1,368 and 1,362 feet (417 and 415
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
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
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)
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
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),
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
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
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
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