Engineering Disaster of WTC Tower 7 on 9/11
Engineering Disaster of WTC Tower 7 on 9/11
A Description of the Engineering Project
Certain vulnerabilities in the design of WTC (World Trade Center) likely contributed to the collapse of Tower 7, as well as the two main towers. While the building was exceptionally strong, it had a number of inherent flaws. The original building was a forty-seven storey building with a red granite frontage. The building was 190 meters (610 feet) tall, having a trapezoidal base forty-three meters (140 feet) wide, and 100 meters (330 feet) long (Gilsang and Willa 42). It was constructed above a substation that had a caisson foundation that had been designed to support the weight of a twenty-five storey building. However, the final design for Tower 7 involved the construction of a larger structure than had originally been planned at the time the substation was built (Craighead 46). Therefore, the structural design of Tower 7 incorporated gravity column transfer girders and trusses that were situated between the fifth and seventh floors, and whose purpose was to transfer the loads to the building’s small foundation.
Caissons that had been installed in 1967, together with new ones, were used to secure the building, with the fifth floor serving as a structural diaphragm to provide lateral stability and distribute loads between the old and the new caissons (Gilsang and Willa 42). A tube-frame design, which consisted of core and perimeter columns, as well as perimeter moment frames to resist lateral loads, was used on all floors above the seventh floor. The building also had a shipping and receiving ramp that was used by the entire WTC complex, and which occupied Tower 7’s eastern quarter. The building was open from the third floor downwards to provide space for truck clearance for WTC shipping ramp. According to the National Institute of Standards and Technology (NIST), the spray-on fireproofing for the building’s structural steel elements had a fire rating of three hours for the columns and two hours for the steel beams, trusses, and girders (NIST 1).
Mechanical equipment was installed on the fourth floor through the seventh floor, including twelve transformers on the fifth floor and a number of emergency generators used by tenants and the Office of Emergency Management. These generators were supplied with fuel (91,000 liters of diesel) which was stored in the basement. Following the WTC bombings of 1993, city authorities decided to locate the fuel tanks and the emergency command center at Tower 7. Even though this decision has been severely criticized following the 9/11 terrorist attacks, it has been established that the fuel stored in Tower 7 did not contribute to the collapse of the building.
A Description of the Failed Aspect of the Project
While the collapse of Tower 1 (WTC 1) and Tower 2 (WTC 2) in the aftermath of 9/11 events were attributed to the impact occasioned by the crash of two airplanes into the buildings, as well as the ensuing flames, the subsequent collapse of Tower 7 (WTC 7) baffled experts since it was never struck directly by airplanes. Moreover, the nature of the collapse, which happened almost seven hours following the collapse of Towers 1 and 2, has led to widespread speculation. Several analyses indicate that even though there were various phases that led to the ultimate collapse of Tower 7, the structure would perhaps have never collapsed if a single critical column had not collapsed. The location of the said column, as well as its role as a central structural constituent, meant that any local failure would have occasioned the global failure of the entire Tower 7 (Gilsang and Willa 42).
As mentioned previously, the original plan for the Con Edison substation included erecting a forty-five storey tower above it. Nevertheless, the final footprint of Tower 7 was tremendously larger than originally planned. Consequently, there were a number of discontinuities between the columns of the substation and those of Tower 7. Braced frames, transfer girders, and transfer trusses at the fifth floor through the seventh floor transferred loads between the building’s discontinuous columns. While these elements served to shift loads between the columns, they in effect “tied” the columns together (Gilsang and Willa 43). The failures of Towers 1 and 2 sent burning debris into the southern facade of Tower 7. The impact of the debris and the fire damage occasioned a sequence that led to the global collapse of the building. The burning debris caused uncontrolled fires that had characteristics similar to those previously observed in tall buildings, and their spread was consistent with common building contents fires (Craighead 46).
It has been argued that had the automatic been properly operational as designed, the fires in Tower 7 would likely have been contained, thereby preventing the global failure of the building. Nevertheless, the collapse of Tower 7 underscores the need to design fire-resistant structures for situations where automatic sprinklers do not function (such as when the water supply is impaired or disconnected), are overwhelmed, or are absent altogether. The absence of a functional sprinkler system led to the spread of the fires to the northeast side of the building. The buckling of a key interior column caused the collapse sequence that occasioned the global collapse of Tower 7 (Craighead 46). The column in question had become unsupported over several stories after a local damage induced by fire led to a series of local floor failures.
The buckling of the column occasioned a vertical sequence of floor failures to the roof, as well as the buckling of the adjoining interior columns to the south of the key column. This was followed by an east-to-west horizontal succession of interior column buckling, which was brought about by the weight of the falling debris, the loss of lateral support to adjoining columns, as well as the redistribution of loads from the already buckled columns (Gilsang and Willa 44). The buckling of the exterior columns when the core of the tower failed led to the redistribution of all loads to the exterior columns, which led to global collapse as the building above the buckled area collapsed downwards as one unit. This was most likely a disproportionate (fire-induced) collapse, which may be defined as the proliferation of local damage resulting from an inducing agent, and spreading from one element to the next, leading to the ultimate collapse of a structure in its entirety or a substantial part of it.
While the collapse of Tower 7 resulted from the failure of one critical column, the causes of failure can be summarized as thermal expansion that occurred at temperatures significantly lower than those normally considered in structural design for establishing fire resistance ratings and considerable magnification of the effects of thermal expansion resulting from long-span floors commonly used in office buildings. Others include the use of connections that were capable of resisting gravity and other loads, but could not prevent fire-induced collapse. It is imperative to note that while the building had emergency generators whose fuel supply were located in the building’s basement, the fires resulting from the fuel oil had no role in the collapse of Tower 7.
Assigning Blame for the Failure of Tower 7
The blame for the failure of Tower 7 lies with the design engineers, as well as the structural engineers. It is increasingly being recognized that the current rigid, prescriptive design approach only offers a comparison of how the various elements of steel structures would perform when exposed to a standard fire. It does not offer any information regarding the load carrying capacity (actual performances) of the elements or the entire assembly in environments of real fires. In this regard, high-rise buildings such as Tower 7, which because of their significance and the longer times required for evacuation, may be required to remain structurally sound beyond the maximum code-required fire resistance without collapsing (Franssen, Venkatesh Kodur, and Raul 3). Regrettably, this was not the case for Tower 7. The Debris from Towers 1 and 2 resulted in damage to Tower 7 due to the effects of both impact and fire. The structural steel was greatly weakened by the fire, causing one or several of the columns on the tower’s eastern side to fail. Therefore, the ultimate responsibility for the collapse of the Tower 7 lies with the design and structural engineers.
Craighead, Geoff. High-rise Security and Fire Life Safety. Amsterdam, NL: Butterworth-Heinemann/Elsevier, 2009. Print.
Franssen, Jean-Marc, Venkatesh Kodur, and Raul Zaharia. Designing Steel Structures for Fire Safety. Boca Raton, FL: CRC Press, 2009. Print.
Gilsang, Ramon, and Willa Ng. “Why the Loss of One Column May Have Led to the Collapse of Wtc 7.” Structure 2007: 42-45. Print.
NIST. “Questions and Answers About the Nist Wtc 7 Investigation.” nist.gov. National Institute of Standards and Technology, 2011. Web. 11 Oct. 2015. <http://www.nist.gov/public_affairs/factsheet/wtc_qa_082108.cfm>.