The Attack of the Pentagon on 9/11

Building History


The Pentagon is home of the U.S. Department of Defense (DoD) and is one of the largest office buildings in the world with about 6.6 million square feet (Delatte 2009). The Pentagon gets its name from its shape of five sides, but it also has five levels and five concentric rings. There are also ten radial corridors that lead away from the center courtyard and connect the rings. Renovations of the Pentagon began in 1993 with an original completion date of 2014.

The Pentagon was constructed in 16 months between September 1941 and January 1943 during World War II and was intended to be a temporary home for the DoD. The Pentagon was intended to be turned into storage for records after the DoD had moved to its permanent location. With this in mind, the designers constructed the floor to withstand a live load of 150 lb/sq.ft. (¹Mlakar et al. 2003, pp. 8-9). The structure is a reinforced concrete construction using mostly 2,500 PSI concrete and an intermediate grade reinforcing steel (40,000 PSI) due to the fact that most of the steel made in the U.S. was being used for the war effort. Columns varied in size from 21 inches square on the first floor to 14 inches square on the fifth floor. All columns supporting more than one floor were constructed using spiral reinforcing with some of the others using tied columns. Floor spans were kept short, with a slab thickness of 5-½ inches spanning in between beams 10 foot on center. Beam spans are between 10 and 20 feet (²Mlakar et al. 2003, p.3).

The Attack


At 9:38 AM on September 11, 2001, a hijacked Boeing 757-200 crashed into the western most wall, roughly 140 feet to the south of the boundary between Wedge 1 and Wedge 2, in an act of terrorism (NTSB 2002). The impact of the plane and the resulting fire killed all 64 people aboard and another 125 people within the building, including military personnel and civilian employees. All five floors of E-Ring (the outer most ring) of the Pentagon collapsed about 20 minutes after the impact in the affected area in Wedge-1.

The Investigation


Shortly after the attack, the Pentagon Building Performance Study (BPS) team was formed. They were permitted access to the site for four hours on October 4, 2001. By that time, most of the debris from the attack had been removed from the affected area.

The path of structural damage was 75-80 feet wide and about 230 feet long, tapering as it went farther into the building (³Mlakar et al. 2003, p. 8). Most of the structural damage was done to the first floor, while the other floors had much less damage. Damage to the structure was severe, including heavy cracking and spalling from the impact and the fire. The condition of the columns varied from being completely missing or detached, to being bent at midpoint as much as three times the diameter of the column, to columns being bent in triple curvature (¹Mlakar et al. 2003, pp. 28-29). One of the most important aspects of the investigation was the performance of the Pentagon during the fire. Tests were conducted in Belgium at the University of Liége on 11.5"x11.5" columns with a 28-day compressive strength of 2,900 PSI, well within the range of concrete found at the Pentagon, in 1997. Using the data from the author's findings, the BPS team estimated that the maximum temperature reach in some locations topped out at 1,740° F. Although the laboratory tests do not exactly match the conditions experienced at the Pentagon, the comparison should provide and indication of the lower bound of the temperature reached inside (⁵Mlakar et al. 2003, p. 4). The initial investigation suggested the following structural performance issues that needed further analysis:

  • response of the concrete columns to impact,
  • load capacity of the floor system, and
  • thermal response of the columns and girders (Mlakar et al, 2003, p. 45).

Almost none of the columns appeared to have failed due to shear or reinforcing bar pull-out (Delatte 2009). Unless the columns were torn from their supports, most of them remained ductile. Where the supports for the columns had failed, the reinforcing bars had considerable necking before they were fractured (¹Mlakar et al. 2003, 45-48).

It is believed that the reason for the partial collapse of the Pentagon is the heat from the fire and high-strain creep and the weight of the water that was pumped into the structure to put out the fires (¹Mlakar et al. 2003 48-50).

An analysis of the thermal response of the columns and girders was conducted as part of the investigation. For the analysis, two cases were considered: the standard building fire (ASTM E-119) and the standard hydrocarbon pool fire (ASTM E-1529), which correspond to the lower and upper boundsof the actual fire condition after the first hour, respectively. A typical column (type 5, 14"x14") and main girder (14"x20") were subjected to the analysis and were subjected to two conditions: undamaged and damaged. The tests indicated that the fire endurances of the column and girder were greater than 90 and 120 minutes, respectfully, and are considered the lower bound values. On the other hand, the damaged column (cover stripped to the spiral reinforcing) had a fire endurance of 25 to 50 minutes and the damaged girder (cover stripped to the rebar) had a fire endurance of 12 to 20 minutes. The endurance times estimated in the analysis were consistent with the observed time to collapse after the initial fire (⁵Mlakar et al. 2003, pp. 9-10)

In an analysis of the response of the columns to impact, comparisons were made between typical "tied" columns and spirally reinforced columns, which the later was used in the Pentagon. The analysis showed that if all of the columns were "tied", the structure would have collapse immediately after impact. The energy absorbing nature of the sprially reinforced columns is one of the factors that allowed the structure to survive for the 20 minutes after the impact of the plane (⁴Mlakar et al. 2003, p. 7).

