The study analysed how the twin towers collapsed

The study by a Cambridge University 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 study suggests a different explanation for how 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 to be 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"

"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"

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.