Twenty years later, engineering students continue to learn from the structural factors that contributed to the collapse of World Trade Center buildings following the airplane impacts on 9/11.
“The World Trade Center was a brilliant design,” said Barzin Mobasher, an Arizona State University civil engineering professor who includes a section on the collapse as a learning tool in his upper-level undergraduate course in design of steel structures.
“It was a marvel given the resources, boldness, confidence and optimism of the engineers who designed and built it. The building worked.” Mobasher said. “It was the pride of New York, and rightfully so.”
But engineers aren’t prescient, and designing around a possible terrorist attack by commercial airliners wasn’t built into the WTC safety package.
Acknowledging that design engineers can’t predict every possible scenario that could have impact on any structure, Mobasher asserts that future engineers need to assess possible derivations and consider possible failures at each point in the design process.
“We study the lessons we learned in terms of the design of structures,” Mobasher said of his course content. “The forensic analyses from the WTC are a window to the importance of evaluating all potential modes of failure.”
Mobasher’s course relies heavily on WTC investigation reports from the National Institute of Standards and Technology (NIST).
He also includes the findings of his own mentor, Zdeněk P. Bažant, a professor of civil engineering and materials science at Northwestern University. Bažant’s paper, “Why Did the World Trade Center Collapse,” was written two days after the collapse and published six days after that.
Critics questioned Bažant’s summaries, noting they were presented too quickly and without specific data about the building’s design.
“Often, a basic understanding of engineering principles and back-of-the-envelope calculations can tell you what you need to know,” Mobasher said. “I want my students to learn to take in the available info, build a model and hypothesis, and then make an evaluation,” he said. “You don’t always have to make a big soup with every ingredient to get a grasp of a situation.
“After all, Bažant’s engineering-based findings were validated by NIST and have stood the test of time after 20 years.
“While it’s impossible to design for every variable that could happen to a structure, our future engineers will need to assess possible derivations, run the mathematics and consider how design failures may occur at any step.”
Without the fires, the towers would still be standing
The physical impact of the planes into the towers didn’t cause the buildings to fall. The fires were ultimately the cause of the progressive collapse.
“The WTC buildings were like huge sails built to resist a hurricane hitting at 225 kilometers an hour,” said Mobasher. “They were designed to resist wind loads at more than 30 times the weight of the aircraft; very few metal components in the aircraft were comparable in strength to the perimeter columns of the buildings.
"There is no way to prepare for the kind of structural damage done when steel is exposed to high-intensity heat that lasts for hours,” Mobasher said. “At 600 degrees, steel cannot even sustain its own weight, let alone the weight of floors above.”
The primary fire blew off all the fire-retarding material from the main structure. Then, the secondary fire — all of the burning office furniture, equipment and paper — caused overheating of all the structural elements and lost connectivity between floors.
“The impact footprint illustrates that the columns around the crash site, coupled with the columns on the other three sides of the building, were enough to sustain the structure," Mobasher said. “Were it not for the fire, engineers could secure the building, strip out the damaged components and repair and retrofit.”
In fact, the building was the first structure to be studied during its initial design stage by considering the impact of a passenger jet airplane, but not at the full traveling speeds and fuel loads of the airplanes on 9/11. Also not considered was the effect of the secondary fire that lasted more than an hour before the collapse.
“Despite the loss of fireproofing materials around the steel members, and the fact that steel columns were damaged, they were still initially transmitting the load,” Mobasher said. “However, once the temperatures reach 600–700 degrees Fahrenheit, the stiffness and strength is practically reduced so much that a column can’t sustain even its own weight any longer.”
In his course, Mobasher’s points out that the explosion of 17,000-plus gallons of fuel and the fireball it initiated prolonged the burning of all flammable materials, which in turn heated a majority of columns that were damaged and stripped of the fire proofing, causing them to buckle under the load.
“That deformation broke the connections of the floor trusses, which bridged the core of the building to the perimeter walls. Once a few floors lost their load-carrying capacity and connectivity, the perimeter columns lost the bracing provided by the trusses, and then even the virgin columns buckled as well,” Mobasher said. “The potential energy stored in the structure due to weight of the building itself at the floor heights was sufficient to create a self-driving and accelerating domino collapse effect much like a landslide that gains speed through the process.
“Losing the connection between the floors meant overall losing the ability to carry the floor loads. This allowed the surviving columns to stand unprotected without any bracing provided,” Mobasher said. “One damaged floor broke all of the end connections in the floor below, which then broke the end connections in the next lower floor, until all of the floors collapsed.”
Focus on design
Mobasher’s course focuses on how the buildings were constructed from an engineering perspective, including the interaction of foundation with the core, the steel columns and the floor systems.
“These topics are the bread and butter of understanding the design process by evaluating how the failure could take place. The concepts are tied to current design learning — the study of tension and compression, fracturing bolts and buckling columns,” Mobasher said. “The principles are the same as they were in the late 1940s when many of the buildings were designed.
“Of course, students today have computers and software to assist with the calculations, but the fundamentals have not changed. The World Trade Center provides a dramatic illustration of what can go wrong when you don’t consider all possible design failures,” he said.
Conspiracy theories cannot displace science
“Before the fake-news mentality was the cultural means of dismissing facts we do not want to accept, I was contacted by many 9/11 ‘truth seekers,’” Mobasher said. “I was asked to review videos, listen to conspiracy theories and go on the record to cast doubt on the actual causes and mechanisms.”
Now, Mobasher uses these conspiracy theories to impress upon his students the importance of using scientific evidence.
“I tell them to build an understanding from the individual components of a puzzle through examination, engineering and the science fundamentals they learn in their first two years of study at ASU,” he said.
“I also use the example of Bažant who, using scientific principles, was able to provide a full and solid understanding of what happened, while years of alterative conspiracy theories have failed to stand the test of the basic engineering concepts we expect our students to wield in their structural design arsenals.”
Graphics created by Alejandro Cabrera/ASU