key structural responses that led to the collapse of the towers were as follows: 1) floor
sagging caused by the failure of thermally-weakened truss members, resulting in pull-in forces between
the floor and the exterior wall, and in some cases, disconnection of the floor from the exterior wall;
2) downward displacement of the core due to aircraft impact damage and shortening of the remaining core
columns from increased load, plasticity, creep of steel at high temperatures, and buckling resulting from
fire-induced high temperatures, and unloading of the core; 3) bowing and buckling of exterior walls
caused by the pull-in forces and loss of lateral support from the sagged floors, and floor/wall
disconnections at high temperatures; and 4) redistribution of gravity loads among the columns locally,
among the exterior walls, and between the exterior walls and the core, resulting from impact damage,
relative thermal expansion, shortening of core columns, tilting of the tower above the impact zone, and
bowing and buckling of exterior walls.
In WTC 1, aircraft impact caused damage to the north and south walls, floors, some core columns, and
fireproofing. The subsequent fires caused sagging of the floors on the south side of the office area, where
fireproofing was damaged, and inward bowing of the south wall. The damage to the core columns
resulted in local load redistribution to the remaining core columns. The subsequent fire-induced high
temperatures caused the core to displace downward from plasticity and high creep strains in high stress
and high temperatures. The downward displacement of the core resulted in load redistribution from the
core to the exterior walls. With continuously increased bowing, the entire width of the south wall buckled
inward. The section of the building above the impact zone tilted to the south as instability progressed
horizontally to the adjacent east and west walls. Global collapse occurred as potential energy of the
falling upper structure exceeded the strain energy capacity in the deforming structural members.
In WTC 2, aircraft impact caused damage to the south and north exterior walls, floors, and columns in the
southeast corner of the core. The floor damage and the subsequent fires caused sagging of the floors and
local floor/wall disconnections, and resulted in bowing and buckling of the east wall. The damage to the
core columns and fire-induced high temperatures resulted in local load redistribution to the remaining
core columns in the southeast corner, which redistributed the core column loads to the east and south wall
columns, as the core leaned towards south and east. With continuously increased bowing, the entire
width of the east wall buckled inward. The section of the building above the impact zone tilted to the east
and south as instability progressed horizontally to the adjacent north and south walls. Global collapse
occurred when the potential energy of the falling upper structure exceeded the strain energy capacity in
the deforming structural members.
The results of global analysis of both WTC 1 and WTC 2 showed that global collapse of both towers was
initiated by the instability of the exterior walls pursuant to their excessive inward bowing which
progressed horizontally to adjacent walls.
