World Trade Center Building 7 (Sept. 11, 2001)
Michael W. Hopper, BAE/MS, Penn State, 2009


World Trade Center Building 7 (WTC 7) was a 47 story office building located north of Vesey Street across from the World Trade Center Complex in the Financial District of New York City. On September 11th, 2001 two commercial airliners filled with jet fuel were flown into World Trade Center Towers 1 and 2 (WTC 1 and WTC 2). At 9:59 a.m. WTC 2 collapsed, which caused little damage to WTC 7. However, WTC 7 was significantly damaged by the collapse of WTC 1 at 10:29 a.m. Fires triggered by the collapse of WTC 1 burned nearly 7 hours until WTC 7 collapsed at 5:20 p.m. The collapse of WTC 7 on September 11th, 2001 was significant because it was the first complete collapse of a tall steel building primarily due to fire.

Key Words: WTC 1 (North Tower), WTC 2 (South Tower), WTC 7, Progressive Collapse, Thermal Expansion, Lateral Bracing.

Building Description

WTC 7 was an irregular trapezoid shaped building in plan view. Its north side dimension was approximately 100 m (329 ft.), the south side was approximately 75 m (247 ft.), it was approximately 44 m (144 ft.) wide, and 186 m (610 ft.) tall. Total usable floor space of the building was around 200,000 sq. m (2 million sq. ft.), with each floor containing roughly the same area as a football field. This tall building was built above a Consolidated Edison of New York (Con Edison) substation, which had been built 20 years prior to the opening of WTC 7 to accommodate the loads from a high-rise building. The building footprint of WTC 7 was offset 13 m (42 ft.) from the foundation of the Con Edison substation, represented in Figure 1.
Figure 1 - Footprints of Con Edison Substation and WTC 7 (NIST 2008)

Structural Systems

The floor system of WTC 7 was a composite system using steel beams and girders with 76 mm (3 in.) corrugated metal decking supporting concrete. Floor slabs varied in total thickness from 356 mm (14 in.) at the 1st and 5th levels to a more typical 140 mm (5.5 in.) at upper levels. Floors 5 and 6 were used for mechanical spaces and also contained large steel trusses and 8 cantilevered steel girders to transfer gravity loads from the above levels to the lower levels of WTC 7 and the foundation of the Con Edison substation. Floors 7 though 47 were supported by 24 interior columns and 58 perimeter columns as shown in Figure 2. Perimeter frames contained moment connections.
Figure 2 - Typical WTC 7 Floor Framing Plan (NIST 2008)

Fire Protection Systems

A sprayed fire-resistive material (SFRM) was used to protect the structural steel of WTC 7 from a fire. Instructions to bidders for WTC 7 recommended using a 3 hour fire rating requirement for columns and a 2 hour fire rating requirement for beams. This requirement was more stringent than the required fire ratingss provided by the NYCBC for Type 1C Construction. Private inspections concluded that the thicknesses of applied SFRM were consistent with the recommended values by designers. There were also several active fire protection systems including: fire sensors and alarms, notification systems, automatic fire sprinklers, water supplies, and smoke management. While WTC 7 was a sprinklered building, only the core of the 5th level was protected by sprinklers and none of the electrical rooms were protected with sprinkler systems (FEMA).

Combustible Components

Given that the function of WTC 7 was an office building, the primary combustible fuel load was the workstations. Comprised of cubicle dividers, desks, and paper, these workstations were highly combustible. There were also emergency power generators for WTC 7. Diesel fuel tanks for the emergency power generators were located beneath and within WTC 7. Figure 3 displays the emergency generator systems and fuel storage tank location within WTC 7. The base building tanks were full on September 11th, 2001, but the amount of fuel present in the three day tanks is still unknown. Fuel amounts in the two SSB system tanks are also unknown, but it was assumed that they were available to feed fires with fuel at either the ground or 5th levels. Although these fuel tanks would have been available to burn, the amount of fuel was insignificant compared to the amount of other combustibles on each floor.

