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Collapse of the World Trade Center

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The South Tower (WTC2) collapsed 56 minutes after United Airlines Flight 175 crashed into its 78-84th floors at 9:03 a.m. American Airlines Flight 11 crashed into the North Tower (WTC1) at 8:46 a.m., at the 94-98th floors; The North Tower collapsed at 10:29 a.m. Photographs, videos, and witness accounts show the collapsed initiated at the fire and impact points, and progressed downward with dust clouds obscuring the view.

September 11, 2001

South Tower

Calls from occupants trapped in the upper floors relayed information via 9-1-1 about conditions. At 9:37, an occupant on the 105th floor of the South tower, reported that floors beneath him "in the 90-something floor" had collapsed.[1] Deteriorating conditions were also reported by the helicopters of the NYPD aviation unit.[2]

  • 9:52 a.m. - the NYPD aviation unit reported over the radio that "large pieces may be falling from the top of WTC 2. Large pieces are hanging up there"
  • 9:58 a.m. - they report that the South Tower is coming down.

In the South Tower, the core was severely damaged at the southeast corner and was restrained by the east and south walls via the hat truss and the floors. The steady burning fires on the east side of the building cause the floors there to sag. Some fire spread to the north face of the building, but the fires continued to burn on the east face.

The floors pulled the heated east perimeter columns inward, reducing their capacity to support the building above. Their neighboring columns quickly became overloaded as columns on the east wall buckled. The top section of the building tilted to the east and to the south and began its descent. The time from aircraft impact to collapse initiation was largely determined by the time for fires to weaken the perimeter columns and floor assemblies on the east and south sides of the building. The South Tower collapsed more quickly than the North Tower because there was more aircraft damage to the building core, including one of the heavily loaded corner columns, and there were early and persistent fires on the east side of the building, where the fire had extensively dislodged insulation from the structural steel.

Damage

Damage to the South Tower from video:



The South Tower collapse initiation, described by NIST:

  • The inward bowing of the east wall induced column instability, which progressed rapidly horizontally across the entire east face.
  • The east wall unloaded and tried to redistribute the loads via the hat truss to the weakened core and via the spandrels ot the adjacent north and south walls.
  • The entire section of the building above the impact zone began tilting as a rigid block (all four faces, not only the bowed and buckled east face) to the east (about 7 to 8 degrees) and south (about 3 to 4 degrees) as column instability progressed rapidly from the east wall along the adjacent north and south walls. The building seciton above impact continued to rotate to the east as it began to fall downward, and rotated to at least 20 to 25 degrees.
  • The change in potential energy due to downward movement of building mass above the buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse then ensued.

This video, taken from the north, is zoomed in on the damaged corner of the south tower, as the columns buckle inward and the collapse happens:


In this series of images from a video shot from outside Trinity Church (to the southeast of the WTC), the columns can be seen bending and bowing inward in the moments before the collapse occurred.








North Tower

The fire initially burned in the vicinity of the impact (north part), but then encircled the building and the fire reached the east and west faces of the building.

NYPD helicopters relayed information to command staff on the ground, regarding the deteriorating conditions of the North Tower.[2]

  • 10:20 a.m. - the NYPD aviation unit reports that the top of the tower might be leaning.
  • 10:21 a.m. - they report that the North Tower is buckling on the southwest corner and leaning to the south.
  • 10:27 a.m. - the aviation unit reports that the roof is going to come down very shortly.
  • 10:28 a.m. - the NYPD reports that the tower is collapsing.

With dispatchers overwhelmed, minimal communication with the NYPD (staging points at entirely different spots), and the FDNY experiencing radio communication problems, this information did not get to firefighters.[3][4]

The North Tower collapsed at 10:28 a.m., after burning for approximately 102 minutes. In the North Tower, fires weakened the core columns and caused the floors on the south side of the building to sag. The floors pulled the heated south perimeter columns inward, reducing their capacity to support the building above. The neighboring columns quickly became overloaded as columns on the south wall buckled. The top section tilted to the south and began its descent. The time from aircraft impact to collapse initiation was largely determined by how long it took the fires to weaken the building core and to reach the south side of the building and weaken the perimeter columns and floors.

