The opinion of many engineers
is that the architects that designed the New
York World Trade Center Twin Towers did a good
job in designing the buildings. The buildings
were safe from the hazards envisioned at the
time. Indeed, they were designed to withstand
the force of aircraft that might crash into
them. They were not however, designed to withstand
the effects of the very high temperatures generated
by the ignition of tons of aviation fuel carried
by the aircraft.
Steel
lightly covered with concrete is the main structural
component of the towers and steel loses a lot of structural
strength when heated to 1100 degrees Centigrade and
beyond, as it was. To shield the steel the towers beams
were coated with a few centimeters of concrete. That
insulation was to provide structural integrity for
at least 1 hour of fire. The towers met that test.
The south tower lasted 62 minutes. The north; 103 minutes.
Tragedy followed when not all could escape prior to
the collapse.
The towers were
built using a “tube within
a tube” design. The inner “tube” is
formed by steel box columns that bear most of the
buildings weight. The outer tube is constructed
of 61 load bearing steel columns on each face.
The two tubes are joined by networks of truss beams
that brace each floor. The result provides the
strength to resist twisting and turning from wind
forces. Most buildings greater than about 70 stories
require such design since the force of wind can
more than equal the force of gravity.
Most experts agree
that the Twin Towers were very well designed.
Many also agree that the people
planning the attacks were very knowledgeable and
aimed the aircraft at a height that would effect
the most damage. The aircraft crash and the resulting
fuel fed fires were aimed at about the 90th floor.
That’s just high enough to insure that the
upper floors would weaken with the heat and “pancake” dropping
into the remaining part of the building.
The mass of the falling top floors created such
a falling load that some experts estimate it exceeded
by 5 to 10 times normal bearing loads. The effect
was like a giant hammer, each floor contributing
more weight to be born by the next floor down.
Were the Trade
Towers well designed? Engineers say yes … but. The “but” refers
to the belief that architects will never again
design a tall building without considering the
effects of terrorist attacks of all potential kinds.
That not only includes the ability of the structure
to withstand attack but the means to allow the
buildings occupants to escape in time to survive.
Architects are already considering passive firebreaks
between floors and vertical spaces that contain
no flammable materials. Lightweight materials could
fill cavities that would absorb the shock of crashes
or explosions. New ceramic or cement-based materials
could better insulate walls and steel columns allowing
them to retain their structural integrity at high
temperatures far longer than at present.
Others are considering
stand-alone structures within or just outside
the tall buildings used
as escape routes. For example the world’s
tallest skyscrapers, Petronas Towers in Malaysia
have stairways inside the building’s pressurized
core that its architects refer to as “a building
within a building”.
The World Trade Towers were far more than well
designed tall buildings. They were noble. They
were awe-inspiring. They were New York. Although
now a Southern Californian, I am still a New Yorker.
I was born there, grew up there, and the bonds
are deep and strong. One memory of the Twin Towers
will not fade. I was having brunch with friends
at the Windows On the World restaurant. It was
a beautiful but slightly gray day with lots of
clouds. Sitting at our table we could see the world
straight out through the glass and the top of the
clouds floating by right below.
No one will bother to ask my opinion about rebuilding
the beautiful Twin Towers so I will state it here.
Do it. Do it soon. Make it bigger, taller, and
more elegant than before. I believe that would
be the monument most of those who died there would
wish for. And the message from New Yorkers to any
one wishing us harm will be clear.
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