The authors attribute each tower’s collapse to three separate
but related "loading events". 61) The first event was a Boeing aircraft
hitting the building, cutting through the exterior structure and creating a
fireball that immediately consumed some of the estimated 10.000 gallons (38
kiloliters) of jet fuel. The high-rises structural systems were sufficiently
redundant. However, that this major damage by itself did not cause the collapse.
According to the report, "most of the load supported by the failed columns is
believed to have been transferred to adjacent perimeter columns through
Vierendeel behavior of the exterior wall frame". 62) The
second event was the continuing tire. fed both by the remaining jet fuel and the
office contents of furniture and paper. 63) This fire heated and weakened
the structural systems. adding stress to the damaged structure. Meanwhile,
the sprinklers were not operating as designed, "Even if these systems had not
been compromised by the impacts," says the report, "they would likely have been
ineffective... 64) the initial flash fires of jet fuel would have opened so
many sprinkler heads that the systems would have quickly depressurized and been
unable to effectively deliver water to the large area of fire
involvement." The third event was a progressive collapse:
65) "As the large mass of the collapsing floors above accelerated and
impacted the floors below, it caused an immediate progressive series of floor
failures, punching each in turn onto the floor below, accelerating as the
sequence progressed. Freestanding exterior walls.., buckled at the bolted
column splice connections and also collapsed."