). The above weights per cubic foot are estimates and need to be validated by a survey of existing buildings. They should be incorporated into the fire safety plan for the building. The time that can be allowed to conduct an interior attack can be based on the weight per cubic foot.
This comparison of weight densities does not mean that the towers were not well thought out or not expertly designed. The designers studied existing (at the time) high-rise construction and the specific needs of the towers very thoroughly. The towers were designed to withstand wind loading as the major criterion, and impact of a Boeing 707 was also considered. The fact that the towers withstood the impacts initially and for some time attests to some success of load transfers, and the American Society of Civil Engineers/Federal Emergency Management Agency study cited this as evidence of good building performance.
But the main thrust of the design was for economic benefit without significant consideration of fire safety. Given that higher structural mass usually correlates to better structural resistance and better safety, the Twin Towers were, therefore, intrinsically subject to reduced strength and safety compared with standard, code-enforced high-rise construction.
Lateral resistance. The lightweight construction and attendant large open areas denied the structure of lateral (or shear) strength that would ordinarily be provided (outside the core) by distributed vertical structural columns. These would have provided increased resistance between the shell and the core. Additionally, lateral stiffness was reduced because the elevator and stair shafts were not strongly enclosed, which does not conform to code. It is unlikely that any building, no matter how strong in shear, could have locally withstood the impact of the high-speed airliners. But the lateral shifting and offset of the impacted and partially impacted floors could have contributed to the eventual total collapse of the towers.
Truss construction and joints. The cores of the towers were connected to the outer walls/skin by long (60 to 70 feet) open-web truss type joists that supported the concrete floors. Such trusses have long been a source of concern for firefighters because their rapid weakening and failure in fires can produce early and unpredictable collapses. Failure of the trusses and their connections at the core and outer walls isolated the core from the skin, causing extreme overloading and loss of lateral support that fostered column buckling. The only way for the trusses to survive structurally in fire is for them to be very well thermally protected, and this was not the case in the towers.
Insufficient thermal protection of structural members. Structural members in high-rise construction must be thoroughly covered (i.e., encased) by fire-resistive insulation of sufficient fire rating (typically four hours) to prevent or minimize loss of strength resulting from the elevated temperatures that occur in fires. This is required by fire codes. That the various steel structural members of the towers were not sufficiently encased in thermal protection was known beforehand.
Certainly on the impact floors, large portions of the insulation that was in place were torn and stripped off (and would have been the case no matter how substantial and well-attached the insulation was). The ongoing raging fires presented extreme high temperature environments that pushed any locally remaining and adjacent floor insulation to or beyond its limits. But the fact that there likely was insufficient insulation on undamaged columns in the core and on the floor trusses would have caused these members to weaken and fail in relatively short time periods, certainly shorter than expected. These members were already bearing higher-than-design loads because other parallel members had been damaged or destroyed.
Since there was no formal inspection and oversight during construction and after, the true nature of the thermal protection was not well known. For example, the state of the steel surfaces at the time of insulation application was not documented; and, if there were already corrosion (rust or scale) on the surfaces, the bonding would not have been proper and firm. The floor bar joists (trusses) are particularly vulnerable because the sprayed-on material adheres to only a small surface area. Weathering and handling during construction can cause damage and stripping. At the WTC, there was evidence of problems resulting from inferior materials and workmanship, documented in photographs that showed areas that were not even sprayed. It is not known how much inferior insulation was uncorrected.
The insulation situation was further complicated partway through the construction of the towers because the use of asbestos was outlawed in 1969. At that time, approximately 30 floors of Tower 1 had been sprayed with asbestos foam insulation. When the switch to non-asbestos coating was implemented, the transition produced further uncertainty as to the amount and quality of the insulation, particularly because there were no standards and no oversight by independent inspectors.
