Immediately following the attacks, a building performance study (BPS) team of engineering specialists was formed by the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) and Federal Emergency Management Agency (FEMA). The BPS team issued its report in May 2002, finding that the aircraft impacts caused “extensive structural damage, including localized collapse” and that the resulting fires “further weakened the steel-framed structures, eventually leading to total collapse”. They also presented recommendations for more detailed engineering studies of the disaster.
The BPS team investigation was later followed by a more detailed investigation conducted by the National Institute of Standards and Technology (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. The NIST investigators did not find anything substandard in the design of the WTC towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. They also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns: “This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers.”
The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors. The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half. It has been suggested, that the fires might not have been as centrally positioned, nor as intense, had traditionally heavy high-rise construction been standing in the way of the aircraft. Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores, which would then not have become unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely. The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above.
Collapse of the South Tower
As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions to 9-1-1 dispatchers. At 9:37 am, an occupant on the 105th floor of the South Tower reported that floors beneath him “in the 90-something floor” had collapsed. The New York City Police Department aviation unit also relayed information about the deteriorating condition of the buildings to police commanders. Only 14 people escaped from above the impact zone of the South Tower after it was hit (including Stanley Praimnath, who saw the plane coming at him), and only four from the floors above it. They escaped via Stairwell A, the only stairwell which had been left intact after the impact. Numerous police hotline operators who received calls from individuals inside the South Tower were not well informed of the situation as it rapidly unfolded. Many operators told callers not to descend the tower on their own, even though it is now believed that Stairwell A was most likely passable at and above the point of impact. At 9:52 am, 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”. With the warnings, the NYPD issued orders for its officers to evacuate. During the emergency response, there was minimal communication between the NYPD and the New York City Fire Department (FDNY), and overwhelmed 9-1-1 dispatchers did not pass along information to FDNY commanders on-scene. At 9:59 am, the South Tower collapsed, 56 minutes after being struck.
Collapse of the North Tower
After the South Tower collapsed, NYPD helicopters relayed information about the deteriorating conditions of the North Tower. At 10:20 am, the NYPD aviation unit reported that “the top of the tower might be leaning”, and a minute later reported that the North Tower, “is buckling on the southwest corner and leaning to the south”. At 10:28 am, the aviation unit reported that “the roof is going to come down very shortly”. The North Tower collapsed at 10:28 am, after burning for 102 minutes.
After the South Tower collapsed, FDNY commanders issued orders for firefighters in the North Tower to evacuate. Due to radio communications problems, firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed. 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings. No one was able to escape the North Tower from the impact zone or above, as all stairwells and elevator shafts on those floors were destroyed or blocked. After the collapse, light dust reached as far as the Empire State Building, located 2.93 miles (4.72 km) away.
Mechanics of Twin Towers’ collapse
Both buildings collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides. In both cases, the section of the building that had been damaged by the airplanes failed, which allowed the floors above the impact zone to fall onto the undamaged structure below. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air from the upper levels. The first fragments of the outer walls of the collapsed North Tower struck the ground 11 seconds after the collapse started, and parts of the South Tower after 9 seconds. The lower portions of both buildings’ cores (60 stories of WTC 1 and 40 stories of WTC 2) remained standing for up to 25 seconds after the start of the initial collapse before they too collapsed.
While the buildings were designed to support enormous static loads, they provided little resistance to the moving mass of the sections above the floors where the collapses initiated. Structural systems respond very differently to static and dynamic loads, and since the motion of the falling portion began as a free fall through the height of at least one story (roughly three meters or 10 feet), the structure beneath them was unable to stop the collapses once they began. Indeed, a fall of only half a meter (about 20 inches) would have been enough to release the necessary energy to begin an unstoppable collapse.
After the planes struck the buildings, but before the buildings collapsed, the cores of both towers consisted of three distinct sections. Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating. The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel. As a result, the core columns were slowly being crushed, sustaining plastic and creep deformation from the weight of floors above. As the top section tried to move downward, however, the hat truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fires, and as the floors began to sag they pulled the exterior walls inwards. In the case of 2 WTC, this caused the eastern face to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse. In the case of 1 WTC, the south wall later buckled in the same way, and with similar consequences.
