2 Morrow
Concrete not good for high tower blocks? Let the MIT experts quoted in the Scientific American article speak:
"One audience member asked the assembled experts whether a reinforced concrete skyscraper such as the current height record-holder, the 452-meter Petronas Towers in Kuala Lumpur, Malaysia, would have better resisted a collision with a fuel-filled airliner. Their response indicated that a concrete structure would have probably lasted for a couple of more hours than did the steel World Trade Center towers."
Possibly discredits Morrow? A very interesting article it is, although to my mind strong on theorising and short on fact. Nevertheless recommended reading. Let's look at the main facts that do emerge:
'The huge inner and outer rectangular tubes "needed to be protected to maintain their structural integrity, so the floors acted as reinforcing diaphragms or bulkheads [the term used in shipbuilding]," said panel member Jerome Connor, professor of civil and environmental engineering at M.I.T. The office floors, which each comprised a 35- to 60-foot clear span from the core to the exterior grid, were panelized structural members supported by open web joists with steel decks above them, he said. The horizontal truss struts, bolted and welded to the exterior grid and the core column structures, included viscoelastic stringers that provided increased damping to help make the structure less lively in the wind, according to Connor. Each steel floor deck was covered with four inches of concrete. "With almost an acre of area for each floor and figuring about 100 pounds per square foot of area," he estimated that "each floor system weighed about 3,200,000 pounds."'
If 1sq ft of deck N+1 weighing 100lbs lands flat on 1sq ft of deck N from a height of 10 ft, does it punch its way through deck N? Not likely.
If 1600 tons of deck N+1 lands flat, more or less equally distributed, on deck N from a height of 10 feet, does it punch its way through deck N? No, hardly. Will it break the joints of the open web truss struts of deck N to the exterior grid and the core column structures, so that decks N+1, N, N-1,... slide acceleratingly down the core and inside the exterior grid? Maybe, but then one expects to see the main vertical core members and perhaps the exterior grid too left stark and forlorn against the skyline, but substantially intact, with a large pile of rubble largely contained at the bottom of the box. Not what I remember seeing!
Of course, the sudden arrival of 150 000 tons of soon-to-be-rubble at the bottom of the rectangular pipe could force the outer grid apart near its base precipitating the collapse of the frame, but that would have been awesomely Titanic-like (in reverse, of course). Again not what I remember seeing!
One wonders whether those bracing wires needed for the demolition experts weren't adequately proxied by the panelized structural members and the visco-elastic stringers.
Disingenuously, one expert, Eduardo Kausel, 'addressed the oft-asked question of why the towers did not tip over like a falling tree. "A tree is solid, whereas building is mostly air or empty space; only about 10 percent is solid material. Since there is no solid stump underneath to force it to the side, the building cannot tip over. It could only collapse upon itself." ' (Anyone hear anything about the position of the centre of gravity here? Turning moments? Would you be persuaded by this argument that an open-mesh waste bin cannot tip over? Perhaps it's only true if the air is hot air.) Think about how much air and 'empty space' there is within the envelope of a tree!
But it makes for an interesting thought-experiment. Imagine a gent with chainsaw climbing up the tree and on his way sawing each branch nearly through at the joint with the trunk, so it can still carry the branch's own weight but not much more. Now imagine a lot of branches tearing free from near the top, maybe the gent cut a little too deep when he got up there, and cascading down, taking the lower branches, twigs and leaves with them. Given initial bilateral symmetry, why should the tree (=stem) tip over anyway? It would remain, standing forth proud and free from the pile of firewood at its roots. Oops, apart from the braches above the decisive level at which the branches were cut too far through. Now it's looking like a coconut palm or a prehistoric gingko. Darn, how do we get rid of that topknot?!? Aah, how about a few strategically-placed explosive charges down the length of the stem to make sure the whole lot collapses and gets turned into matchwood?
Clearly the MIT "have-a guess" session wasn't the final report, but it supplied enough material to fuel more doubt rather than douse what already was glowing.
And the 757 that hit the Pentagon must have been one of the rare swing-wing models; you know, the ones where the kamikaze pilot presses a button and then they don't damage the building left and right of the fuselage's entrance hole, nor leave their wings behind on the lawn. Much neater that way. Considerate.
Oh yes, the Pentagon. How many days did it take between impact and letting of the contract to repair? How many weeks, more likely months, does it take to prepare contract documents for a job like that? How do the two timescales compare? Having answered that, you would find it easy to state how long BEFORE impact the contract documents had to start being prepared...
Look, wherever there is politics, from the village level to the global village, there is a nasty smell. We're all used to that. This smells just the same, only more so. Much, much more.
Biting the hand that feeds IT
