The Collapse of a 47-Story Building - WTC7

At 5:20 p.m. on the evening of September 11th, 2001, the Salomon Brothers Building, also known as 7 World Trade Center (WTC7), the northernmost building on the World Trade Center complex, uniformly collapsed in 6.5 seconds. It produced a rubble pile with a radius similar its original dimensions and approximately seven stories tall [1].

WTC7 had trapezoidal measurements of 246 feet on its south length, 329 feet on its north length, 144 feet in width, and 610 feet in height, and it was built according to widely proven construction, fireproofing, and engineering regulations [2]. WTC7 was supported at its foundation by 71 steel columns, which helped share the load of the 47-story building with 159 caissons deeply anchored into the underground bedrock [3]. In 1989, renovations to floors 5 through 7 were made, and 375 tons of steel was added in the form of three transfer trusses and eight transfer girders, which helped distribute the load of the upper portion of the building with the structure below [4]. Above the 8th floor, the steel frame of the building consisted of 24 interior core columns and 57 perimeter columns [2]. Further horizontal framing of the building occurred on each floor level using steel beams and girders, connected by metal decking, wire mesh, and concrete slabs.

Government analyses have mostly focused upon the probable initiation of the collapse sequence of WTC7. A FEMA (Federal Emergency Management Agency) investigation explored a hypothesis in which WTC7 collapsed due to a combination of structural and fire-related damage on the south side of the building (closest to the collapse of WTC1 350 feet away) and possible ruptured diesel fuel lines and tanks [3]. WTC7 housed four large underground fuel tanks supplying five day-tanks located on floors 2, 5, 7, 8, and 9, which implemented an emergency pump shutoff system in the case of the fuel system breach. In the NIST (National Institute of Standards and Technology) analysis of the collapse, their major hypothesis focuses upon the failure of a single interior column related to the 5th floor transfer truss [2]. It is their assertion that a total failure along the vertical axis of the building in line with this column was followed by a second total failure along the horizontal axis of the building, causing the uniform total collapse.

However, the set of characteristics surrounding the collapse of WTC7 is contained in the set of controlled demolition characteristics. Characteristic observations of controlled demolitions are: a lack of structural resistance to the upper portion of the falling structure - allowing for a uniform and near free-fall speed of the total collapse, a contained rubble pile radius with a small height and rubble pieces, and the appearance of a “kink” and/or “squibs”, which is a dip in the roof line, and powerful dust-like clouds exiting the sides of the building, respectively. Indeed, the total collapse of WTC7 clearly indicated these same characteristics, including: a lack of lower-floor structural resistance – which allowed for a uniform 6.5 second collapse, a rubble pile contained in the original dimensions of the building which measured approximately seven stories high, and relatively small rubble pieces. Also, an observable kink and numerous squibs were evident during the collapse of WTC7. Moreover, the characteristics of WTC7’s collapse are not found in similar steel structures with structural and/or fire-related damage, in which weakened portions may collapse locally, but do not affect the entire structure, and which have never resulted in a uniform total collapse with a foundation-wide rubble pile.

[1] Anonymous Ground Zero worker. Pictures of WTC7 Rubble.

[2] Sunder, Shyam, and William Grosshandler, et. al. “Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center - Appendix L - Interem Report on WTC7”. NIST. June 2004.

[3] Gilsanz, Ramon, and Edward DePaola, et. al. “Chapter 5: WTC7”. Federal
Emergency Management Agency (FEMA).

[4] McCain, Mark. "The Salomon Solution; A Building Within a Building, at a Cost of $200 Million", The New York Times, February 19, 1989.