HaCKeRz Kr0nlcKLeZ 1/HaCKeRz Kr0nlcKLeZ.iso/anarchy/essays/term/skyscrap.txt

Text File | 1996-04-27 | 12.8 KB | 248 lines

=======================================================================
Tall Stories                                                 NEWSCIENCE
-----------------------------------------------------------------------

Picture in your mind the skyline of downtown Toronto. There's the CN
Tower, of course, and the 72-floor First Canadian Place, the city's
tallest skyscraper. Cascading from there are the assorted banks and
hotels and insurance towers.

    Now, use your imagination to construct some new buildings, these
ones reaching three, four and five times higher than the others. Top it
all off with a skyscraper one mile high (three times as high as the CN
Tower).

      Sound fanciful? It did 30 years ago when Frank Lloyd Wright
proposed the first mile-high building.

     But not today. We are now said to be entering the age of the
superskyscraper, with tall buildings poised to take a giant new leap
into the sky.  Skyscrapers approaching the mile mark may still be
awhile off, but there are proposals now for megastructures soaring 900
m -- twice as high as the world's tallest building, the 110-story Sears
Tower in Chicago.

      Suppose that you were asked to erect such a building. How would
you do it? What are the obstacles you'd face? What materials would you
use? And where would you put it?

    Building a superskyscraper, the first thing you would need is a
considerable slice of real estate. Tall buildings require a large base
to support their load and keep them stable. In general, the height of a
building should be six times its base, so, for a skyscraper 900-m tall,
you'd need a base of 150 square m.

     That much space is hard to come by in, say, downtown Toronto,
forcing you to look for an undeveloped area, perhaps the Don Valley
ravine, next to the Science Centre. Bear in mind though that the Don
Valley is overlain by loose sand and silt, and tall buildings must
stand on firm ground, or else risk the fate of edifices like the
Empress Hotel in Victoria. This grand dowager, completed in 1908, long
before the science of soil mechanics, has since found herself slowly
sinking into the soft clay.

     Soil analysis is especially critical in facing the threat of
earthquakes. The Japanese have learned many times the hard way what
happens when an earth tremor shakes a high-rise constructed on soft,
wet sand. The quake's enormous energy severs the loose connections
between the individual grains, turning the ground into quicksand in
just seconds and swallowing up the building. .

      Engineers have actually built machines that condense loose
ground.  One machine pounds the earth with huge hammers. Another
plunges a large vibrating probe into the ground, like a blender in a
milk shake, stirring up the sand so that its structure collapses and
the individuals grains fall closer together.

    Anchoring a skyscraper in the Don Valley would best be solved by
driving long steel piles down through the sand and silt into the
underlying hard clay till. Or, if the clay till lies too far
underground, inserting more piles into the sand. The friction between
sand and so much steel would then be sufficient 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 cut costs is with high-strength concretes. Ordinary concrete
is much cheaper than steel, but lacks steel's rigidity, and could not
withstand the huge burdens in a superskyscraper.  But recent
experiments with chemical additives, called superplasticizers, have
whipped up double and triple-strength concretes that could make
superskyscrapers an economic reality.

     Once you've built your superskyscraper, there still remains the
job of servicing it -- providing water, electricity, fire protection,
ventilation and cooling. Servicing also means controlling stack effect.
If you've ever been up in a skyscraper and heard the wind moaning and
whistling by the elevator -- that's stack effect. In any tall building,
the difference in temperature and air pressure between the outside and
inside the structure pushes air up the stairwells and elevators, like
smoke up a chimney. Strong, cold  drafts blowing up the building create
heating problems and make it difficult to open doors into stairwells.
To control stack effect, buildings must be as airtight as possible,
with ventilation ducts extending only part way up the building, and
revolving doors at ground level.

     The one invention that, above all, has enabled buildings to climb
higher is the elevator. As skyscraper populations have grown, elevator
manufacturers have handled larger loads with double-decker  cars -- one
car piggybacking another, with each one stopping at alternative floors.
Another innovation is the sky lobby system, in which passengers take
one car to a floor part way up the building, and then transfer the next
flight up to another car in the same elevator shaft for the rest of the
journey.

     Elevators will probably never move any faster than they do today,
since the human ear can only endure a descent speed of 600 m per
minute. So, an elevator ride in a superskyscraper might be comparable
to a subway trip, with several transfer points and extended waits
between cars.

    Which brings designers to the inevitable question: Will office
staffs and tenants stand for such long rides? Indeed, will they
tolerate all the other shortcomings of skyscrapers -- the feelings of
entrapment inside them, the dark, windy canyons between them, and the
congested traffic below -- made worse by higher heights.

     Developers now claim they've worked most design bugs out of the
new megastructures Whether or not people will actually want to occupy
them should prove if the sky is really the limit.

    Don Valley -- loose deposits of sand and silt overlying deep
deposits of cllay. Soft deposits. -- or is sand cover on top of clay.
terms: loose sand, loose silt, soft clay. Increase surface area of
piles.

    Perhaps the most critical servicing job is protecting the
building's occupants from fire and smoke. Today's skyscrapers are
equipped with ultra-sophistated  fire-control systems: automatic
sprinklers help douse the fire while exhaust fans suck out the smoke
from burning areas, preventing it from escaping into other floors and
stairwells.

    Feeding the sprinkler systems are huge water storage tanks that sit
on the top floor or roof. These are the same tanks that Paul Newman
blew up to douse the rampaging fire in "The Towering Inferno".
Exploding tanks undoubtedly made for exciting climax, but they could
never contain that much water to put out a skyscraper fire.

Built in the early Seventies by I.M. Pei,  one of America's foremost
architects, the "John Hancock" towers majestically over the Back Bay
area of Boston.

Over time,  it developed the bad habit of letting its windows fall out
on windy days. This problem grew so serious, that police had to cordon
off  the leeward side of the skyscraper to keep unsuspecting
pedestrians from getting beaned  by falling glass. In fact, the
situation became so dangerous that doormen were escorting workers in
and out of the building during the daily invasion and exodus, keeping a
wet  finger  to the wind and an eye peeled for falling glass.

And what was the foundation of this perplexing and disturbing
window-popping habit? As it turned out, the foundation was to blame; it
and what is known as Bernoulli's Principle, ( which states that the
pressure of a gas falls as its velocity increases.)

What happens is this:  a light wind comes along and has to get around a
large slab of  building. It pushes against the front of the tower, and
then speeds up to get to the edges of the building so it can keep up
with the rest of the wind, (this is why the areas around tall buildings
and groups of tall buildings become very windy). The back side of the
skyscraper,  because of all the fast air on its sides, develops an area
of low pressure, as predicted by Bernoulli's Principle, and because the
air pressure inside the wall is suddenly higher than that outside,
there is the potential for windows blowing out

This is obviously what was happening to Mr. Pei's building; but why was
it happening with such frequency? After all, this building was becoming
a lethal weapon! The search for the solution would have to start from
the ground up, and the design team began with the history of the
site...

As is the case with many cities built beside a body of water, Boston's
downtown area expanded rapidly during the last century,  and its bay
was filled in to provide more building space.  Because this land was
built on more or less right away, it didn't have the chance to compact
and provide as much support as land that had been settling for
thousands of years.

The design of the "John Hancock" took into consideration the condition
of the soil on which it was built, and the engineers did their best to
allow for settling. What they couldn't accurately predict was how the
building would settle, so they  planned for a uniform settling of the
building. Instead, they found that  the building had settled unevenly!

The result of this settling caused an unequal surface tension on the
curtain wall, which, as all curtain walls are, had been designed only
to serve as an envelope for the building, and to support no weight
other than its own. This meant that it was nearing its maximum strength
limit  even without any wind blowing on it. The suction of the low
pressure area on  the leeward side of the  building caused the  wall to
billow out and pop windows like buttons.

The mechanical engineers, realizing that the negative air pressure was
too much for the wall, decided to fight that negative pressure with
negative air pressure of their own.  Using the fact that all
skyscrapers are completely sealed, the perimeter air supply system of
the whole building was monitored with regards to the exterior air
pressure, and then air was supplied or removed to balance the tension
on the curtain wall.  Quite literally, they would make the building
suck in its billowing stomach to keep from popping buttons.

Simple, huh?

This tale ends with a moral and with a warning: the moral of the story
is to  look up when you're around  tall buildings on very windy days ;
the warning (for local folks) is that all the land south of Front
Street is infill!