Figure 3 - Table Displaying Emergency Power Systems (NIST 2008)

Debris Impact Damages and Fires

Due to the sudden collapse of WTC 1 and 2, the extent of damage to the south facade of WTC 7 could not be documented. Tower 2 collapsed at 9:59 a.m. and is believed to have caused minor damage to WTC 7. Investigators used a variety of photographs and video evidence to locate various locations of debris impact damage from the collapse of Tower 1, which occurred at 10:29 a.m. It is apparent in photographs taken after the collapse of WTC 1 that the southwest corner of WTC 7 at floors 8 to 20, 24, 25, and 39 to 46 were damaged.

It is probable that the fires started in WTC 7 as a result of debris impact from the collapse of WTC 1. These fires are believed to have began as single floor fires near the southwest corner of the building. Evidence shows fires burning on the upper floors of WTC 7 between 12:10 to 1:00 p.m., but it is believed this fire was extinguished by a sprinkler system whose water supply came from storage tanks on the 46th floor. After 1:00 p.m., there are no accounts of fires at the upper levels on the east, west, and north faces of the building. However, there are also accounts of fires on the 7th through 13th levels. These levels were protected using an automatic sprinkler system supplied by city water, which was lost when the towers collapsed, and therefore allowed the fires to spread. Fire intensities were greater on the 11th through 13th levels than the 7th through 9th levels due to a larger floor area and more combustible materials.

It was formerly known that steel subjected to extreme heat experienced a significant reduction in strength due to material degradation. However, tests of steel beams with composite concrete decking found that the response of the structure is dominated by thermal expansion, and that the effects of material degradation and gravity loading are secondary (Gillie, M., Usamani, A. S., and Rotter, J. M. 2001). Thermal expansion is typically resisted in composite construction by shear studs embedded into the concrete decking and by the connections to supporting members. This resistance results in axial stresses which cause steel members to yield or buckle.

Collapse and Investigation

At 5:20 p.m. WTC 7 collapsed in 8.2 seconds and was recorded by several video cameras from the northeast and north west. These recordings were studied and used to create a visual external sequence of events (Gilsanz, R., and Ng, W. 2007). A kink formed in the east penthouse before it fell into the building. After this occurred, the west penthouse fell into the building, followed by a kink in the entire north facade, and then total collapse of the structure. To investigators, this sequence of events suggested an interior failure. It was also believed that the kink in the east penthouse was caused by the failure of an interior line of columns directly beneath the east penthouse. This columns are labeled 79 though 81 in Figure 2 and have significantly large tributary areas.

This possible collapse mechanism was tested by the National Institute of Standards and Technology (NIST). Engineers created a finite element computer model using the original structural drawings and the construction shop fabrication drawings. The computer model sequentially applied gravity loads, debris impact damage from the collapse of WTC 1, and the fire induced temperatures on the structure prior to the analysis. Global structural analyses were performed to study the progression of the initial failure to the total collapse. Calculated failure sequences from the model were compared to documented photographs and videos displaying the actual collapse behavior.

NIST found the initial failure event to be the buckling of Column 79 labeled in Figure 2. Fires weakened the steel structure between the 8th and 14th levels, with the most substantial damage occurring in the east region of floors 12 though 14 (NIST 2008). Temperatures did not exceed 750 degrees Fahrenheit, which is well below temperatures associated with significant loss in steel strength. However, combined thermal expansion and thermally degraded material properties caused resisted floor framing to buckle and also weakened beam and girder connections. Floor sections between Columns 79 to 81 failed on levels 13 and 14 and fell to the floors below. These weakened floors could not sustain the impact from the above floors, which led to the progressive collapse of the floor framing around Columns 79 to 81. As the floor beams and girders failed, the unbraced length of Columns 79 to 81 increased and eventually reached critical conditions for buckling. After these columns buckled, the above floors fell downward creating a vertical progression of floor collapses.

Localized failures began at other interior columns after the vertical progressive collapse. Debris from the above failures damaged columns and transfer trusses that were already weakened by fire. As the floor framing failed the lateral bracing of the interior columns was lost, which eventually led to the buckling of all interior core columns between floors 9 and 14. Collapse of the core columns was followed by the buckling of the exterior columns between floors 7 and 14. After all lower lever column support was lost, the structure fell as a single unit.

