World Trade Center puzzle lingers

World Trade Center puzzle lingers

Investigations began as the wreckage was cleared

Better construction could have saved hundreds of lives in the World Trade Center (WTC), but investigators are still puzzling over the evidence.

The WTC towers "did very well" to stand as long as they did, the head of the official inquiry into their collapse said in a BBC Horizon programme broadcast in the UK on Thursday.

But other structural engineers say use of more robust materials would have definitely allowed some people trapped on the upper floors of the centre to escape and may have prevented their collapse entirely.

The issue is of vital interest because the WTC's design was revolutionary and its construction methods influenced skyscrapers across the world.

Designed for impact

Leslie Robertson designed the structural elements of the WTC towers to withstand the impact of the largest airliner then in service, the Boeing 707.

Would they ultimately have collapsed? Maybe not

Charles Thornton Structural engineer

"With the 707 however, to the best of my knowledge, the fuel load was not considered in the design," he told Horizon.

But as the programme explained, it was the fuel aboard the hijacked Boeing 767s that led to the towers' collapse.

Each tower had a strong central column and a strong external steel skeleton.

Bound to fall

But neither could ever be strong enough to stand by itself, so the key to stability was the steel floor trusses which linked the two.

Investigators believe that foam fireproofing surrounding these trusses blew away when the planes hit the towers, leaving the steel to buckle in the heat of the fires that followed.

Once the crucial trusses began to fail, the towers were bound to fall.

In the minutes that they remained standing, most of the people below the levels where the planes hit were able to escape.

But those above were trapped as fire blocked the central columns and their stairways.

Only four people from either tower managed to escape from the levels above the impacts.

Squeegee escape

Some investigators believe that better fireproofing of the central columns would have saved hundreds of them, even if the towers had still collapsed in the end.

The drywall fireproofing surrounding the central columns was highly fire-resistant but not very strong.

Researchers believe much of it was dislodged on impact.

And it was weak enough for a man trapped between floors in a lift to hack his way through it with the squeegee he used for cleaning windows.

Gene Corley, head of the official inquiry into the towers' collapse, says that the towers did well not to fall down immediately.

I cannot escape the people who died there

Leslie Robertson WTC structural engineer

"The fact that one of them lasted 55 minutes and the other about an hour and 40 minutes says they did very well," he told the programme.

Mr Corley is due to publish his interim findings in April.

But other engineers appearing in the programme speculate that a better tower design could have survived.

"Had the floor system been a more robust floor system with much stronger connections between the exterior and the inside, I think the buildings... would have lasted longer," said Charles Thornton.

"Would they ultimately have collapsed? Maybe not."

Mr Robertson is plagued by the wisdom of hindsight and deeply distressed by the memory of what happened to those inside his buildings.

"I cannot escape the people who died there... that still to me somehow up there in the air are burning.

"I cannot make that go away," he said.

Twin Towers showed 'remarkable' resistance

Twin Towers showed 'remarkable' resistance

The towers' survival "astonished" engineers

The World Trade Center showed "remarkable" resistance in the 11 September attacks and its design contributed to saving many lives, a US government report has found.

The Towers could have survived the impact had fire not broken out

The report, jointly issued by the Federal Emergency Management Agency and the American Society of Civil Engineers, found that the length of time the buildings remained upright after being hit by two passenger planes "astonished most observers, including knowledgeable structural engineers".

It claimed this resistance was testament to its good construction and undoubtedly saved the lives of many within the buildings.

"The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely," the report said.

Immense heat

Investigators believe the WTC would probably even have survived being hit by two passenger jets on 11 September had fires not weakened the buildings' support structures.

Up to 3,000 people died in the attack on the World Trade Center

The report said engineers now believe that it was a combination of three factors: the force of the aircraft impacts, the heat from burning jet fuel, and the heat from the burning of much of the buildings' contents that caused the towers to collapse.

But most crucially, the team of civil, structural and fire protection engineers found that building fire systems in general are not equipped to deal with simultaneous fires breaking out over an entire building, as happened at the Twin Towers, Reuters news agency reported.

The report found that heat from the many fires ignited in the minutes immediately following the planes hitting the buildings rapidly became equivalent in temperature to "the power produced by a large commercial power generating station".

Designers exonerated

This heat gradually induced additional stresses into the already damaged structural frames while simultaneously softening and weakening them, it said.

The resulting damage then inevitably led to the collapse of both structures.

The findings also exonerated the buildings' designers, stating that many of the structural and fire protection procedures in place in the buildings were "superior" to other constructions.

However it also warned that it may not be "technically feasible" to design a building that could withstand the force of a plane hitting it and, even if possible, the costs would be prohibitive.

Up to 3,000 people are thought to have died in the Twin Towers attack.

