Sunday, October 28, 2001		Article

THE glinting twin towers of the World Trade Center (WTC), New York, were completed in 1973, but the WTC was officially put into commercial use from 1975 onwards. It took 8 years and 750 million dollars to complete the Twin Towers Complex, situated in a 16 acre plot in Manhattan. It met a violent end on the morning of September 11, 2001.

These towers were considered to be the symbols of American economic power and had 35 tenants from 26 different countries and there were about 43,000 people employed in the complex.

On the pavement of the beautiful promenade, known as the Battery Park, the message of the well-known French architect, Le Corbusier had been inscribed. It read:-

"A hundred times I have thought: New York is a catastrophe and fifty times: it is a beautiful catastrophe."

The words of Corbusier proved to be prophetic after the Black Tuesday of September 11 this year.

From the top of the towers, which became the centre of attraction for the tourists who visited New York, one could have a breath-taking view of New Jersey.

The 110-storey building was 1362 feet high and its foundation went more than 70 feet below the ground. There were 70 department stores and restaurants located in its basement and above ground. The building came to designed after the famous IBM building of Seattle and its facade of 208 feet was built with prefabricated steel on steel columns, which served to resist wind storms of speeds up to 90 miles per hour.

The designers had to comply with the requirements of the New York State Water Power and Control Commission, that the water pumped from underground for cooling and air-conditioning had to be returned to the aquifer from which the water had been drawn. The measures so taken by the commission had stabilisation effect on the ground water table. The groundwater available from the aquifers was found to be soft i.e. of low mineral content and so could be used for drinking purposes after being treated.

The grills of the towers were assembled with the help of special purpose hoist cranes imported from Australia. The pre-fabricated grills were shipped from far-off places like Seattle and Los Angles. When bolted together they created more than 130 vertical steel support columns which were 40 inches apart.

The horizontal steel beams emerged from the outer vertical columns or tubes and were tied to the inner core or tube which housed 99 elevators. The floor provided was a 4-inch thick reinforced slab capable of carrying a load of 9000 tonnes. The inner core comprised of another set of steel columns.

The set-up had tremendous strength with its foundation resting on solid rock at a depth of 70 feet.

The north tower stood for more than one and half hours after it was hit by the American Airlines Flight 11. But the south tower could hold for only one hour or so after it had been struck by the United Airline Flight 175. The planes, Boeing 757s, weighed about 178.57 tonnes and were travelling at a speed of around 300 mph. The towers swayed back and forth due to the initial impact and then stopped. Meanwhile fires broke out due to the spillage of 90,850 litres of highly inflammable aviation fuel. The terrorists had converted the planes into deadly FAE (fuel-air explosive) bombs which are considered to be far more destructive than the conventional TNT or chemical explosive bombs.

The collapse mechanism witnessed has taught the engineers several lessons. The phenomenon witnessed can be termed as a Volcanogenic Earthquake measuring 6.0 on the Richter’s Scale.

The explosion on account of the aviation fuel resulted in fires which created extremely high temperatures, as high as 2000°C, which resulted in melting of the steel and sagging and breaking down of the reinforced concrete floors. The molten steel resembled lava and started flowing down.

Due to the fire, the floors sagged first by two to three feet but thereafter fractures occurred, breaking them loose from the outer steel frame and the inner core that supported the building. A single floor of the tower weighed as much as 3000 tonnes and each floors had been designed to hold the load of three floors or so, with gradual load addition without impact. But with the temperature around 1500°C the steel reinforcement lost its power to support more than 50 per cent.

So the debris started falling with a top-down domino effect termed by some engineers as the pancaking phenomenon. The building finally imploded. This contributed to the fact that there was relatively little damage to the surrounding buildings which would otherwise have been pulverised if the towers had toppled over instead of imploding.

It was a pitiable end for an engineering marvel which was considered to be a model of stability and strength.