NCJ Number
192788
Date Published
October 2001
Length
9 pages
Annotation
This article highlights the vulnerabilities of ultra-tall buildings to fire and points out steps that could be taken to lessen them.
Abstract
One month after the attack on the World Trade Center, a panel of Boston area-based civil and structural engineers convened to discuss how and why the towers came down. The main structural engineering design criteria for the World Trade Center’s 1,362-foot-tall south tower and 1,368-foot-tall north tower centered on two things: ensuring resistance to the gigantic gravity loads of the buildings themselves as well as to sideways or lateral forces caused by high winds and earthquakes, which can generate huge overturning forces at the bases. Each twin tower was strongly framed in structural steel. The central core, formed of massive vertical steel columns that held most of the building’s weight, contained elevator shafts, stairways and utility spaces. The support structures of both twin towers withstood the initial hits of the two airliners despite the breaching of many levels of framing. They stood for an hour or so despite the damage to a major potion of the gravity system. However, the buildings were not designed for the massive explosions or the intense jet fuel fires that came next – “a key design omission.” The towers collapsed after the kerosene fuel fire compromised the integrity of their structural tubes. The aviation fuel fires burned at a much hotter temperature than the typical contents of an office, which was the type of fire they were designed for. The sprinkler system, which was probably compromised, would have been useless against this kind of fire. The huge gray dust clouds that covered lower Manhattan after the collapse were probably formed when the concrete floors were pulverized in the fall. Despite the expert panel’s preliminary musings on the failure mechanisms responsible for the twin towers’ fall, the definitive cause has yet to be determined. Recommendations to lessen these vulnerabilities include retrofitting existing skyscrapers with some additional safety measures, providing improved fire protection for structural elements, and improving the effectiveness of building safety systems.