Twin Towers 'never fire-tested'

Twin Towers 'never fire-tested'

A two-year study is investigating the towers' collapse

The floors of the World Trade Center had not been tested for their ability to withstand fire, an interim report has found.

Extreme heat generated by the many fires that broke out after two aircraft were flown into the buildings on 11 September is seen as one potential factor in their collapse.

The government agency that built the towers never tested the fireproofing insulation on the WTC's floors - meant to withstand two hours of fire - according to the National Institute of Standards and Technology.

Arden Bement, NIST director, said the report did not draw conclusions or make recommendations.

"That's for later, when we have a much more complete picture," he said.

The NIST's two-year investigation was launched last year, and will analyse the collapse and evacuation of the WTC in an effort to make buildings safer in the future.

Despite the widely accepted notion that no skyscraper could have withstood a fuel-laden jet slicing into its core, relatives say there are still many technical questions to be answered.

Fire-proofing upgraded

Previous reports have found that the towers showed "remarkable resilience", standing for more than an hour after being hit by the aircraft.

But some experts say the buildings may even have withstood the strikes, had fires not weakened their support structures.

In 1999, the building's owners, the Port Authority of New York and New Jersey, issued guidelines to upgrade the fireproofing by tripling the thickness on the flooring.

As recently as 2000, officials were still redoing the fireproofing, noting in one property assessment that some areas could withstand only one hour in a fire.

By 11 September 2001, the NIST report found, fireproofing had been upgraded on 29 floors in the areas of the towers where the hijacked planes struck.

Glenn Corbett, a US fire science expert, was quoted by AP saying that the upgraded system should have been tested.

But the fast-moving fires in the towers on 11 September may have been far more severe than the typical conditions assumed in the fire resistance test, he added.

9/11 demolition theory challenged

9/11 demolition theory challenged

The study analysed how the twin towers collapsed

An analysis of the World Trade Center collapse has challenged a conspiracy theory surrounding the 9/11 attacks.

The study by a Cambridge University, UK, engineer demonstrates that once the collapse of the twin towers began, it was destined to be rapid and total.

One of many conspiracy theories proposes that the buildings came down in a manner consistent with a "controlled demolition".

The new data shows this is not needed to explain the way the towers fell.

Over 2,800 people were killed in the devastating attacks on New York.

After reviewing television footage of the Trade Center's destruction, engineers had proposed the idea of "progressive collapse" to explain the way the twin towers disintegrated on 11 September 2001.

This mode of structural failure describes the way the building fell straight down rather than toppling, with each successive floor crushing the one beneath (an effect called "pancaking").

Resistance to collapse

Dr Keith Seffen set out to test mathematically whether this chain reaction really could explain what happened in Lower Manhattan six years ago. The findings are published in the Journal of Engineering Mechanics.

Previous studies have tended to focus on the initial stages of collapse, showing that there was an initial, localised failure around the aircraft impact zones, and that this probably led to the progressive collapse of both structures.

Once the collapse began, it was destined to be "rapid and total"

In other words, the damaged parts of the tower were bound to fall down, but it was not clear why the undamaged building should have offered little resistance to these falling parts.

"The initiation part has been quantified by many people; but no one had put numbers on the progressive collapse," Dr Seffen told the BBC News website.

Dr Seffen was able to calculate the "residual capacity" of the undamaged building: that is, simply speaking, the ability of the undamaged structure to resist or comply with collapse.

His calculations suggest the residual capacity of the north and south towers was limited, and that once the collapse was set in motion, it would take only nine seconds for the building to go down.

This is just a little longer than a free-falling coin, dropped from the top of either tower, would take to reach the ground.

'Fair assumption'

The University of Cambridge engineer said his results therefore suggested progressive collapse was "a fair assumption in terms of how the building fell".

"One thing that confounded engineers was how falling parts of the structure ploughed through undamaged building beneath and brought the towers down so quickly," said Dr Seffen.

Conspiracy theorists see evidence of a "controlled detonation"

He added that his calculations showed this was a "very ordinary thing to happen" and that no other intervention, such as explosive charges laid inside the building, was needed to explain the behaviour of the buildings.

The controlled detonation idea, espoused on several internet websites, asserts that the manner of collapse is consistent with synchronised rows of explosives going off inside the World Trade Center.

This would have generated a demolition wave that explained the speed, uniformity and similarity between the collapses of both towers.

Conspiracy theorists assert that these explosive "squibs" can actually be seen going off in photos and video footage of the collapse. These appear as ejections of gas and debris from the sides of the building, well below the descending rubble.

Other observers say this could be explained by debris falling down lift shafts and impacting on lower floors during the collapse.

Dr Seffen's research could help inform future building design.