RECOMMENDATIONS
The unthinkably huge loss of firefighters at the Twin Towers resulted from collapses that occurred much sooner than anyone expected, based on experience with high-rise fires. The FDNY acted in accord with established procedures but did not have sufficient information about the building characteristics to impose modifications to its strategy and tactics. Had the buildings been constructed and maintained according to code, it is quite likely that there would have been added time that would have minimized or prevented loss of firefighters and other rescue personnel.
With these considerations in mind, we offer the following recommendations.
• Design and construction of all high-rise buildings must be governed by applicable codes and standards that account for structural survivability and safety of egress in fires and other disasters. These codes must be based on the most reliable data and analyses available and must provide for a well-defined minimum time to collapse (of at least four hours).
• The codes and standards must be approved or developed by structural specialists (engineers/building department officials) working in conjunction with fire service experts (chiefs, engineers, inspectors).
• Building and fire inspectors must have access and authority throughout the construction of new high-rise buildings and full access to existing high rises.
• Based on information compiled by inspectors and input from the fire department and outside independent consultants, as needed, a safe time to collapse must be defined for every high-rise building. This information must be clearly documented and transmitted to every applicable fire chief and line officer.
• The “Time-to-Collapse” rating should be determined by a committee or board administered by the building department and consisting of structural engineers and fire service representatives experienced in the behavior of high-rise structure fires. The “Time-to-Collapse” rating should be based on
-the weight density of the structure;
-the amount, quality, and condition of the fire-resistive encasement of the structural members;
-the presence of hazardous materials and special hazards including fuel tanks;
–the existence of portions of the building where the weight density might be significantly lower than the average;
-the potential effectiveness of the sprinkler system; and
–structural and thermal analysis and engineering simulations performed by the National Institute of Standards and Technology or similar independent specialists.
• Structural members must be thoroughly and properly encased in fire-resistive coatings applied in accordance with standards and codes by sufficiently skilled labor. The design and application must be overseen by a certified independent consultant who (a) supervises and certifies that all proper procedures were followed and (b) certifies that the application is in compliance with code at the time of covering up.
• The building structure and insulation must be formally inspected periodically every 10 years or less and refurbished as necessary.
• Actual installed performance of fire-resistive coatings must be brought up to the level of the ratings specified by manufacturers based on laboratory testing. Currently, the idealized ratings obtained with even coverage and optimum adhesion are not usually approached in the field because of improper application, inadequate or uneven coverage, weathering, handling, and inappropriate surface condition of the structural members at the time of application.
• Improved methods of encasing open-web trusses should be developed. Spray-on foam methods usually provide inadequate and nondurable coverage because of the small surface area for adhesion. An alternate method of encasing, for example, would be to enclose the sides with gypsum board of sufficient thickness for at least a four-hour fire rating and coat the chords with rigid foam of sufficient depth.
• Stairwells and elevator shafts must be strongly enclosed to provide protection for evacuation and contribution to lateral rigidity of the building.
• Specifically, the replacement complex for the WTC must adhere to these principles. ■
■ ELMER F. CHAPMAN, a 38-year veteran of the Fire Department of New York (FDNY), retired in 1984 as a deputy chief. He served as an adjunct instructor at the National Fire Academy and the Nassau County (NY) Fire Academy and as liaison and consultant to FDNY in matters concerning codes and standards, smoke movement in high-rise buildings, HVAC systems, and elevators. He was a member of the advisory board for the Fire Science Institute of John Jay College, the National Fire Protection Association (NFPA) Smoke Control Committees 92A and 92B, ASHRAE Committee 5.6 for Fire and Smoke Control, and NIST Task Group for Smoke Control Manual.
■ DR. EDWARD KUSH is an engineering consultant involved in the aerospace propulsion, fire science, and energy fields. He has been a volunteer firefighter for more than 20 years as a member of four Long Island departments and has served as a consultant to Nassau County (NY) Fire Service Academy on training fire dynamics and turnout gear performance. He maintains a special interest in applying technology to firefighter safety. He has a Ph.D. in aerospace engineering from Virginia Polytechnic University.
