Much more must to be done to protect super-tall buildings and their inhabitants from another 9/11 style catastrophe, leading structural engineers told NCE in the week that marks 10 years since the shocking terrorist attacks.
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The warning came after it emerged that less than half of recommendations from the official investigation into the World Trade Center disaster have been fully applied in the six years since its report.
The latest update from the US National Institute of Standards and Technology (Nist) shows that 18 of its 30 recommendations have been acted on. But only 14 have convincing outcomes, leaving 16 to be properly addressed. Significantly, that includes the key recommendation demanding that buildings be designed against progressive collapse.
Have the Nist recommendations been fully implemented?
Engineers told NCE in the immediate wake of the disaster that the US had previously failed to take the threat of progressive collapse seriously. Yet it has not made substantial progress in the intervening 10 years.
Arup principal David Scott, who is based in New York and leads the firm’s buildings practice for the Americas, said that at the time of the disaster he was of the view that the US must have progressive collapse codes. “The building process is particularly slow — and particularly in super tall buildings,” he said.
“Looking back, what strikes me is how little has changed in how we practically do things.”
The Nist report called on the American Society of Civil Engineers to revise its ASCE-7 standard Minimum Design Loads for Buildings and Other Structures to tackle progressive collapse, but this has not yet happened. The US-based International Building Code has been revised to improve structural integrity, but Nist’s update makes clear that this is not intended to eliminate progressive collapse.
It’ll be a slow process but we’re making progress
Bob Smilowitz, Weidlinger Associates
Consultant Weidlinger Associates principal Bob Smilowitz is also chairman of the ASCE committee that is studying progressive collapse.
Smilowitz told NCE the ASCE Committee is working towards developing a consensus document that is approved by industry.
Relevant UK codes and Eurocodes are being reviewed and are likely to have an influence. But this work has yet to be completed.
“It’ll be a slow process but we’re making progress…” he said. “There is more work to be done — and even associations in the UK are looking at requirements for resilience. Things may have been working fi ne but have they really been tested and taxed to the extent they’ll need to be?”
Experts also warned against a fixation on progressive collapse.
“Would the World Trade Center have performed better with progressive collapse design? The answer for [towers] 1 and 2 is ‘no’,” said Scott. “But maybe for WTC7 [which caught fire and suffered thermal expansion to its structural steel beams leading to its progressive collapse following the Twin Towers destruction], it could have made a difference.”
I look at the WTC and think it performed extremely well in the initial impact. But how did it shed its load? It’s not easy to determine.
David Scott, Arup
He added that when the fuel-laden jets crashed into the WTC towers, many more structural columns were knocked out than the one redundant column at ground level stipulated by such codes subsequently. But the towers remained standing for some time. “I look at the WTC and think it performed extremely well in the initial impact,” said Scott. “But how did it shed its load? It’s not easy to determine.”
But this is not unique to the Twin Towers, said Scott. “I know many buildings that could perform that well,” he said, even if they have not been designed for progressive collapse.
More important could be the way that structural connections behave in fire, a factor which has had less attention. He said that the belief is that problems occur here when they heat up and they lose their strength. This has an impact but is not as significant as thermal expansion, which can be solved by introducing ductile components, according to Scott.
“Small details can make a huge difference,” agreed Smilowitz. “We can allow building systems to deform and not fail, but the connections have to be able to cope too.”
There is not prescriptive change
Alastair Soane, Cross
Structural connection design is the subject of meetings, hypotheses and presentations, Scott said, but more is still to be done. Yet extensive “resiliency”, which is the new buzz word according to Smilowitz, will still only be applied in special circumstances.
“Some buildings warrant more attention than others,” he said. “The intent is to put out a menu of different requirements — more stringent for higher and iconic buildings.
“There is not prescriptive change — such as how many columns must be redundant — the idea is that each building is looked at in its own right and assessed for vulnerability, risk level etcetera,” added Confidential Reporting on Structural Safety director Alastair Soane.