On the morning of the 11th of September 2001 hijacked commercial aircraft crashed into each of the
World Trade Center towers in New York, USA. The towers subsequently collapsed resulting in the worst building disaster in history. 146 people died on board the planes, 2,604 died either in or at the site of the World Trade Center Complex and another 24 remain listed as missing. The US Government led by President George W Bush responded by declaring “war on terror”, tightening travel security measures and taking military action around the world. Conspiracy theories about on what occurred on the day that has become known as 9/11. This article examines what happened to the buildings and why the towers collapsed.
At 8.46 a.m., American Airlines Flight 11 hit the World Trade Center's North Tower. At 9.03am United Airlines Flight 175 flew into the South Tower. Both impacts resulted in catastrophic fires in the buildings. The South Tower fell first at 9.59am after burning for 56 minutes. The North Tower burned for 102 minutes before collapse at 10.28am. Debris from the collapse of the North Tower severely damaged and set fire to the adjacent building 7 World Trade Center and it subsequently collapsed at 5.20pm.
Numerous other neighbouring buildings were also severely damaged and were subsequently condemned and demolished.
Conspiracy theories surrounding 9/11 abound. Although the arguments put forward by conspiracy theorists have been largely disproved by solid scientific and engineering research, the internet has given them a strong voice. The theories tend to revolve around the theme that the US government either knew about the impending attacks and did nothing to prevent them or that the US government went so far as to be responsible for the attacks themselves.
Part of these conspiracy theories includes the idea that the towers would not have been able to collapse in the way they did through the jet impacts and subsequent fires. Believers in these theories say the towers collapsing were a result of controlled explosive demolition.
Detailed reports published by the National Institute of Standards and Technology reject the idea of controlled demolition, missile attack and other theories. The findings put forward by NIST have subsequently been examined by and backed by the wider community of civil engineers.
The National Institute of Standards and Technology has conducted a 3-year building and fire safety investigation to study the factors contributing to the probable cause (or causes) of post-impact collapse of the WTC Towers (WTC 1 and 2) and WTC 7.
Chronological Damage Events
The NIST investigation team has formulated the following chronological sequence of major events leading to the eventual collapse of the towers:
• Aircraft impact damaged the perimeter columns, causing redistribution of column loads to adjacent perimeter columns and to the core columns via the hat truss (the steel structure that supported the antenna atop the towers and was connected to the core and perimeter columns).
• After breaching the building’s exterior, the aircraft continued to penetrate into the buildings, damaging core columns with redistribution of column loads to other intact core and perimeter columns via the hat truss and floor systems.
• The subsequent fires, influenced by the post-impact condition of the fireproofing, weakened columns and floor systems (including those that had been damaged by aircraft impact), triggered additional local failures that ultimately led to column instability.
• Final column instability resulted when redistributing loads could not be accommodated any further.
Contributing Factors in the Collapse
Among the factors relevant to the condition and collapse of the WTC towers were:
• The innovative structural system at the time they were built, incorporating many new and unusual features, including: a composite floor system, using open-web bar joist elements and the use of wind tunnel testing to estimate lateral wind loads in the design
• The relative roles of the aircraft impacts and subsequent fires
• The post-impact condition of the fireproofing on the floor systems
• The qualities and properties of the structural steel used.
Innovative Structural System
The fire protection of a truss-supported floor system by directly applying spray-on fireproofing was innovative and not consistent with prevailing practice at the time of construction.
The fireproofing thickness (specified to meet a 2-hour fire endurance rating) was 1/2 inch at construction and was upgraded on some floors to 1-1/2 inches prior to Sept. 11, 2001.
Unrelated to the WTC buildings, a model code evaluation system service recommended in June 2001 a minimum thickness of 2 inches for a similar floor system to achieve the 2-hour fire rating.
The three-to-four-fold difference (between 1/2 inch and 2 inches) in specifying the fireproofing thickness to meet the required fire rating is extraordinarily large and confirms the lack of technical basis in selecting a thickness.
While the building designers recognized the benefits of conducting a full-scale fire endurance test to determine the required fireproofing thickness, no such tests were conducted on the floor system used in the WTC towers (NIST will be conducting this test later this summer).
