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I was in study hall when it happened. An announcement came
over the loud-speaker that a plane had hit the World Trade Center. I didn't
even know much about what that building was, except that it was big and
important; I couldn't really grasp what they had said. But shortly after, we
were all sent home from school. I watched the rest of the horrible events
unfold on TV for hours that day.
In tribute to this day of remembrance and those who lost
their lives in the tragedy, I thought it would be fitting and appropriate to
take a look at the engineering achievement behind the Twin Towers.
The Project
The conception of the world trade center is attributed to
David Rockefeller, a grandson of the famous John D. Rockefeller. In the 1950s
and '60s, Rockefeller was looking to bring new life to lower Manhattan through
new construction, and constructing a trade center would no doubt bring about economic
growth. It would also enhance the value of the Chase Manhattan Bank tower,
another one of Rockefeller's projects.
Rockefeller commissioned The Port of New York Authority to
head up the project in 1960; their own Guy Tozzoli managed the entire design
and construction process. Minoru Yamasaki, the team's chief architect, came up
with the twin tower concept and the basic layout for the entire complex, which
consisted of office and hotel space, an exhibit hall, a securities and exchange
center, and shops.
Design and
Construction
The World Trade Center towers, the masterpieces of the
complex, used a unique "tube" design, where all support columns would be
located at the perimeter and the core of the building. Basically, each tower
was a box within a box joined by horizontal trusses on each floor. On each
floor, the outer box was made of 14-inch wide steel columns with aluminum
facing, while the inner box consisted of 47 heavy steel columns which went all
the way down to a spread footing structure beneath the tower basement. Each column
in the spread footing design rested on a cast-iron plate above steel grillage
and a concrete pad. Once in place, this whole bottom structure was drowned in
concrete.


(Images Credit: HowStuffWorks)
This innovative design provided incredible stability. The
inner structure was completely dedicated to supporting the incredible vertical
loads, while the outer structure reinforced all the horizontal loads resulting
from wind. In addition, the tube concept allowed for a more open floor plan,
since all the columns and support was located in the center and edges of the
building.
Constructing the Twin Towers was also a true feat,
considering it was a logistical nightmare. Not only was the engineering
incredibly difficult, but the space for the construction site and materials was
limited. In order to manage this and keep the project moving at a decent pace, the
200,000 tons of steel had to be supplied by just-in-time delivery. The process
moved chunk by chunk from the inside out; inner steel tubing was built to a
certain height first, followed by perimeter wall, the floors, and the anodized
aluminum facing. On April 4, 1973, the WTC complex opened its doors - 13 years
or more after the project began.
September 11
It took about an hour for Tower 2 to collapse after being
struck by an airliner on the morning of September 11. Tower 1 followed only 40
minutes later.

A combined effort by FEMA and SEI/ASCE was coordinated to
investigate what exactly happened and what caused the collapse. The entire
report can be found here.
To sum up, the crashes involved two 395,000 pound Boeing 767s flying about 470
mph (Tower 1) and 590 mph (Tower 2). The impact wrecked floors, damaged columns
at each building's core, and destroyed as many as 36 (Tower 1) and 32 (Tower 2)
vertical columns around each tower's perimeter. In each crash, the jet fuel
ignited and resulted in a massive fireball. This caught much of the office
equipment and building materials on fire, which continued to fuel the flames
until the collapse.
(Image Credit: BBC News -->)
Most skyscrapers would presumably have toppled within
seconds just from the initial crash. But remarkably, the twin towers managed to
hold up against the incredible stress put on the remaining columns of the
building. The FEMA report claims that without the onset of the fire or any
additional loads, the building would have remained intact indefinitely. But the
+2,000°F
fire, which spread across multiple floors and over a large area, was more heat
than the columns and trusses could handle.
When enough steel had been weakened and stressed from the heat, the result
was catastrophic failure. The remaining supports gave way, sending some twenty
stories crashing down on the intact portions below where the planes had hit.
The rest, as they say, is history.

Looking Back
The Twin Towers were certainly built in a spirit of
excellence. They were remarkable technological achievements that showcased the
hard work and driving spirit of this great country. It is in this spirit that
we honor the victims of these attacks and remember the bravery shown by rescue
workers on that day, from the first responders to those involved after the
collapse. It's unconditional sacrifice like this which we should look to,
sacrifice which gives us hope for the future.
References
How
the World Trade Center fell - BBC News
The
World Trade Center - HowStuffWorks
World
Trade Center Disaster - Engineering.com
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