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Originally Posted by duckznutz
willravel, apologies for the confusion . . .. the steel lattice I refer to is not the external lattice of the facade (which purpose is to take out wind loads), but the prefabricated lattice trusses which made up the floor plates. These were light-weight steel sections, prefabricated into zig-zag truss beams (to save weight and allow services to pass through) and were connected to the central core. It was supposedly the failure of these light-weight floor plate trusses (at their connection with the core)which resulted in the floor collapse. Once just one part of one floor starts crashing down, the whole thing is going to go down like a pack of cards. Strong as the building was . . . the large span floor plates were not designed to withstand the impact of a concrete floor from above dropping down 12' . . and even if it could, you would now have the weight of TWO concrete floors bearing on weakened connections with fires still raging. Stunned as I was at the time . . in hindsight, the collapses are not really hard to understand.
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So you are refering to the theory that the lateral trusses became detached by the heat of the fire and fell down on each other, leaving the columns bare and without the latteral support the columns fell. Of course this makes no sense. Even if you believe this idea that the peremeter columns, lacking lateral support, would somehow buckle and fall, you certianally can't apply that logic to the core structure because the core columns are not free standing. The core was also a lattuce of steel. This was a crossbraced structure with these huge steel columns, the dimensions of which were 36" by the outside dimensions and they were fabricated of steel that was 4" thick at it's base. So if your theory is true, then
the outside would have fallen around the core, leaving the core standing. On top of that, when you examine pictures of the collapse, you do not see free standing portions of perimeter columns that are buckling. You see the building systematically being exploded from top to bottom.