23-08-2013, 06:42 PM
Lauren Johnson Wrote:Jeffrey proposes the hat truss system distributes load into the vertical columns. My understanding is that the spandrals (sp?), the horizontal steel beams which tie the vertical columns into a kind of web surrounding the buildings, serve to distribute load in a very significant way. How would this load distribution system compare with that of the hat truss system?
The twin tower walls were vierendeel trusses... almost like the entire side was a single plate made up of the 10'x36 panels. The weakest link here was the connections themselves.. the bolts used to connect one panel columns and spandrels to the other.
The hat truss was a multi story structure which spanned the entire footprint and all the core columns were integrated into this structrure. Think of the core as the trunk... the hat truss as the branch system... almost.
The hat truss was designed to support the massive antenna which put a 360 ton concentrated load on 3 of the smallest core columns. So the hat truss spread the load to the rest of the core and even some of the facade since it was the entire foot print in plan. It also acted like an end plate for the tube structure of the facade and the entire top 3 floors where NOT framed with the lightweight open web trusses by very heavy steel beams as all the mech floors were. So the entire top 3 floors were a rather massive steel frame with many disagonal members providing rigidity like a truss bridge structure but in 3 dimensions.
When the core columns at floors 93-5 lost their ability to carry load the columns above would be hangin (in tension) from the hatt truss. This meant that instead of the loads bearning DOWN.. they were being HUNG from the hat truss. This meant that those loads had to find (and did for a while) alternate load paths to bedrock... through the remaining columns... some core and some facade. As the core columns "dropped out* of the support *game* more and more load was transferred to remaining columns. When the hat truss could no carry the loads the core columns broke free and all hell broke loose... the facade was pulled downward... and it was translated
THERE WERE NO COLUMN TO COLUMN COLLISIONS... the entire top was rotated and distorted at that instant...the entire upper fls had broken free INSIDE the dropping facade and were what became the initiating ROOSD mass.