Globe and Boston.com coverage from September 11, 2001
List of victims
World Trade Ctr.
AA Flight 11
AA Flight 77
United Flight 93
United Flight 175
9/11 on the Web:
An archive of Websites, e-mails, photos, video, audio, and discussion groups.
A library of Web content from around the world. sept11.archive.org/
Engineers soak up what they can about building safety by studying Sept. 11
By Sharon L. Crenson, Associated Press
NEW YORK — By the end of the day last Sept. 11, skyscraper experts already were guessing how and why the World Trade Center's towers fell.
A few said it was raging fire, not the impact of the airplanes, that brought 1.7 million tons of steel and concrete crashing to the ground.
They were right.
A year of intensive study by academics, engineers and government officials also revealed a number of other key points.
They've concluded, for instance, that even if the towers hadn't collapsed, it would have been difficult, maybe impossible, to rescue victims from the higher floors because exit stairwells were severely damaged or blocked.
And they've learned that thousands of gallons of jet fuel burned off within minutes of each plane hitting its target. It was the resulting office fires that actually weakened the towers enough to fell them.
But much remains unknown -- including whether any other design could have changed the result.
"All bets are off when you run an airplane into a building," said Gene Corley, a lead engineering investigator on both the Oklahoma City bombing and the Sept. 11 tragedy.
Experts say they will be studying and debating the possibilities for years to come.
So far, the most important thing they have learned may be simply that the exit stairways inside the towers were too close to one another to allow people on upper floors to escape after the jetliners hit. The exits were all located in the buildings' central cores, and were blocked or damaged in the attacks and subsequent fires.
Placing exits farther apart could save lives.
"I hate to put it that way, because clustering them together like that is perfectly legitimate by today's codes," Corley said. "But nobody envisioned an airplane knocking out stairwells."
Other questions have yet to be answered.
Did the floors cave first, or did the supporting steel columns give way?
Could more concrete have held the twin towers up longer?
What if the fireproofing had been upgraded as planned?
Experts such as University of California-Berkeley professor Abolhassan Astaneh-Asl, who received an early grant from the National Science Foundation to study the towers' collapse, are working on those questions.
Astaneh is fine-tuning a computer model to study the reaction of various building designs subjected to the type of impacts the trade center sustained. He's also testing the possibility of retrofitting existing buildings with steel cable connections to reinforce their structures.
However, both Astaneh and Frederick W. Mowrer, a fire protection professor at the University of Maryland who also received funding from the science foundation, say their efforts were hampered by restricted access to ground zero.
Astaneh resigned from the investigation team put together by the Federal Emergency Management Agency and the American Society of Civil Engineers because he didn't agree with the group's decision to keep findings secret until the initial inquiry was complete. Without FEMA's backing, the National Science Foundation team was shut out.
That, Mowrer said, has meant a key question has gone unanswered: whether the fire-protection on the trade center's steel columns was as effective as possible.
For now, New York is considering upgrading the building code to include some features of the trade center's design that experts say did work well. Proposals include requiring clearer emergency exit signs and lighting, and mass evacuation plans for high-rise buildings. Standard procedure in most tall buildings requires only that people leave nearby floors when a fire sparks.
All of those safety measures were implemented at the trade center after the 1993 bombing in which seven people were killed. Sept. 11 survivors say the improvements helped save lives last year.
Other ideas being explored in New York -- such as requiring hundreds of the city's older high-rises to install sprinkler systems -- don't appear to have made any difference in the twin towers' fate.
FEMA reported with near certainty that sprinkler systems at the trade center failed, either because of the impact of the airplanes or because those that did work quickly depressurized as the system lost water.
Still, people around the country are analyzing what happened in an effort to better protect their own cities.
Ted Krauthammer, professor of civil engineering at Penn State and director of the Protective Technology Center, advises planners that the first thing they should do is assess risk. Protective measures are, and should be, different for an FBI facility in Los Angeles than for a farm agency in Nebraska.
Simple measures such as installing protective glass and anchoring light fixtures add safety to low-risk buildings.
It's still uncertain, however, whether different fire suppression or detection systems or better smoke management should be implemented in response to Sept. 11.
"We don't really know," Corley said. "These are things that may still
come out in the future."