Three buildings collapsed on 9/11, fire brought down Building 7 says report

NO PLANE flew into Building 7 at the World Trade Centre. But seven hours after the Twin Towers collapsed in flames on September 11, 2001, this third skyscraper fell too.

Like its larger neighbours, it fell rapidly, vertically, almost symmetrically, like an implosion. It took 5.4 seconds for its 47 storeys to complete their fiery descent.

Building 7 has preoccupied conspiracy theorists ever since. Many believe it was brought down by controlled explosions. And if it was, so were the Twin Towers. And if they're willing to believe that, it is not such a big leap to conclude that the whole atrocity was a US Government plot. They have not been silenced by an official report that concludes their theories are bunkum. It didn't help that it was released almost seven years later, in August last year.

The National Institute of Standards and Technology spent three years investigating Building 7. It names fire as the culprit. Fire - fuelled by office furnishings, aided and abetted by the thermal expansion of structural elements.

"Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down," said the lead investigator, Shyam Sunder. The conclusion that this was an ''extraordinary event'' - the world's first known total collapse of a tower caused by fire - only emboldened the doubters.

Explosives? The institute concludes that the smallest blast capable of crippling the third tower's critical column would have produced a "sound level of 130 to 140 decibels at a distance of half a mile''. No witness reported it.

The 9/11 Truth Movement points to the discovery of thermite, a potential explosive. The institute replies that the same metal compounds would have been present in the construction.

The institute's finding is less sensational, but perhaps more alarming for people who frequent towers. Debris from the collapse of the first tower ignited fires on at least 10 floors of Building 7. These uncontrolled fires caused thermal expansion of steel beams on lower floors, damaging floor framing on multiple floors.

''Eventually, a girder on floor 13 lost its connection to a critical interior column that provided support for the long floor spans … Floor 13 [collapsed], beginning a cascade of floor failures …''

The really scary part? This happened at hundreds of degrees below what had been anticipated in fire-resistance ratings. The institute recommended urgent evaluation of towers, improved thermal insulation and resistance for building materials, and structural systems to prevent ''pan-caking'' or progressive collapse.

- smh