INTENSE HEAT from burning aviation fuel is likely to have been the ultimate cause of failure to structural steelwork leading to the progressive collapse of both of New York's World Trade Center towers on Tuesday.
Collapse of a section of each building's central steel core is believed to have led to an entire floor collapse in both cases, overloading floors below, causing catastrophic sequential failure of both buildings.
Leading engineers said there was little which could have been done to avert the collapse of both towers in the world's worst ever terrorist attack. But they said that design codes for skyscrapers are likely to be reviewed after the disaster.
Just before 9am New York time on Tuesday morning, a hijacked Boeing 767 passenger jet crashed into World Trade Center 1 (WTC1) - the south tower - just above the 80th storey of the 110 floor structure.
The plane left two gaping holes in the side of the tower which burst into flames.
Shocked onlookers watched as smoke billowed out of the building, rising above the city skyline.
Eighteen minutes later an American Airlines Boeing 757, with 64 people on board, then struck the north tower, WTC2, above floor 60.
This caused a similar fire.
Emergency crews rushed to the scene and evacuations of surrounding buildings began.
An hour later, WTC2 spectacularly collapsed in on itself, spreading huge plumes of dust and debris across the southern tip of Manhattan.
The destruction was complete when 20 minutes later WTC1 also collapsed, leaving a large gap in the New York skyline, and panic and disorder below.
In the meantime reports had started to come in that a plane had also crashed into the Pentagon. All flights in and out of the United States were suspended and telephone lines closed.
'I would expect that there will be a change in codes for high rise buildings, ' said Bob Campbell, chief executive of consultant Waterman, structural designer of Canary Wharf.
'But we don't design for aircraft loading - nobody does, ' he added.
Another structural engineer said there was nothing wrong with the design of the towers.
'An aeroplane flying into the building is such an unusual circumstance that we wouldn't design for it. Then there is the massive increase in fire loading provided by aviation fuel, ' said WS Atkins director and head of structural engineering Mike Otlet. 'A bomb explosion will shift elements in a building and the loading will change, but it will not cause a fire like that.
'It is such an incredible set of circumstances that it is not a structural engineering issue - it is not reasonable to design for this type of load, ' he added.
Otlet said that each building consisted of a fully welded external frame forming a torsionally rigid framed tube with an internal steel core. The external frame would resist lateral and torsional loads, while the core would resist gravitational loads only.
The external columns consisted of welded plate sections of varying grade and thickness, with lighter sections for less loading higher up the building.
Glazing was enclosed between the columns which were connected with moment connections. The internal core was surrounded by dry walling or sheetrock while the columns were surrounded by a vermiculite fireproofing skin.
'The aeroplane would easily puncture the columns - you would expect that. But because of the rigid columns and Vierendeel beam arrangement it would cope well with that, ' said Otlet.
'But it is an extremely light building. Because of the slender sections in the core, the massive fire load would weaken the columns to their yield point and they would buckle. This would have brought down a floor, onto the floor below, overloading more columns in a sequential failure. The building came down vertically so it is column failure, ' he added.
David Bennett, technical writer and author of the book Skyscraper, added: 'I would think that there were bombs on the planes and heat from the huge explosions eventually sheared the structures' columns.
'The floors would then collapse on the floor below and so on down the building, which would explain why the towers fell as if they had been designed to by controlled demolition.'
Timetable 8.58am New York time: Jet airliner crashes into World Trade Centre's south tower, WTC1.
9.16am: A second airliner crashes into the north tower, WTC2, causing an explosion.
9.43am: Reports emerge that a plane has crashed into the Pentagon, Washington DC.
9.50am: All airports across the US shut down.
10.07pm: WTC2 collapses.
10.27am: WTC1 collapses.