The collapse initiation of the North Tower was as follows, as described by NIST:

  • The inward bowing of the south wall induced column instability, which progressed rapidly horizontally across the entire south face.
  • The south wall unloaded an tried to redistribute the loads via the hat truss to the thermally weakened core and via the spandrels to the adjacent east and west walls.
  • The entire section of the building above the impact zone began tilting as a rigid block (all four faces, not only the bowed and buckled south face) to the south (at least about 8 degrees) as column instability progressed rapidly from the south wall along the adjacent east and west walls.
  • The change in potential energy due to downward movement of building mass above the buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse then ensued.

Once the collapse initiated on the one floor, the kinetic energy of the top part of the tower impacting on the floor beneath was 8.4 times larger than the plastic energy absorption capability. At that point, the subsequent progressive collapse was inevitable. The amount of resistance or absorption capability that each floor beneath could provide was minuscule in comparison to the amount of kinetic energy from the above structure collapsing. This explains why the collapse happened as quick as it did -- not free fall speed, but perhaps twice the amount of time than free fall.[5]

Damage to the North Tower from video:




The collapse of the towers is often said to have occurred in free fall. The time it took the first exterior panels to hit the ground on for the North Tower was estimated at 11 seconds. Portions of the building's core remained standing a bit longer.

North Tower collapse from an NYPD helicopter
North Tower collapse from an NYPD helicopter


Initial expert opinions

Main article: Collapse of the World Trade Center - Initial expert opinions

In the immediate aftermath of 9/11, numerous articles were published in newspapers, with the journalists consulting experts for their opinions on what caused the towers to collapse. The experts included engineers who worked on the design and construction of the towers, and structural engineer experts. They pointed to the fact that steel is substantially weakened at high temperatures, and other factors.

Richard M. Kielar, who was construction manager on the project and spokesperson for Tishman, said that the failure probably occurred with the floor structure, then leading to "catastrophic failures of the rest of the buildings... As the structure warped and weakened at the top of each tower, it -- along with concrete slabs, furniture, file cabinets and other materials -- became an enormous consolidated weight that eventually, progressively crushed each tower below," he said in a statement.[6]

British engineer professor Alastair Soane, who worked on the towers before their official opening in 1973, said "They were extremely robust buildings and built to withstand a tremendous amount. But this was of course a completely abnormal situation and one which would not have been envisaged by the people who built it. The strength of the towers was enormous but they would not have been designed for aircraft strikes."[7]

The enormous heat from the jet fuel fire probably caused the steel trusses holding up concrete-slab floors and vertical steel columns to bend like soft plastic, said Jon Magnusson, chairman and chief executive of Skilling Ward Magnusson Barkshire in Seattle, a structural engineering firm that worked out the original design.[6] "The subsequent fires caused the collapse," said Jon Magnuson, "What happens in a fire is steel at 1,500, 1,600 degrees Fahrenheit, steel loses its strength. Then it just collapses." He said nobody could design a building and account for the possibility of heat generated by burning jet fuel. The impact of the plane could have damaged the sprinkler system, allowing the fire to burn uncontrolled. The combination of the plane crashes and the heat from the fires on the steel columns probably began the collapse. A structural engineer said that after the upper floors gave way, each then overloaded the floor beneath it, continuously to the ground.[8] The skyscrapers had two means of defense against normal fire damage, Mr. Magnusson said. One, thick layers of insulation sprayed onto the steel beams, could have been breached by the initial crash, he said. Another, the building's sprinkler system, may have been disabled as well, or it may simply have been useless in the heat of the jet fuel fire.[6]

Ronald O. Hamburger, a structural engineer and member of the initial ASCE investigative team, explained "Steel is born of fire. As it's reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit, it loses a significant amount of its strength." He said the extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle. Heat also may have caused the steel flooring to separate from the columns, or the columns themselves may have heated up and buckled outward.[9]

