A Cambridge University academic has shattered conspiracy theories surrounding the September 11 terrorist attacks that took place in New York six years ago today with a new mathematical analysis of the collapse of the World Trade Centre.

Keith Seffen, a senior lecturer in Cambridge's engineering department, used established engineering models to demonstrate that once the collapse of the twin towers began it was destined to be rapid and total.

While the causes that initiated the collapse of the towers are now well understood, engineers continue to speculate about the speed and totality with which the buildings were demolished during the fateful attacks.

Conspiracy theorists claim US government involvement in the catastrophic events that followed two planes being flown into the buildings. They suggest "controlled demolition" was the reason behind the speed, uniformity and similarity between the collapse of both towers.

But according to Dr Seffen's analysis of engineering principles, the way the towers collapsed was "quite ordinary and natural".

"The World Trade Centre towers were designed to absorb an aircraft impact but an accidental one with much less fuel and speed," he said.

"It is widely acknowledged that the impacts on September 11th were extraordinary, which led to consequences well in excess of the design capacity for the buildings. The original design of both towers must be praised for standing as long as they did, saving more lives than might have been expected."

Dr Seffen's research showed many studies focused on the phase just before collapse begins.

"They rightly show that the combination of fire and impact damage severely impaired those parts of the building close to where the aircraft hit to hold the weight of the building above. The top parts were bound to fall down but it was not clear why the undamaged building should have offered little resistance to these falling parts," he said.

His new analysis, which will be published in the American Society of Civil Engineers' Journal of Engineering Mechanics, calculates the average strength of a given storey of the building away from the impact area as it was being squashed flat.

This allowed him to define the "residual capacity" of the building, which he then used to develop a dynamic model of the collapse sequence, simulating the successive squashing of individual storeys based on the residual capacity already identified.

From this, Dr Seffen predicted that the residual capacity of both buildings was limited and once collapse had started it would take only 10 seconds for the building to go down.

This shows that the speed of the collapse as actually occurred was consistent with a "pancaking" effect caused by the dual impacts of the planes. As such, the mechanics of this pancaking process were exactly the same as a controlled demolition, but starting from the top and moving downwards, he said.