Height:
1,368 and 1,362 feet (417
and 415 meters)
Owners: Port Authority of New
York and New Jersey.
(99 year leased signed in
April 2001 to groups including Westfield
America and Silverstein Properties)
Architect: Minoru Yamasaki, Emery
Roth and Sons consulting
Engineer: John Skilling and
Leslie Robertson of Worthington,
Skilling, Helle and Jackson
Ground Breaking: August 5, 1966
Opened: 1970-73; April 4, 1973
ribbon cutting
Destroyed: Terrorist
attack, September 11, 2001 |
The
Structural System:
Yamasaki
and engineers John Skilling and Les Robertson
worked closely, and the relationship between the
towers’ design and structure is clear. Faced
with the difficulties of building to unprecedented
heights, the engineers employed an innovative
structural model: a rigid "hollow tube"
of closely spaced steel columns with floor trusses
extending across to a central core. The columns,
finished with a silver-colored aluminum alloy,
were 18 3/4" wide and set only 22"
apart, making the towers appear from afar to have
no windows at all.
Also unique to the engineering design were its
core and elevator system. The twin towers were the
first supertall buildings designed without any
masonry. Worried that the intense air pressure
created by the buildings’ high speed elevators
might buckle conventional shafts, engineers
designed a solution using a drywall system fixed
to the reinforced steel core. For the elevators,
to serve 110 stories with a traditional
configuration would have required half the area of
the lower stories be used for shaftways. Otis
Elevators developed an express and local system,
whereby passengers would change at "sky
lobbies" on the 44th and 78th floors, halving
the number of shaftways.
The
structural system, deriving from the I.B.M.
Building in Seattle, is impressively simple. The
208-foot wide facade is, in effect, a
prefabricated steel lattice, with columns on
39-inch centers acting as wind bracing to resist
all overturning forces; the central core takes
only the gravity loads of the building. A very
light, economical structure results by keeping the
wind bracing in the most efficient place, the
outside surface of the building, thus not
transferring the forces through the floor membrane
to the core, as in most curtain-wall structures.
Office spaces will have no interior columns. In
the upper floors there is as much as 40,000 square
feet of office space per floor. The floor
construction is of prefabricated trussed steel,
only 33 inches in depth, that spans the full 60
feet to the core, and also acts as a diaphragm to
stiffen the outside wall against lateral buckling
forces from wind-load pressures."
Typical
Floor Plan of the World Trade Center:
A
perimeter of closely spaced columns, with an
internal lift core. The floors were
supported by a series of light trusses on rubber
pads, which spanned between the outer columns and
the lift core.
Why Did
It Collapse?
Tim
Wilkinson, Lecturer in Civil Engineering
(This
is an initial suggestion on one possible reason
for failure, and should not be regarded as
official advice)
|
 The
structural integrity of the World Trade
Center depends on the closely spaced
columns around the perimeter.
Lightweight steel trusses span between
the central elevator core and the
perimeter columns on each floor.
These trusses support the concrete slab
of each floor and tie the perimeter
columns to the core, preventing the
columns from buckling outwards.
After
the initial plane impacts, it appeared
to most observers that the structure had
been severely damaged, but not
necessarily fatally.
It
appears likely that the impact of the
plane crash destroyed a significant
number of perimeter columns on several
floors of the building, severely
weakening the entire system.
Initially this was not enough to cause
collapse.
However,
as fire raged in the upper floors, the
heat would have been gradually affecting
the behaviour of the remaining material.
As the planes had only recently taken
off, the fire would have been initially
fuelled by large volumes of jet fuel,
creating potentially enormously high
temperatures. The strength of the steel
drops markedly with prolonged exposure
to fire, while the elastic modulus of
the steel reduces (stiffness drops),
increasing deflections.
|
|
 Modern
structures are designed to resist fire
for a specific length of time.
Safety features such as fire retarding
materials and sprinkler systems help to
contain fires, help extinguish flames,
or prevent steel from being exposed to
excessively high temperatures.
This gives occupants time to escape and
allow fire fighters to extinguish
blazes, before the building is
catastrophically damaged.
It
is possible that the blaze, started by
jet fuel and then engulfing the contents
of the offices, in a highly confined
area, generated fire conditions
significantly more severe than those
anticipated in a typical office fire.
These conditions may have overcome the
building's fire defences considerably
faster than expected.
Eventually,
the loss of strength and stiffness of
the materials resulting from the fire,
combined with the initial impact damage,
would have caused a failure of the
truss system supporting a floor, or the
remaining perimeter columns, or even the
internal core, or some combination.
Failure of the flooring system would
have subsequently allowed the perimeter
columns to buckle outwards.
Regardless of which of these
possibilities actually occurred, it
would have resulted in the complete
collapse of at least one complete storey
at the level of impact.
Once
one storey collapsed all floors above
would have begun to fall. The huge
mass of falling structure would gain
momentum, crushing the structurally
intact floors below, resulting in
catastrophic failure of the entire
structure. |
The
only evidence so far are photographs and television
footage. Whether failure was initiated at the
perimeter columns or the core is unknown. The
extent to which the internal parts were damaged
during the collision may be evident in the rubble if
any forensic investigation is conducted.
Since the mass of the combined towers is close to
1000000 tons, finding evidence will be an enormous
task.
Perimeter
columns, several storeys high, and still linked
together, lie amongst all the debris on the ground.
This
photograph shows the south tower just as it is
collapsing. It is evident that the building is
falling over to the left. The North Tower
collapsed directly downwards, on top of itself.
The same mechanism of failure, the combination of
impact and subsequent fire damage, is the likely
cause of failure of both towers. However, it
is possible that a storey on only one side of the
South Tower initially collapsed, resulting in the
"skewed" failure of the entire tower.
The
gigantic impact forces caused by the huge mass of
the falling structure landing on the floors below
travelled down the columns like a shockwave faster
than the entire structure fell. The clouds of
debris coming from the tower, several storeys below
the huge falling mass, probably result from the
sudden and almost explosive failure of each floor,
caused by the "shockwave".
(Pictures
taken from various news sources on the Internet)
|