Investigations

Building Performance Study

Main article: World Trade Center Building Performance Study

In the aftermath of the 9/11 attacks, the Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers (ASCE) conducted a Building Performance Study for the World Trade Center and nearby buildings. W. Gene Corley served as the lead investigator on the World Trade Center Building Performance Study. This preliminary investigation was initially organized by the Structural Engineers Institute (SEI) of the American Society of Civil Engineers, with the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, and the Society of Fire Protection Engineers also involved.[10] ASCE also invited FEMA to join the investigation, which later became a joint ASCE-FEMA effort.[10] The investigative team released its report on the collapse of the World Trade Center in May 2002.

NIST investigation

Main article: NIST - Collapse of the World Trade Center

In August 2002, NIST announced that they would lead a multi-year investigation into the collapse of the World Trade Center, as well as building #7. The NIST investigation was to look at structural fire protection, life safety, and engineering practice. NIST would build upon the initial work done by FEMA and the American Society of Civil Engineers (ASCE). The investigation would result in "lessons learned" and suggest revisions to building codes, standards, and practices. The investigation included extensive, ranging from heat tests on structural steel members, computer simulation and modelling of smoke and fire movement, as well as interviews with survivors.

NIST determined that the impact of the aircraft caused fireproofing to be knocked off the structural steel, making the columns more vulnerable to the heat from the resulting fires. The heat drastically reduced the structural strength of the steel; That factor, combined with the fact that 33 external columns and 11 core columns were severed and others damaged significantly, means that the remaining columns took on substantial added load to support the building structure above the impact zone. Other critical structural elements also were extensively damaged, including floor structures. The added load on the remaining, weakened steel columns and floor structure, eventually caused the structure to fail.

Fireproofing

Performance of the fireproofing has been cited as a critical factor in the collapse. Fireproofing likely knocked off some steel columns and floor trusses due to the plane impacts.

The Port Authority also had problems keeping up with maintenance and repair of fireproofing in the World Trade Center buildings. Thus, it is likely there were many gaps in the fireproofing, with some steel elements exposed. Frederick W. Mowrer, an associate professor in the fire protection engineering department at the University of Maryland, looked through an archive of photographs and other evidence. The photographs were taken during inspections relating to asbestos lawsuits. He found that large areas of fireproofing were missing from the core columns. Roger G. Morse, an architect who took the photos, said "the problems were far more widespread than that, probably because the fireproofing had been applied improperly to rusty steel."[11]

Alan Reiss, a Port Authority official since 1984 who rose to director of the department in charge of day-to-day operations at the trade towers, said engineers had long been aware of difficulties in keeping some of the fireproofing, in the core of the building around the elevator shafts, on the large structural columns there. Reiss said the problems were caused by the swaying of the buildings in the wind and the impact of elevator cables against the beams. "It was an ongoing maintenance headache," he said. Although measures were repeatedly taken to prevent the problem, he said, "every March and April when you had these windstorms and the building rocked back and forth, you would still knock some of the fireproofing down."[11]

References

  1. ↑ "FDNY McKinsey Report - Executive Summary". FDNY / McKinsey & Company (August 2002). Retrieved on 2007-07-10.
  2. ↑ 2.0 2.1 Lawson, J. Randall, Robert L. Vettori (September 2005). "NIST NCSTAR 1-8 - The Emergency Response" p. 37. National Institute of Standards and Technology.
  3. ↑ "McKinsey Report - Emergency Medical Service response". FDNY / McKinsey & Company (August 9, 2002). Retrieved on 2007-07-12.
  4. ↑ "McKinsey Report - NYPD" (August 19, 2002). Retrieved on 2007-07-10.
  5. ↑ http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/466.pdf
  6. ↑ 6.0 6.1 6.2 Glanz, James, Towers Believed to Be Safe Proved Vulnerable to an Intense Jet Fuel Fire, Experts Say. The New York Times. September 12, 2001
  7. ↑ The terrible transformation of the world's most celebrated skyline stands as a tragic testament to an act of unimaginable violence from which America may never recover, Western Daily Press, September 12, 2001
  8. ↑ "30 Questions, Sorting it Out, Looking Ahead", St. Petersburg Times (September 12, 2001). 
  9. ↑ Shwartz, Mark, Structural engineer describes the collapse of the World Trade Center towers, Stanford Report, December 3, 2001
  10. ↑ 10.0 10.1 "Experts Debate Future of the Skyscraper in Wake of Disaster", Engineering News-Record (September 24, 2001). 
  11. ↑ 11.0 11.1 Glanz, James and Michael Moss (December 13, 2001). "Faulty Fireproofing Is Reviewed as Factor in Trade Center Collapse", The New York Times. 

