Environmental Engineering Reference
In-Depth Information
Boeing 767-200, and the structures performed as in-
tended, even though the impact velocity of the hijacked
planes was more than three times higher (262 m/s vs.
80 m/s) than that of a slowly flying plane close to
landing.
As a result, the kinetic energy at impact was about 11
times greater than envisaged in the original design, or
roughly 4.3 GJ vs. 390 MJ. But because each tower had
a mass of more than 2,500 times that of the impacting
aircraft, the enormous concentrated kinetic energy of
the planes acted much like a bullet hitting a massive tree.
It penetrated instead of pushing; it was absorbed by
bending, tearing, and distortion of structural steel and
concrete; and the perimeter tube design redistributed
lost loads to nearby columns. Consequently, it was the
more gradual flux of the ignited fuel rather than an in-
stantaneous massive kinetic insult that weakened the col-
umns of structural steel. (Each 767 airplane carried more
than 50 t of kerosene, whose heat content was more than
2 TJ.)
Unfortunately, the eventuality of such a fire had not
been considered in the original WTC design. Moreover,
no fireproofing systems were available at that time to
control such fires. Once the jet fuel spilled into the build-
ing, it ignited an even larger mass of combustible materi-
als (mainly paper and plastics) inside the structures, and
the fires burned with diffuse flames at low power den-
sities of less than 10 W/cm
2
. The fuel-rich, open-air fire
could not reach the 1,500
C needed to actually melt the
metal. This left enough time for most people to leave the
buildings before the thermally weakened structural steel,
thermal gradients on outside columns, and nonuniform
heating of long floor joists precipitated the staggered
floor collapse that soon reached free-fall speed (hitting
bottom at 200 km/h) as the towers fell in only about
10 seconds (Eagar and Musso 2001).
But massive mobilization of energies is no guarantee
of the outcome. Rapid deployment of U.S. economic
might, energized by a 46% increase in the total use of
fuels and primary electricity between 1939 and 1944,
was clearly instrumental in winning WW II. In contrast,
the Vietnam War showed that in order to win it is not
enough to have advanced weapons and to use an enor-
mous amount of explosives (nearly three times as much
as all bombs dropped by the U.S. Air Force on Germany
and Japan in WW II). And the attacks of September 11,
2001, illustrate the perils of the aptly named asymmetri-
cal threats as enormous damage is inflicted by expending
relatively little energy. Nineteen Islamic terrorists, at the
cost of their lives and an investment of perhaps less than
$100,000, caused about 3,000 virtually instantaneous
deaths as well as direct and indirect economic disloca-
tions that led to costly and open-ended deployments of
military and covert power.
Finally, a few notes on energy resources as a justifica-
tion for war. Japan's attack on the United States in
December 1941 is often cited as a classic case of a coun-
try's going to war to preserve its access to energy re-
sources (R. Stern 2006). In July 1940, President
Roosevelt terminated export licenses for aviation gaso-
line, and the attack on Pearl Harbor was said to clear
the way for Japan's control of Sumatran and Burmese oil
fields. Declining oil supplies undeniably figured in the
decision to attack the United States, but it is indefensible
to depict Japan's aggression as solely an energy-driven
quest. The attack at Pearl Harbor was preceded by nearly
a decade of expansive Japanese militarism, (beginning
with the conquest of Manchuria in 1933 and escalated
