The hijacked planes that destroyed New York's World Trade Center crashed into the twin towers at their most vulnerable spot, which suggests the suicide attackers may have known where to strike, an expert said on Wednesday.

The jetliners smacked into the towers about three-quarters up their 110-storey height, an impact probably equal to twice the force of the biggest likely hurricane, a top British structural engineer, Gordon Masterson, said.

Structural damage from this impact, and the searing heat of blazing aviation fuel, then combined to bring down the top 20 or so floors, each weighing 2 500 tonnes.

The accumulating weight would have caused both buildings to collapse, floor by floor, faster and faster, he said.

"If they had struck right at the top of the tower that would have had a greater effect on the foundations, but the fire would have been more contained," Masterson, chairman of the structural and building board at Britain's Institution of Civil Engineers, said.

"However, they struck in both cases at an extremely vulnerable point.

"Once the fire broke out at level 90 and softened the steel supports to the upper floors, which were already damaged by the impact, then the uppermost levels would have fallen down.

"The weight of this would have been far beyond anything that this floor (at level 90) was capable of supporting, and this would have caused the whole building to go down progressively, floor by floor by floor.

"That zone is probably the vulnerable point," Masterson said from London.

Asked whether this meant the hijackers knew where to hit, Masterson said, "it's entirely possible."

"The papers on the design and construction of the World Trade Center are readily available in the public domain and the perpetrators appear to have had access to skilled resources in all sectors, including piloting planes perhaps, so nothing would surprise me."

A French architect, Marc Mimram, said very tall buildings were prey to a phenomenon called moment tensor inversion - a shock from a seismic wave or impact that ripples through the building but has a whiplash, leveraged effect on its higher floors.

The two towers, one 417m and the other 415m high, were designed in the mid-1960s, and were completed in 1973 after a six-year endeavour.

They basically comprise a square-sided steel tube - the external metal frame of the building - with a central core.

According to repute, the buildings were designed to withstand the impact of a Boeing 707, the predominant airliner at the time.

Masterson said he had never heard of any skyscraper being deliberately designed to take a hit from an aircraft. Such buildings were intended for civilian use and were built to withstand natural phenomena such as high winds and earthquakes as well as a certain amount of accidental damage, he said.

He presumed the Boeing 707 scenario must have been an incidental calculation, perhaps out of curiosity, to see whether a building would immediately topple over if it were hit by a big aircraft.

On that score, both towers had withstood this test, as they did not immediately fall over, for it was the resultant fire that probably inflicted the mortal blow, he said.

Whether the plane was a 707 or a somewhat bigger modern twinjet was largely irrelevant. What mattered was that a very large object impacted the building, projecting aviation fuel inside that caused an "instant inferno," he said.

The temperature of the fire would be in the region of 800-1,000 deg C, easily enough to destroy the remaining integrity of the steel supports, which would soften at 600 deg C, he said.

Masterson said it was impossible to design skyscrapers to withstand such an event.

"What took place was way beyond the reasonable contemplation of the building's original designers. These high-rise buildings are designed for peacetime civilisation. We choose to live in cities, not in underground nuclear bunkers." Sapa-AFP
Trade Centre's steel and concrete design couldn't sustain the hit: Architects

The image of the World Trade Center's 110-story twin towers crumbling seemed a scene of impossible destruction.

But the miraculous steel and concrete architecture that made them could not withstand the power of Tuesday's attack and ensuing fire. No building designed today could, said Masoud Sanayei, a civil engineering professor at Tufts University.

Experts in skyscraper construction said video of the collapse led them to believe the towers were perhaps weakened by the initial impact of the airplanes that hit them Tuesday, but that heat from the resulting fire was likely the most punishing blow.

Hyman Brown, a University of Colorado civil engineering professor and the Trade Center's construction manager, speculated that flames fuelled by thousands of litres of aviation fuel melted steel supports.

"This building would have stood had a plane or a force caused by a plane smashed into it," he said. "But steel melts, and 90 850 litres of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."

Brown said the twin towers were originally designed to survive powerful impacts, but that such construction couldn't make them fire- or bombproof. He said it appeared the attack was meticulously planned.

"If they did it lower in the building the fire department could have gotten to it sooner. In its simplicity, it was brilliant," he said.

The towers have staircases in all four corners of the buildings and were designed to be evacuated in an hour, but it appeared that since the planes crashed into the corners, escape was cut off for those on the floors above, Brown said.

Sanayei said the heat may have disconnected one of the towers' concrete floors from the tubular steel columns that ringed the buildings. If one or two floors collapsed, it would have created a pancake effect of one massive floor caving into the next.

"In my opinion, the fire weakened the connection between the floor system and the columns on the higher floors and caused a couple of the floors to collapse," Sanayei said. "The floors are very heavy, made of reinforced concrete, so when one hits the next, they cause a domino effect ... and it can go all the way down to the first floor."

The glass and steel towers were the highlight of the career of Minoru Yamasaki, who was born in Seattle in 1912 and died in 1986.

Yamasaki worked with engineers John Skilling and Leslie E. Robertson to design the fabled towers.

At the Detroit architectural company Yamasaki founded, there were condolences Tuesday. Minoru Yamasaki Associates "will provide any assistance we can to aid the rescue efforts," the company said.

"Words cannot adequately express the depth of our grief, and our thoughts and prayers go out to the victims and their families," the company added in a statement, declining further comment.

In his 2000 book "Building Big," architect David MaCaulay described the towers' engineering as "a series of load bearing exterior columns spaced a meter apart and tied together at every floor by a deep horizontal beam, creating a strong lattice of square tubing around each tower."

The core surrounding the elevators inside was much the same, with a giant lattice work of steel covered by poured concrete connecting the interior columns to the exterior ones. The design was free enough for each of the towers to hold 360 000m² of space unencumbered by columns or load bearing walls.

Sections of exterior wall were wrapped around the outside in 7.2- and 10.8-meter high sections, creating a sort of patchwork so that not all the floor joints would meet walls at the same height, according to MaCaulay.
- Sapa-AP