If a “structural frame” approach (considering that the floor truss was connected to the interior and perimeter columns, essentially forming a single structural unit) had been used, the needed fire rating would likely have been 3 hours, as it was for the perimeter columns alone.
NIST computer simulations indicate that flames in a given location lasted about 20 minutes before spreading to adjacent, yet unburned combustibles, and that this spread was generally continuous because of the even distribution of combustibles throughout the floors and the lack of interior partitions.
The results of two sets of wind tunnel tests on the WTC towers conducted by independent laboratories in 2002 and provided to NIST show large differences – as much as 40 percent – in resultant forces on the structures. Additionally, the wind loads estimated from these tests are about 20-60 percent higher than those apparently used in the original design of the WTC towers.
Wind load capability is a key factor in determining the overall strength of a tall building and important in determining its ability to withstand not only winds but also its reserve capacity to withstand unanticipated events such as a major fire or impact damage.
NIST is conducting an independent analysis to establish the baseline performance of the WTC towers under the original design wind loads and will compare those wind load estimates with the then-prevailing code requirements.
Relative Roles of Aircraft Impact and Fires
The two WTC towers withstood the initial impact of virtually identical aircraft (Boeing 767 200ER) during the terrorist attacks of Sept. 11, 2001. The robustness of the perimeter structural system and the large dimensional size of the WTC towers helped the buildings withstand the aircraft impact.
Following impact, the WTC towers displayed and withstood vibrational forces that were as much as half the levels (in extreme wind conditions) for which the buildings were designed.
Preliminary aircraft impact damage analysis indicates that the impact of a fuel filled wing section resulted in extensive damage to the exterior wall panel, including complete failure of the perimeter columns.
Fires played a major role in further reducing the structural capacity of the buildings, initiating collapse. While aircraft impact damage did not, by itself, initiate building collapse, it contributed greatly to the subsequent fires by:
• compromising the sprinkler and water supply systems
• dispersing jet fuel and igniting building contents over large areas
• creating large accumulations of combustible matter containing aircraft and building contents
• increasing the air supply into the damaged buildings that permitted significantly higher energy release rates than would normally be seen in ventilation-limited building fires, allowing the fires to spread rapidly within and between floors
• damaging ceilings that enabled “unabated” heat transport over the floor to ceiling partition walls and to structural components.
The jet fuel, which ignited the fires, was mostly consumed within the first few minutes after impact. The fires that burned for almost the entire time that the buildings remained standing were due mainly to burning building contents and, to a lesser extent, aircraft contents, not jet fuel.
The typical WTC office workstation furnishings were able to sustain intense fires for at least an hour on a given WTC floor.
Role of Fireproofing Conditions
Most of the floor systems in WTC 1 impacted by the aircraft crash and fires had upgraded or thicker (1-1/2 inches) fireproofing while most of the affected floors in WTC 2 had the original (1/2-inch) thickness.
The response of a structural component to fire is sensitive to variability in fireproofing thickness along its length.
As applied – both in the original spraying and in later upgrades – the fireproofing was found to be thermally equivalent to uniform thicknesses that were greater than the specified minimums required by the building owner.
It was found that the acceleration of a structural component would have to be about 100-150 times the acceleration due to gravity to dislodge 1-inch-thick fireproofing similar to that used in the WTC towers. NIST is currently conducting analytical studies to estimate the magnitude of accelerations of the structural components due to aircraft impact. This will help identify those regions where fireproofing may have been dislodged.
Analysis of Recovered WTC Steel
• The collection of 236 pieces of steel in NIST’s possession is adequate for analyzing the quality and properties of the steel for the investigation, emphasizing regions of impact and fire damage. Pieces of all specified grades of steel (for the exterior panels, core columns and steel trusses in the floor systems) were acquired.
• Analysis of the recovered steel indicates that each of the structural components of the WTC towers had the grade specified in the design drawings.
• Metallography and mechanical property tests indicate that the strength and quality of the steel was as specified, typical of the era and likely met all qualifying test requirements.
• The room-temperature strength of the steel used in the towers met the relevant standards and, in many instances, exceeded the requirements by 5-10 percent.
• Analysis is ongoing of the performance of the steel components under impact and fire conditions up to the starting point of the total building collapse.