Wednesday September 12, 2001
Situated at the southern end of Manhattan island, the 110-storey, 1,350ft silver slabs towered over Wall Street and provided office space to around 40,000 employees. Ninety thousand more visited on a daily basis. The towers were the largest commercial complex in the world, complete with their own subway station.
The ground excavated for the foundations was shoved into the Hudson river and used to create Battery Park, a green space facing Staten Island that was yesterday swamped beneath the shattered towers.
The towers were initially derided as white elephants, but the economic boom of the 1980s ensured near total occupancy, and with their distinctive appearance they became an instantly recognisable landmark as well as an economic success.
Graham Masterton, a structural engineer, said towers on this scale are almost impossible to protect from the impact of two passenger planes ploughing into them. The only building capable of remaining standing after such an impact would have to be designed "like a nuclear bunker".
"Initially you have localised damage at the point of impact, although the scale of this would have been beyond what an architect usually takes into account," said Mr Masterton, a senior spokesman for the Institute of Civil Engineers.
"This massive force would have reverberated through the building, above and below ground, and this would have a significant weakening effect.
"The twin towers didn't collapse immediately, but the structural strength was affected by the subsequent explosion and the progressive effects of the fire seem to have triggered the final collapse.
"Of course it is up to investigators to identify the exact cause of the collapse but it seems to have been a combination of catastrophic events beyond any reasonable expectations."
Duncan Steel, a physicist at the University of Salford, said the impact of a laden passenger airliner, even at several hundred miles an hour, is less than the force imposed on a tall building by normal high winds. However, he calculated that the subsequent explosion of fuel would have dwarfed the initial impact.
"I would imagine that the major damage that caused the building to collapse is the release of that fuel," he said. "It was a delayed explosion in each case."
"It's a very large explosion," said Scott Steedman, a civil engineer and an expert in natural disasters. "You have the energy in the plane itself, its weight and speed, which is absorbed by the building on a monumental scale.
"Typically that deflection would be likely to be less than the wind deflection. The impact of an aeroplane on the side of the building will also cause it to deflect, and then it will bounce back again, and the energy will be absorbed into the structure.
"But of course if it is full of aviation fuel then that impact is absolutely monumental. In this case, it is obviously clear that the impact was so enormous it has knocked the building sideways."
Tall buildings are designed to absorb horizontal shock. The World Trade Centre towers, built on rubber bases, normally swayed by a metre or more in the wind.
The architect and the engineers developed a rigid "hollow tube" design of closely spaced steel columns with floor trusses extending across to a central core.
The foundations of the centre extend some 70ft underground in huge concrete piles that connect with the unusually hard bedrock of Manhattan island. This rock is ideally suited to take the load of tall buildings. Without it the island would not be able to support even its more modest buildings, and the World Trade Centre towers could never have been contemplated.
The towers became a potent symbol of America's global dominance, as well as a tourist attraction.
In 1993 Ramzi Ahmed Yousef and Eyad Ismoil, motivated by hatred of Israel and America, planted a bomb in the underground car park of Tower 1, the second to collapse yesterday. The subsequent explosion killed six people but failed to fulfil Ismoil's ambition of toppling one tower into the other and killing 250,000 people. Yesterday's attacks succeeded where he failed.