BBC News Online: Sci/Tech

The destructive forces unleashed

Some want the skyline to be restored

Frank Moscatelli, from Swarthmore College, Pennsylvania, said a rough estimate of the figures involved showed why the twin towers never stood a chance.

The professor said the sheer size of the World Trade Center towers was their undoing and America should think carefully before building similar skyscrapers again.

"Being from New York, I understand the desire to rebuild those towers," he said. "But we might consider for a moment the wisdom of reconstructing buildings that stick out as such clear targets and hold such destructive potential as well."

Mass and height

Professor Moscatelli said that of the three sources of energy delivered to New York City last Tuesday - the kinetic energy due to the motion of the two aircraft, exploded jet fuel and gravitational potential energy due to the falling building material - the last was the most devastating.

"My calculations show that the largest component by far was the latter," said Professor Moscatelli, a native New Yorker. "This is due to the large mass and height of the towers. Their splendour was their undoing."

According to Professor Moscatelli, the numbers, in joules, are:

1. 1.9x10⁹ (both planes)
2. 6.9x10¹⁰ (both planes)
3. 6.8x10¹¹ (both towers plus sundry other collapses)

"This is equivalent to the detonation of 0.2 kilotons of TNT," the professor said. "As comparison, the Hiroshima bomb was 10 kilotons TNT equivalent." Professor Moscatelli estimated his numbers to be accurate to within about 25%.

Structural engineers are agreed that once the central steel cores in the WTC towers melted, the buildings would have started to collapse down on themselves. Massive and increasing forces would have been inflicted on even those floors unaffected by the collisions and the fires that followed.

A comparison with some other approximate energy values: