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Canon Powershot Camera Answering Machines
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Canon Powershot Camera Answering Machines canon powershot camera
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Canon Powershot Camera Answering Machines canon powershot camera Information
to hold the concrete foundation above in place. The next
obstacle in
erecting a superskyscraper, and perhaps the biggest
one, is wind. Tall
buildings actually sway in the breeze, in much the
same way that a diving
board bends under the weight of a diver.
Building an edifice that doesn't
topple over in the wind is easy
enough. The real challenge is keeping the
structure so stiff that it
doesn't swing too far, cracking partitions,
shattering windows and
making the upper occupants seasick. As a rule, the
top of
skyscraper should never drift more than 1/400 of its height at a
wind velocity of 150 km/h. Older buildings, like the Empire State
Building, were built so that their core withstood all bending
stresses.
But structural engineers have since found that by shifting
the bracing and
support to the perimeter of a building, it can better
resist high winds. The
most advanced buildings are constructed like
a hollow tube, with thin, outer
columns spaced tightly together and
welded to broad horizontal beams.
Toronto's First Canadian Place
and New York's World Trade Center towers are
all giant, framed
tubes. A superskyscraper would undoubtedly need extra
rigidity,
which you could add by bracing its framework with giant diagonal
beams. You'll see this at Chicago's John Hancock Center where
the
architect has incorporated diagonal braces right into the look of
the
building, exposing five huge X's on each side to public view.
Alternatively,
you might design your building like a broadcasting
tower, and tie it to the
ground with heavy, sloping guy wires
extending from the four corners of the
roof to the ground. A
control mechanism at the end of each cable would act
like a fishing
reel, drawing in the cable whenever the sway of the building
caused it to slacken. Tall buildings also encounter the problem of
vortex shedding, a phenomenon that occurs as the wind swirls
around the
front corners of the building, forming a series of eddies
or vortices. At
certain wind speeds, these vortices vibrate the
building, threatening to
shake it apart. In New York City's Citicorp
Center, engineers have tackled
vortex shedding with a 400-tonne
concrete block that slides around in a
special room on one of the
upper stories. Connected to a large spring and a
shock absorber,
and riding on a thin slick of oil, the big block responds to
oscillations
of the building by moving in the opposite direction. Other ways
to
disrupt vortex shedding include making several large portals in the
upper part of the tower, through which the wind passes freely. In
New
York City's World Trade Center, vibrations are dampened
with special
spongelike pads sandwiched in its structure. The price
tag on a
superskyscraper is going to be enormous, but one way to
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