LONG
Perhaps of interest to those into the mechanics
of tower design. By a professor of mechanical/structural
engineering:
"The twin towers were designed by Leslie Robertson, structural
engineer from NYC and his firm during the 1970's. They used
the best practices at that time. It's about 1250 feet high and
used the framed tube system, with columns spaced about 3'-3"
(about 1 m) centers. This exterior column-girder grid forms a
square tube in plan to resist the wind and earthquake loads, in
tandem with the interior core, which was made up of braced frames,
enclosing the elevator-stairwells.
The building was column free, from the core to the exterior tube
frames. The gravity loads were carried by open-web joist trusses
from the core to the exterior frames. The towers also had over
several thousand visco-elastic dampers, at the connections
between the core and the exterior tube, absorbing vibrations
due to lateral wind sway.
The building had a period of oscillation of 11 sec. Thus its natural
frequency is 0.0909 Hertz, and hence it was designed to carry
wind gust loading spectrum centered around this frequency and
its higher harmonic frequencies. Wind gust spectrum has only
small amounts of energy beyond 2 Hertz. Wind tunnel studies
were made by Prof. Alan Davenport at UWO, London, Ontario.
Similar studies were made for earthquake loads.
However, the impact loads caused by the aircrafts, which carried
about 24,000 gallons of aviation fuel, caused a huge fireball, and
its resultant explosions essentially caused all the exterior columns
and the core to buckle under thermal loads, with temperatures well
over 800 degrees centigrade, which no structural steel element
can sustain.
The first tower had the aircraft impact at the level of about
the 67th floor, roughly 0.609H, where H is the height. The
second tower received the second aircraft, a few minutes
later, at about 0.5H, midheight of the tower.
When the columns failed at these levels, the portion of the
building above the impact, essentially enveloped in flames,
acted as a large compression
load on the tower, like a pile driving hammer. The towers then
collapsed under this load, in a progressive failure mode, much
like the Ronan Point Towers in England during the 1960's, due to
an explosion at the upper level. Progressive collapse was
researched since then.
It's a catastrophic event in the lifetime of such tall building
towers. The probability of such events are often very small,
often of the order of one in a million.
The structural integrity of the beam-column framed tube and the
core may be now in some question. Structural engineers are
wondering that we should use more diagonal trusses on the
exterior, like in the John Hancock Center in Chicago, and in
the Sears Tower, which uses a bundled tube concept, with its
nine tubes, braced by external outrigger trusses, located at 75th,
50th and at 30th levels or so, tying the tubes all around with
belt-trusses.
These buildings of periods of about 8 seconds, with a natural
frequency of about 0.125 Hertz. They are stiffer.
The Empire State building withstood an impact from a B-20
bomber or so during the 1940's, on a foggy night and rainstorms.
Essentially, it damaged only one floor. But the plane did not
have much fuel as the passenger liners had, due to their trans-
continental flights. This building is even stiffer than the Sears
Tower, with a period less than 8 seconds.
It's the fire loading and blast which caused the structural
collapse -- not the inadequacy of structural design principles.
We should have had some redundancy in design for such a
catastrophic loading. The diagonal trussed tube provides this
additional stiffness.
Even then, the buildings performed remarkably well. They
withstood the buckling for over an hour or more, while the
second tower succumbed to the vibrations caused by the
first one, in a lesser time span.
The prime responsibility of any structural engineer is to prevent
loss of life in extreme loads of this type.
Even though we design structures such as offshore oil platforms,
nuclear power plants, and missile silos and aircraft carriers for such
explosions, we seldom do these for tall buildings.
Corporations and government agencies, which commission us
to design tall buildings, are not willing to pay for such large loads.
It's unfortunate. We lost thousands of innocent women, children
and men, working and living in these twin towers. They will be in
my thoughts and prayers."
The site whereon I found this material does not identify the school/
University source of these statements. The piece was signed by a
"Prof. Jay".
Fascinating reading, though I do not understand all the framed
and/or bundled tube construction explanations.
73, Jim KH7M
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