World Trade Centre - New York - Some Engineering Aspects
General | Structural
System | Why Did It Collapse | Other
Links
General Information:
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
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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.
(Taken from www.skyscraper.org)
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."
Taken from www.greatbuildings.com
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.
?
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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)
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 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.
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 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.
(US readers note:? storey
is the Australian/English spelling of story)
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Sydney
Morning Herald graphic
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)
?
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Other Links
TenLinks
(http://www.tenlinks.com/NEWS/special/wtc/index.htm)
and ICivilEngineer
(http://www.icivilengineer.com/News/wtc.php)
have a wide selection of articles collected from the world on engineering
aspects of the WTC.
The best and most complate source of information on the towers is the official FEMA report, given at
http://www.fema.gov/library/wtcstudy.shtm
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