Vous êtes sur la page 1sur 4

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

Why Did It Collapse?

Tim Wilkinson, Lecturer in Civil Engineering

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 structures 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 behavior of
the remaining material. As the planes had only
recently taken off, the fire would have been initially
fueled by large volumes of jet fuel, which then

ignited any combustible material in the building.

While the fire would not have been hot enough to
melt any of the steel, 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 defenses considerably faster than expected. It is likely that the water
pipes that supplied the fire sprinklers were severed by the plane impact, and much of the fire
protective material, designed to stop the steel from being heated and losing strength, was blown off by
the blast at impact.

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. While the columns at say level 50
were designed to carry the static load of 50
floors above, once one floor collapsed and the
floors above started to fall, the dynamic load of
50 storeys above is very much greater, and the
columns at each level were almost instantly
destroyed as the huge upper mass fell to the
When this article was first written on 9/11, the only evidence Was 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.
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

While the ways the two towers

fell were slightly different, the
basic cause is similar for both - a
large number of columns were
destroyed on impact, and the
remaining structure was gradually
weakened by the heat of the fire.
Not much significance should be
taken from the fact that one tower
fell in 45 minutes and the other in
90 minutes.

The gigantic dynamic impact

forces caused by the huge mass of
the falling structure landing on
the floors below is very much
greater than the static load they
were designed to resist.