Further reading

  • Abbout, N., M. Levy, D. Tennant et al (2003) "Anatomy of a Disaster: A Structural Investigation of the World Trade Center collapses", Forensic Engineering.
  • Bazant, Z.P., Y. Zhou (2002a) "Why Did the World Trade Center Collapse? - Simple Analysis", Journal of Engineering Mechanics, 128(1), pp. 2-6.
  • Bazant, Z.P., Y. Zhou (2002a) "Addendum to: Why Did the World Trade Center Collapse? - Simple Analysis", Journal of Engineering Mechanics, 128(3), pp. 369-370.
  • Beyler, C., D. White, M. Peatross et al (2003) "Analysis of the Thermal Exposure in the Impact Areas of the World Trade Center Terrorist Attacks", Forensic Engineering
  • Corley, W.G. (2004) "Lessons Learned on Improving Resistance of Buildings to Terrorist Attacks", Journal of Performance of Constructed Facilities, ASCE, Vol 18(2), May 1, 2004, pp. 68-78.
  • Why Did the World Trade Center Collapse? - Simple Analysis - Zdenek P. Bazant, and Yong Zhou (Journal of Engineering Mechanics, September 2001)
  • Gayle, Frank W; Banovic, Stephen W; Foecke, Tim; Fields, Richard J; Luecke, William E; McColskey, J David; McCowan, Chris; Siewert, Thomas A (October 2004) The structural steel of the World Trade Center towers, Advanced Materials & Processes. Vol. 162, no. 10, pp. 37-39.
  • The Towers Lost and Beyond - Eduardo Kausel, MIT Civil Engineering
  • Lane, Barbara (2003) "Letter to the Editor", Fire Safety Journal, Vol 38, pp. 589-91.
  • Langer, Arthur M., Roger G. Morse (2001) "The World Trade Center Catastrophe: Was the Type of Spray Fire Proofing a Factor in the Collapse of the Twin Towers?", Indoor and Built Environment. 10(6):350-360.
  • Mahmoodi, P., Robertson, L.E., et al (2002) "Performance of Viscoelastic Dampers in World Trade Center Towers", Dynamics of Structures, p. 632-644.
  • Monahan, Bernard (2002) "World Trade Center Collapse - Civil Engineering Considerations", Practice Periodical on Structural Design and COnstruction, ASCE, Vol. 7(3), August 1, 2002.
  • Newland, D.E., D. Cebon (2002) "Could the World Trade Center have been Modified to Prevent its Collapse?", Journal of Engineering Mechanics, SCE, Vol. 128(7), July 1, 2002, pp. 795-800.
  • Omika, Y., E. Fukuzawa et al (2005) "Structural Responses of World Trade Center under Aircraft Attacks", Journal of Structural Engineering, ASCE, Vol 131(1), January 1, 2005.
  • Quan, X., N. K. Birnbaum (2002) "Computer simulation of impact and collapse of New York World Trade Center North Tower on September 11" 20th International Symposium on Ballistics, Orlando, 22-27 September 2002
  • Quintiere, J.G., M. di Marzo, R. Becker (2002) "A suggested cause of the fire-induced collapse of the World Trade Towers", Fire Safety Journal. 37(7):707-716.
  • Wierzbicki, T., X. Teng (2002) "How the airplane wing cut through the exterior columns of the World Trade Center", International Journal of Impact Engineering.

External links

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