Conspiracy Theory

Conspiracy theorists argue that WTC 7 was brought down by a controlled demolition (Editor 2005) and this method was also addressed by NIST (NIST 2008). This analysis focused on a single location blast scenario. It was determined that 4 kg (9 lb) of RDX explosives would be required to cut Column 79. Other scenarios required more explosives and would have been detected. Pressures associated with the detonation of 20 percent of the explosives required to structurally damage a single column would have been large enough to shatter the exterior glass. Visual evidence does not support this glass breaking pattern.

Another argument conspiracy theorists have used to defend the idea that WTC 7 was brought down by a controlled collapse is the collapse time. They state that the building must have been a controlled demolition because video evidence suggests the structure fell at a near free fall rate. NIST also covered this issue and determined that the building took approximately 40 percent longer to fall than the computer free fall time (NIST 2008). Video recorded collapse times begin at the collapse of the penthouse and followed by the entire building facade. In reality, the collapse time was much longer than this, as the internal collapse mechanism that caused the progressive collapse is not visible in any evidence.

NIST also determined that a blast would have propagated sound waves outward from the building. It was determined that the charge sizes for the hypothetical blast scenario would have produced sound levels of 130 to 140 db up to 1 km (0.6 mi) away if unobstructed. In southern Manhattan, these sound waves would have reflected off of hard building exteriors and echoed through channeled streets. All video recordings of the collapse of WTC 7 did not record any sounds near the intensities of a blast large enough to demolish WTC 7. Therefore, it was determined that there was no demolition type blast that caused the collapse of WTC 7.


Although no building designs can be expected to withstand terrorist attacks such as the events of September 11th, 2001, several lessons can be learned from the collapse of WTC 7. These lessons are:
  • steel framing connections should be designed to resist thermal expansion,
  • progressive collapse should be prevented through a nationwide adoption of codes and standards,
  • thermal insulation should be increased to protect structural steel from both thermal expansion stresses and steel strength loss,
  • all automatic fire sprinklers should have a reliable primary and secondary source of water supply, and
  • improved compartmentation in open tenant spaces to reduce the spread of fires.
NIST also provides a list of recommendations for the building industry. Some of the key recommendations include:
  • evaluating the technical basis of classifying appropriate fire rating requirements, especially for tall buildings,
  • improve fire resistance testing of materials through a nationwide effort,
  • fireproofing all members, including lateral bracing, that contribute to the structural integrity of buildings,
  • structures should have the ability to allow fires to burnout before local collapses occur, and
  • develop programs to further educate architects and engineers about computational fire dynamics and thermostructural analysis.
These recommendations are intended to create safer buildings and structures. However, many engineers feel the recommendations are unnecessary, vague, and are based on an extremely rare event (Post 2008).


Editor. (2005). "Debunking the 9/11 Myths: Special Report," Popular Mechanics. 5. March.

Flint, G. et al. (2007). "Structural Response of Tall Buildings to Multiple Floor Fires," Structural Journal. American Society of Civil Engineers. Volume 133. Issue 12. 1719 - 1732.

Gillie, M., Usmani, A. S., and Rotter, J. M. (2001) "A Structural Analysis of the Cardington British Steel Corner Test," Journal of Constructional Steel Research. Volume 58. Issue 4. 427 - 442.

Gilsanz, R., DePaola, E., Marrion, C., and Nelson, H. (2002). "FEMA - World Trade Center Building Performance Study," Federal Emergency Management Agency, Chapter 5, 1 - 32.

Gilsanz, R., and Ng, W. (2007). "Single Point of Failure: How the Loss of One Column May Have Led to the Collapse of WTC 7," Structure Magazine, 42 - 45.

NIST. (2008). "Final Report on the Collapse of World Trade Center Building 7," National Institute of Standards and Technology.

NIST. (2008). "Global Structural Analysis of the Response of World Trade Center Building 7 to Fires and Debris Impact Damage," National Institute of Standards and Technology.

NIST. (2004). "Key Findings of NIST's June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster," Online Fact Sheet from the National Institute of Standards and Technology, Accessed September 18th, 2009.

Post, N. (2008). "Engineers' Reaction Strong to Seven WTC Collapse Report," ENR / McGraw-Hill Construction. September 3.