Pentagon Renovations and Retrofitting


As part of the Pentagon Renovation Project, the exterior facade and windows were being upgraded in Wedge 1 to help in the case of a blast event at the time of the attack. The windows were upgraded with thicker panes and heavier construction. The glazing is framed with steel tubes that transfer the loads to the floor slab. A geotextile membrane was placed over the interior surface of the masonry wall to prevent the masonry from becoming a debris hazard during a blast event. Although the window construction was designed for a vehicle bomb attack, the facade held up remarkably well under the impact of the plane and subsequent fire and is attributed to the survival of many lives at the Pentagon (Hall et al. 2002, pp. 2-3). The reconstruction of the affected area in the Pentagon, named the Phoenix Project, was completed in September of 2002, one year after the attack and well ahead of the projected schedule.

Lessons Learned


The BPS team attributed the response of the structure to the following:

  • redundant and alternative load paths of the beam and girder framing systems,
  • short spans between columns,
  • continuity of beam and girder bottom reinforcement through the supports,
  • design for 150 lb/sq.ft warehouse live load in excess of the service load,
  • significant residual capacity of damaged spirally reinforced columns,
  • ability of the exterior walls to act as transfer girders (⁶Mlakar et al, 2003, p. 2).

In the end, the partial collapse was attributed to the fire and its effects on the structure after all or most of the protective covering for the reinforcing steel had been stripped from the impact. The BPS team also reported recommendations for measures to reduce the risk of collapse due to severely abnormal loading conditions such as:

  • Continuity, as in the extension of bottom reinforcing of beams and girders through the columns,
  • Redundancy, as in the two-way beam and girder system,
  • Energy-absorbing capcity, as in the spirally reinforced columns,
  • Reserve strength, as in provided by the original design of floor live loads in excess of service (⁶Mlakar et al. 2003, p. 3).

The BPS team recommends those practices should be considered in the design and construction of structures required to resist progressive collapse (⁶Mlakar et al 2003, p. 3).

The attack on the Pentagon supports the need for further research and development in progressive collapse and extreme lateral column response. The following topics are of great interest:
  • consolidation of informations on prevention of progressive collapse,
  • influence of extreme column deformations on load-carrying capacity,
  • influence of extreme column deformations on loads within a statically indeterminant structure,
  • energy-absorbing capacity of reinforced-concrete elements,
  • ability of a structure to withstand extreme impact (⁶Mlakar et al. 2003, p. 3)

Bibliography


Delatte, N. J. (2009). “The Pentagon Attack.” Beyond Failure: Forensic Case Studies For Civil Engineers, American Society of Civil Engineers (ASCE), Reston, VA, 162-167.

¹Mlakar, P. F., Dusenberry, D. O., Harris, J. R., Haynes, G., Phan, L. T., Sozen, M. A. (2003). The Pentagon Building Performance Report, ASCE, Reston, VA. 76 pages

²Mlakar, P. F., Dusenberry, D. O., Harris, J. R., Haynes, G., Phan, L. T., Sozen, M. A. (2003). “The Pentagon Crash of September 11.” ASCE Technical Council on Forensic Engineering (TCFE) Third Forensic Congress, ASCE TCFE, Reston, VA. 8 pages.

National Transportation Safety Board (NTSB) Office of Research and Engineering (2002). Flight Path Study – American Airlines Flight 77, NTSB, Washington, DC. 7 pages.

³Mlakar, P. F., Dusenberry, D. O., Harris, J. R., Haynes, G., Phan, L. T., Sozen, M. A. (2003). “Structural Damage Induced by a Terrorist Attack on the Pentagon.” ASCE Technical Council on Forensic Engineering (TCFE) Third Forensic Congress, ASCE TCFE, Reston, VA. 12 pages.

⁴Mlakar, P. F., Dusenberry, D. O., Harris, J. R., Haynes, G., Phan, L. T., Sozen, M. A. (2003). “Structural Analysis of the Damaged Structure at the Pentagon.” ASCE Technical Council on Forensic Engineering (TCFE) Third Forensic Congress, ASCE TCFE, Reston, VA. 10 pages.

⁵Mlakar, P. F., Dusenberry, D. O., Harris, J. R., Haynes, G., Phan, L. T., Sozen, M. A. (2003). “Thermal Response of the Pentagon Structural Elements.” ASCE Technical Council on Forensic Engineering (TCFE) Third Forensic Congress, ASCE TCFE, Reston, VA. 12 pages.

Hall, R. L., Baylot, J. T., Dinan, R. J., Dove, R., Hayes, J., Hossley, W. M., O’Daniel, J. L., Slawson, T. R., Woodson, S. C. (2002). Terrorist Threats Against the Pentagon. Doboku Kenkyujo, Japan. 9 pages.

⁶Mlakar, P. F., Dusenberry, D. O., Harris, J. R., Haynes, G., Phan, L. T., Sozen, M. A. (2003). “Findings and Recommendations from the Pentagon Crash.” ASCE Technical Council on Forensic Engineering (TCFE) Third Forensic Congress, ASCE TCFE, Reston, VA. 3 pages.