21-09-2011, 11:38 AM
(This post was last modified: 21-09-2011, 06:13 PM by Jeffrey Orling.)
Phil is essentially correct and it should be understood that in a commercial CD there is some prep aside from explosive and cutter charge placement. This prep may even include some *pre weakening* such as cutting into some columns with a torch, or removing some bolts. And then not every column is targeted... only the ones necessary to cause the failure to progress per design so that the building structure usually fails entirely in the center and the building comes down into itself.
The twin towers AND bldg 7 were very unique structures similar with long span column free floors... structurally they were a series of floors suspended between two arrays of columns - at the facade and at the perimeter of the core.
A CD always takes out the columns and at the base so the upper mass comes crashing down by gravity and destroys itself... no more explosives are needed.. gravity does the job. But the structure MUST fall far enough so that the impact energy shatters the frame's joints and the columns and beams break apart like dropped pick up sticks.
Attacking selected key columns at the base is a much more efficient CD then exploding all the floor masses AND all the columns. If the frame is not broken at its joints and connections it will stand absent the floors slabs. But the floors slab in the case of the WTC buildings were part of the composite with the floor trusses and the destruction of the floors destroyed the trusses leaving the facade with no lateral bracing. The inside of the core lost it's bracing in the collapsing floors within it. So the floor destruction was not impeded by the columns or their strength but it did WEAKEN them... make them unstable... as they beams/bracing was ripped off the core's columns. So there was no rigid steel frame left to stand. The core then toppled from its internal instability and broke apart at its connection/splices.
A short structure's columns might have stood without the bracing... but above a certain height it could not. A standard 3D lattice structure as most towers are.. would also mitigate a global floor collapse and still have many braces connecting the columns. The column free floor design was the undoing of the 3 buildings. The destructive mass was provided by several dropping floor masses and building components.
There was some inward bowing of the facade in the twins. This was likely not from sagging trusses (NIST) pull the facade in.. but rather a core failure which led to the facade taking too much load and buckling... as the core was slowing losing strength as its columns failed one by one.
The twin towers AND bldg 7 were very unique structures similar with long span column free floors... structurally they were a series of floors suspended between two arrays of columns - at the facade and at the perimeter of the core.
A CD always takes out the columns and at the base so the upper mass comes crashing down by gravity and destroys itself... no more explosives are needed.. gravity does the job. But the structure MUST fall far enough so that the impact energy shatters the frame's joints and the columns and beams break apart like dropped pick up sticks.
Attacking selected key columns at the base is a much more efficient CD then exploding all the floor masses AND all the columns. If the frame is not broken at its joints and connections it will stand absent the floors slabs. But the floors slab in the case of the WTC buildings were part of the composite with the floor trusses and the destruction of the floors destroyed the trusses leaving the facade with no lateral bracing. The inside of the core lost it's bracing in the collapsing floors within it. So the floor destruction was not impeded by the columns or their strength but it did WEAKEN them... make them unstable... as they beams/bracing was ripped off the core's columns. So there was no rigid steel frame left to stand. The core then toppled from its internal instability and broke apart at its connection/splices.
A short structure's columns might have stood without the bracing... but above a certain height it could not. A standard 3D lattice structure as most towers are.. would also mitigate a global floor collapse and still have many braces connecting the columns. The column free floor design was the undoing of the 3 buildings. The destructive mass was provided by several dropping floor masses and building components.
There was some inward bowing of the facade in the twins. This was likely not from sagging trusses (NIST) pull the facade in.. but rather a core failure which led to the facade taking too much load and buckling... as the core was slowing losing strength as its columns failed one by one.