6.14.2 Results of Global Analysis of WTC 1
After the aircraft impact, gravity loads that were
previously carried by severed columns were redistributed
to other columns. The north wall lost about 7 percent
of its loads after impact. Most of the load was
transferred by the hat truss, and the rest was redistributed
to the adjacent exterior walls by spandrels. Due
to the impact damage and the tilting of the building to
the north after impact, the south wall also lost
gravity load, and about 7 percent was transferred by
the hat truss. As a result, the east and west walls and
the core gained the redistributed loads through the hat truss.
Structural steel expands when heated. In the early stages
of the fire, structural temperatures in the core rose,
and the resulting thermal expansion of the core was greater
than the thermal expansion of the (cooler) exterior walls.
About 20 min. after the aircraft impact, the difference
in the thermal expansion between the core and exterior walls,
which was resisted by the hat truss, caused the core column loads
to increase. As the fires continued to heat the core areas
without insulation, the columns were thermally weakened and
shortened and began to transfer their loads to the exterior walls
through the hat truss until the south wall started to bow inward.
At about 100 min, approximately 20 percent of the core loads were
transferred by the hat truss to the exterior walls due to thermal
weakening of the core; the north and south walls each gained about
10 percent more loads, and the east and west walls each gained
about 25 percent higher loads. Since the hat truss outriggers
to the east and west walls were stiffer than the outriggers to
the north and south walls, they transferred more loads to the east
and west exterior walls.
The inward bowing of the south wall caused failure of exterior
column splices and spandrels, and these columns became unstable.
The instability spread horizontally across the entire south face.
The south wall, now unable to bear its gravity loads, redistributed
these loads to the thermally weakened core through the hat truss
and to the east and west walls through the spandrels. The building
section above the impact zone began tilting to the south as the
columns on the east and west walls rapidly became unable to carry
the increased loads. This further increased the gravity loads
on the core columns. Once the upper building section began
to move downwards, the weakened structure in the impact and fire
zone was not able to absorb the tremendous energy of the falling
building section and global collapse ensued.
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