14-08-2013, 10:48 AM
Tony Szamboti Wrote:Phil Dragoo Wrote:The hat trusses said to have weakened and sagged forty inches, in tests sagged two to six, and could not have initiated collapse absent significant weakening of core columns--likely by charges, as no other cause presents.Phil, I think you meant to say it was the floor trusses under the floor slabs between the core and perimeter which NIST claimed sagged 40+ inches and pulled in the south exterior wall of WTC 1 (North Tower) over several stories around the 98th floor. Of course, there are serious problems with this theory as they couldn't get the sagging trusses to pull the exterior columns inward in their FEA model and the trusses did not sag anywhere near what they claimed in their actual floor slab fire testing, as you state. The only legitimate mechanism to pull in the perimeter columns was a falling core and there was not enough heat to weaken the core and cause the acceleration observed over the first story of the fall.
In case you aren't aware, the hat truss was a large truss at the top three floors of the building, with A-frame outriggers connecting the core to the perimeter, intended to spread the antenna wind load induced moments out to the perimeter which cut down the vertical force required to counteract them due to the increased lever arm. Lauren put pictorials of it in post #292 of this thread.
The hat truss was a 3 story space frame interconnected with all the columns and bracing. It was a very rigid structure which carried and spread the 360 ton concentrated antenna loads to the building's columns. It was also part of the strategy to transfer/distribute wind shear loads.
The 360 tons attenna sat above the 3 weakest core columns in the center of the tower. To support this concentrated load they used the hat truss instead of using a 4th column and beefing them all up. the 3 columns were used to accomodate the service elevator which went from sub basement to the top mech floor.
The hat truss like DID transfer loads when the core columns beneath were severed and others weakened and the loads from say 98 up were hanging (in tension) from the hat truss. The column connections in the tower were not designed for tension but were to maintain position / alignment for axial load transfer. When core columns were severed the connections were not strong enough and they failed leading to the core above 98 or so to come apart. All floor loads above 98 were then cantilevered from the facade columns and the slabs began breaking free and dropping probably in huge sections which built to threshold ROOSD mass. The transition from static to unstable once it began happened very quickly... destruction of remaining capacity.