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NEW YORK – In the aftermath of the horrific events of Sept. 11, 2001, the American Society of Civil Engineers () and organizations like the conducted in-depth investigations to understand what triggered the towers’ progressive collapse.

A recent article tells the stories of the engineers who not only contributed to the understanding of what happened at the World Trade Center and the Pentagon, but about their own experiences during the events of that day and the lessons learned.

The Daily Commercial News has gathered some important elements from the article, but the

The need for a blast standard

On the morning of Sept. 11, 2001, Paul Mlakar, an engineer emeritus with the U.S. Army Corps of Engineers, was preparing for a telephone call with colleagues to discuss the need for a blast-resistant design standard. He had a news show muted in the background when the first plane hit the North Tower at 8:46 a.m. EDT.

Mlakar said 9/11 underscored the importance of developing what would become the Blast Protection of Buildings standard.

“Not all buildings need to be blast-resistant,” he said. “But for public buildings, we now have the ASCE standard to provide a basis for that. It could help designers better protect those structures in the future.”

Designing to avoid progressive collapse

Gerard Schwartz, who became president of ASCE the month after the attacks, first visited Ground Zero in late October 2001.

He said most people do not realize how much structural engineers, particularly SEAoNY members, contributed to rescue and cleanup, the article notes.

Schwartz, as the new head of ASCE, helped co-ordinate the civilian forensic teams that would do the building performance assessments and analyses at the World Trade Center and the Pentagon. He said that work has helped the field better understand what factors contributed to the resulting collapses and can be used to inform future designs.

“The planes hit the World Trade Center tower at an angle, cutting through several floors,” he explained. “The building was actually strong enough structurally to withstand that hit. But it wasn’t strong enough to withstand the fire that resulted from all the jet fuel. That lit everything inside the building on fire, buckling important joists that provided important vertical and horizontal support.”

The Pentagon’s older design helped protect it from a larger progressive failure.

“There was a lot about the way the Pentagon was designed and built (beginning) in 1941 that made it more resistant to the progression of collapse,” Mlakar said in the article. “There was a relatively small area of the building that collapsed 20 minutes after impact, but the extent of it was fairly minimal, thanks to the continuity of the structural system and the way the columns were reinforced. We can learn from that.”

A broader view of risk

Vicki Arbitrio, an associate partner at , headquartered in Manhattan, was secretary of SEAoNY when the twin towers fell. She said in the article, “strong engineering judgment” ensured no one died from physical injuries during the removal process.

Twenty-five years later, she believes thanks to the ASCE and NIST investigations after 9/11, building safety in general has improved. She also credits the tragedy with a cultural shift in structural engineering, with more collaboration across different disciplines.

She states the World Trade Center was, in fact, designed for a plane impact.

“Planes have gotten so much bigger since the building went up (in 1973), and no one thought any impact would be intentional,” she said. “We understand now there are other technologies we need to learn more about so we can understand their potential impact on a structure, depending on where a building is and what’s around it. There are a lot of possibilities that you want to consider as you work through a design.”