Link to Index Page

ANSYS model of inward bowing of the south face of WTC 1 at 6000 seconds after initial impact: Floors 101 - 93

The image and text are from:
http://www.nist.gov/el/disasterstudies/wtc/upload/Media_Public_Briefing_040505_final.pdf
http://fire.nist.gov/bfrlpubs/fire05/art119.html
Public briefing by Sivaraj Shyam-Sunder of NIST

From NIST: Collapse Sequence for WTC 1
1. Aircraft Impact Damage:
a. Aircraft impact severed a number of exterior columns on the North wall from floors 93 to 98, and the wall section above the impact zone moved downward.
b. After breaching the building’s perimeter, the aircraft continued to penetrate into the building, severing floor framing and core columns at the North side of the core. Core columns were also damaged toward the center of the core and, to a limited extent on the South side of the core. Fireproofing was damaged from the impact area to the South perimeter wall, primarily through the center of WTC 1 and at least over a third to a half of the core width.
c. Aircraft impact severed a single exterior panel at the center of the South wall between floors 94 and 96.
d. The impact damage to the exterior walls and to the core resulted in redistribution of severed column loads, mostly to the columns adjacent to the impact zones. The hat truss resisted the downward movement of the North wall, and rotated about the East-West axis.
e. As a result of the aircraft impact damage, the North and South walls each carried about 7 percent less gravity loads after impact, and the East and West walls each carried about 7 percent more loads. The core carried about 1 percent more gravity loads after impact.

2. Effects of Subsequent Fires and Impact Damaged Fireproofing:
A. Thermal Weakening of the Core:
a. The undamaged core columns developed high plastic and creep strains over the duration the
building stood, since both temperatures and stresses were high in the core area. The plastic
and creep strains exceeded thermal expansion in the core columns.
b. The shortening of the core columns (due to plasticity and creep) was resisted by the hat truss which unloaded the core over time and redistributed loads to perimeter walls.
c. As a result of the thermal weakening (and subsequent to impact and prior to inward bowing of the South wall), the North and South walls each carried about 10 percent more gravity loads, and the East and West walls each carried about 25 percent more loads. The core carried about 20 percent less gravity loads after thermal weakening.

B. Thermal Weakening of the Floors:
a. Floors 95 to 99 weakened with increasing temperatures over time on the long-span floors and sagged. The floors sagged first and then contracted due to cooling on the North side; fires reached the South side later, the floors sagged, and the seat connections weakened.
b. Floor sagging induced inward pull forces on the South wall columns. 
c. About 20 percent of the connections to the South perimeter wall on floors 97 and 98 failed due to thermal weakening of the vertical supports.

C. Thermal Weakening of the South Wall:
a. South wall columns bowed inward as they were subjected to high temperatures and inward
pull forces in addition to axial loads.
b. Inward bowing of the South wall columns increased with time.

3. Collapse Initiation
a. The inward bowing of the South wall induced column instability, which progressed rapidly horizontally across the entire South face.
b. The South wall unloaded and tried to redistribute the loads via the hat truss to the thermally weakened core and via the spandrels to the adjacent East and West walls.
c. The entire section of the building above the impact zone began tilting as a rigid block (all four faces; not only the bowed and buckled South face) to the South (at least about 8o) as column instability progressed rapidly from the South wall along the adjacent East and West walls.
d. The change in potential energy due to downward movement of building mass above the buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse then ensued.

Page 68 / 114