The conspiracy theory that the World Trade Centre (WTC) towers were demolished by explosive charges previously placed within them is probably impossible, the French monthly Le Monde Diplomatique reported on Monday.
According to the report, any demolitions expert concocting a plan to hit a tall building with a plane and then using pre-placed explosives to ensure the undetectable collapse of the building would never place the explosives 20, 30 and 60 floors below the impact point. He would put them on one or more floors as close as possible to the planned impact level.
The paper said that it was inconceivable that the demolitions expert would time his surreptitious explosions to blow hours after the impact. He couldn’t be certain that the impact fires would even last an hour, so to mask the booster explosions he would have to time them to follow right after the impact, the report said, adding that sizeable charges, large enough to do a lot more than emit the puffs of smoke cited as evidence for the hypothesis, were needed to collapse a major building.
Explaining the collapse of buildings 1 and 2, Herman Soifer, a retired structural engineer, said that the towers were tubes, essentially hollow. Tubes can be efficient structures, strong and economical, he said, adding that the WTC tubes resisted vertical loads, wind loads and vibrations and could probably have done well against earthquakes. However, he said the thin skin of the hollow tube must be braced at intervals to prevent local buckling of the skin under loads, otherwise the tube could go out of shape and lose its strength.
For their interior bracing, the thin-walled tubes depended primarily on the interior floors being tied to the outer wall shells. These floor beam structures were open web joists, adequate for the floor loads normally expected. The joist ends rested on steel angle clips attached to the outer walls.
As the floors at the level of impact caught fire, the open web joists, which could not be expected to resist such heat, softened, sagged and pulled away from their attachments to the walls. Their weight, and the loads they carried, caused them to drop on to the floor below, which then carried double loads and was exposed to heat. It collapsed; so it went on down. As the floors dropped, they no longer braced the main tubes. This loss of bracing permitted the walls to buckle outward in successive sections, hence the house of cards effect.
There was no other major bracing as in a more conventional structure, or as might have been introduced if there were a fear of potential loss of the floors. There were no stiff horizontal trusses in the perimeter to act as bracing ribs every few floors. There was no system of vertical trusses to provide integrity, nor anything that could be considered a frame or skeleton of columns and attached girders to keep the tube intact.
The conspiracists’ last accusation is the collapse of WTC building number 7 hours after the attacks. But the explanations offered by US government agencies (the National Institute of Standards and Technology and, for Building 7, Fema) are more than adequate. The blast of hot debris from Building 1 kindled fires in 7 and caused an emergency power system to feed the burning to the point of collapse. One of the building’s major bridging supports was heated to exhaustion by the burning of an abundant store of diesel fuel, pumped from the sub-basement by the back-up system and spraying through a torn pipe into the fire next to two of the three major structural trusses. The types of steel in the WTC (plain carbon and vanadium) lose half their strength when heated to about 570 C, and even more as temperatures rise, as they did in WTC 1 and 2, to 1,100 C.
Physicist Manuel Garcia Jr calculated that the fuel spraying at 75 gallons a minute in Building 7 released energy equivalent to an explosion of 367 tons of TNT.
(Original headline: Conspiracy theory debunked: Previously placed explosives didn’t demolish WTC towers )
