Initial opinions and analysis Immediately after the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse.
Abolhassan Astaneh-Asl, a structural engineering professor at the
University of California at Berkeley, explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become "soft and mushy", eventually leaving them unable to support the structure above. Astaneh-Asl also suggested that the fireproofing became dislodged during the initial aircraft impacts. He also explained that, once the initial structural failure occurred,
progressive collapse of the entire structure was inevitable.
César Pelli, who designed the
Petronas Towers in Malaysia and the
World Financial Center near the Twin Towers, remarked, "no building is prepared for this kind of stress." On September 13,
Zdeněk P. Bažant, professor of civil engineering and materials science at
Northwestern University, circulated a draft paper with results of a simple analysis of the World Trade Center collapse. Bažant suggested that heat from the fires was a key factor, causing steel columns in both the core and the perimeter to weaken and experience deformation before losing their carrying capacity and buckling. Once more than half of the columns on a particular floor buckled, the overhead structure could no longer be supported and complete collapse of the structures occurred. Bažant later published an expanded version of this analysis. Other analyses were conducted by MIT civil engineers Oral Buyukozturk and
Franz-Josef Ulm, who also described a collapse mechanism on September 21. They later contributed to an MIT collection of papers on the WTC collapses edited by
Eduardo Kausel called
The Towers Lost and Beyond. 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 legislators. 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 small fraction 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 the first anniversary of the disaster, there was growing public pressure for a more thorough investigation. Congress passed the National Construction Safety Team bill in October 2002, authorizing the NIST 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.
NIST report in the foreground was crushed when both towers collapsed. The outer shell of the South Tower (tower 2) of the WTC is still standing behind and to the right of the Marriot. 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
Fire Dynamics Simulator software. 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 were
static or quasi-static, including deformation but not the motion of structural elements after rupture. James Quintiere, professor of fire protection engineering at the University of Maryland, called the fact that only a small portion of the building's steel was preserved for study "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.
7 World Trade Center In May 2002, FEMA issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under leadership of
W. Gene Corley, FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1 WTC and 2 WTC but by fires on multiple stories ignited by debris from the other two towers that continued unabated due to lack of water for sprinklers or manual firefighting. The report did not reach conclusions about the cause of the collapse and called for further investigation. In response to FEMA's concerns, the National Institute of Standards and Technology (NIST) was authorized to lead an investigation into the structural failure and collapse. The investigation, led by Shyam Sunder, drew upon in-house technical expertise and the knowledge of several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers (ASCE), the
Society of Fire Protection Engineers, the
National Fire Protection Association, the American Institute of Steel Construction, the
Council on Tall Buildings and Urban Habitat, and the
Structural Engineers Association of New York. The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the collapse of the World Trade Center twin towers. In June 2007, Shyam Sunder explained, "We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible. The 7 WTC investigation is in some respects just as challenging, if not more so, than the study of the towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers." In August 2008, NIST released a draft report on the causes of the collapse, and after a period for public comments, NIST determined that diesel fuel did not play an important role, nor did the structural damage from the collapse of the twin towers, nor did the transfer elements (trusses, girders, and cantilever overhangs), but the lack of water to fight the fire was an important factor. The fires burned out of control during the afternoon, causing floor beams near Column 79 to expand and push a key girder off its seat, triggering the floors to fail around Column 79 on Floors 8 to 14. With a loss of lateral support across nine floors, Column 79 soon buckled – pulling the East penthouse and nearby columns down with it. With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the entire building above to fall downward as a single unit. From collapse timing measurements taken from a video of the north face of the building, NIST observed that the building's exterior facade fell at free fall acceleration through a distance of approximately 8 stories (32 meters, or 105 feet), noting "the collapse time was approximately 40 percent longer than that of free fall for the first 18 stories of descent." The fires, fueled by office contents, along with the lack of water, were the key reasons for the collapse. When the NIST report was published, Barbara Lane, with the UK engineering firm
Arup, criticized its conclusion that the loss of fire proofing was a necessary factor in causing the collapses; "We have carried out computer simulations which show that the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact."
Jose L. Torero, formerly of the
BRE Centre for Fire Safety Engineering at the University of Edinburgh, pursued further research into the potentially catastrophic effects of
fire on real-scale buildings. ==Aftermath==