Total progressive collapse:
The collapse of the World Trade Center has been called “the most infamous paradigm” of progressive collapse, also called “Pancaking”. Once the collapse initiated, the mass of failing floors overwhelmed the floors below, causing a progressive series of floor failures which accelerated as the sequence progressed. Soon, large portions of the perimeter columns and possibly the cores were left without any lateral support, causing them to fall laterally towards the outside pushed by the increasing pile of rubble. The result was the walls peeling off and separating away from the buildings by a large distance (about 500 feet in some cases), hitting other neighboring buildings. Some connections broke as the bolts snapped, leaving many panels randomly scattered. Significant parts of the naked cores (about 60 stories for the North Tower and 40 for the South Tower) remained standing for a few seconds before they also collapsed.
Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation. While there are clear procedures for the investigation of aircraft accidents, no agency had been appointed in advance to investigate building collapses. A team was quickly assembled by the Structural Engineers Institute of the American Society of Civil Engineers, led by W. Gene Corley, Senior Vice President of CTLGroup. It also involved the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, and the Society of Fire Protection Engineers. ASCE ultimately invited FEMA to join the investigation, which was completed under the auspices of the latter.
The investigation was criticized by some engineers and lawmakers in the U.S. It had little funding, no authority to demand evidence, and limited access to the WTC site. One major point of contention at the time was that the cleanup of the WTC site was resulting in the destruction of the majority of the buildings’ steel components. Indeed, when NIST published its final report, it noted “the scarcity of physical evidence” that it had had at its disposal to investigate the collapses. Only a fraction of a percent of the buildings remained for analysis after the cleanup was completed: some 236 individual pieces of steel, although 95% of structural beams and plates and 50% of the reinforcement bars were recovered.
FEMA published its report in May 2002. While NIST had already announced its intention to investigate the collapses in August of the same year, by September 11, 2002 (a year after the disaster), there was growing public pressure for a more thorough investigation. Congress passed the National Construction Safety Team bill in October 2002, giving NIST the authority to conduct an investigation of the World Trade Center collapses.
FEMA building performance study:
FEMA suggested that fires in conjunction with damage resulting from the aircraft impacts were the key to the collapse of the towers. Thomas Eagar, Professor of Materials Engineering and Engineering Systems at MIT, described the fires as “the most misunderstood part of the WTC collapse”. This is because the fires were originally said to have “melted” the floors and columns. Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires. As Eagar said, “The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.” This led Eagar, FEMA and others to focus on what appeared to be the weakest point of the structures, namely, the points at which the floors were attached to the building frame.
The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the buildings’ contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures.
After the FEMA report had been published, and following pressure from technical experts, industry leaders and families of victims, the Commerce Department’s National Institute of Standards and Technology conducted a three-year, $16 million investigation into the structural failure and progressive collapse of several WTC complex structures. The study included in-house technical expertise, along with assistance from several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers, Society of Fire Protection Engineers, National Fire Protection Association, American Institute of Steel Construction, Simpson Gumpertz & Heger Inc., Council on Tall Buildings and Urban Habitat, and the Structural Engineers Association of New York.
The scope of the NIST investigation was focused on identifying “the sequence of events” that triggered the collapse, and did not include detailed analysis of the collapse mechanism itself (after the point at which events made the collapse inevitable). In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these using the software program Fire Dynamics Simulator. NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a global model of the towers as a whole which is less detailed. These models are static or quasi-static, including deformation but not the motion of structural elements after rupture as would dynamic models. So, the NIST models are useful for determining how the collapse was triggered, but do not shed light on events after that point.
James Quintiere, professor of fire protection engineering at the University of Maryland, called the spoliation of the steel “a gross error” that NIST should have openly criticized. He also noted that the report lacked a timeline and physical evidence to support its conclusions. Some engineers have suggested that understanding of the collapse mechanism could be improved by developing an animated sequence of the collapses based on a global dynamic model, and comparing it with the video evidence of the actual collapses. The NIST report for WTC 7 concluded that no blast sounds were heard on audio and video footage, or were reported by witnesses.
Read The Full NIST Report Here: NIST NCSTAR 1 – Collapse Of The Twin Towers (PDF)