When the twin
towers of the World Trade Center were struck on
September 11, 2001, it seemed at first that they might
remain standing. But in less than two hours, both towers
had collapsed to the ground. Click
here to find out how the remarkable structural
support system in these buildings eventually gave
way.
Throughout the
history of architecture, there has been a continual quest for
height. Thousands of workers toiled on the pyramids of ancient
Egypt, the cathedrals of Europe and countless other towers,
all striving to create something awe-inspiring. People build
skyscrapers primarily because they are convenient -- you can
create a lot of real estate out of a relatively small ground
area. But ego and grandeur do sometimes play a significant
role in the scope of the construction, just as it did in
earlier civilizations.
Photo courtesy Wayne Lorentz: Glass, Steel
& Stone The two
towers in New York's World Trade Center stood 1,360-feet
(415-meters) tall, with a massive steel truss at their
core.
Up until relatively recently, we could only go so high.
After a certain point, it just wasn't feasible to keep
building up. In the late 1800s, new technology redefined these
limits. Suddenly, it was possible to live and work in colossal
towers, hundreds of feet above the ground.
In this edition of HowStuffWorks,
we'll look at the innovations that made these incredible
structures possible. We'll examine the main architectural
issues involved in keeping skyscrapers up, as well as the
design issues involved in making them practical. Finally,
we'll peer into the future of skyscrapers to find out how high
we might go.
Fighting Gravity The main obstacle in building upward is the downward pull
of gravity. Imagine carrying a friend on your
shoulders. If the person is fairly light, you can support them
pretty well by yourself. But if you were to put another person
on your friend's shoulders (build your tower higher), the
weight would probably be too much for you to carry alone. To
make a tower that is "multiple-people high," you need more
people on the bottom to support the weight of everybody above.
This is how "cheerleader pyramids" work, and it's also how
real pyramids and other stone buildings work. There has to be
more material at the bottom to support the combined weight of
all the material above. Every time you add a new vertical
layer, the total force on every point below that layer
increases. If you kept increasing the base of a pyramid, you
could build it up indefinitely. This becomes infeasible very
quickly, of course, since the bottom base takes up too much
available land.
The Empire State Building in New York City.
The view from the building's 86th-floor
observatory is one of New York City's top tourist
attractions.
In normal buildings made of bricks and mortar, you
have to keep thickening the lower walls as you build new upper
floors. After you reach a certain height, this is highly
impractical. If there's almost no room on the lower floors,
what's the point in making a tall building?
Using this technology, people didn't construct many
buildings more than 10 stories -- it just wasn't feasible. But
in the late 1800s, a number of advancements and circumstances
converged, and engineers were able to break the upper limit --
and then some. The social circumstances that led to
skyscrapers were the growing metropolitan American centers,
most notably Chicago. Businesses all wanted their offices near
the center of town, but there wasn't enough space. In these
cities, architects needed a way to expand the metropolis
upward, rather than outward.
The main technological advancement that made skyscrapers
possible was the development of mass iron and steel
production (see How Iron and
Steel Work for details). New manufacturing processes made
it possible to produce long beams of solid iron. Essentially,
this gave architects a whole new set of building blocks to
work with. Narrow, relatively lightweight metal beams could
support much more weight than the solid brick walls in older
buildings, while taking up a fraction of the space. With the
advent of the Bessemer process, the first efficient
method for mass steel production, architects moved away from
iron. Steel, which is even lighter and stronger than iron,
made it possible to build even taller buildings.
Giant Girder Grids The central support
structure of a skyscraper is its steel skeleton. Metal
beams are riveted end to end to form vertical columns.
At each floor level, these vertical columns are connected to
horizontal girder beams. Many buildings also have
diagonal beams running between the girders, for extra
structural support.
Click "Build" to see how the parts of a skyscraper
work together.
In this giant three-dimensional grid -- called the super
structure -- all the weight in the building gets
transferred directly to the vertical columns. This
concentrates the downward force caused by gravity into the
relatively small areas where the columns rest at the
building's base. This concentrated force is then spread out in
the substructure under the building.
In a typical skyscraper substructure, each vertical column
sits on a spread footing. The column rests directly on
a cast-iron plate, which sits on top of a
grillage. The grillage is basically a stack of
horizontal steel beams, lined side-by-side in two or more
layers (see diagram, below). The grillage rests on a thick
concrete pad poured directly onto the hard clay under the
ground. Once the steel is in place, the entire structure is
covered with concrete.
The pieces of a skyscraper's spread
footing
This structure expands out lower in the ground, the same
way a pyramid expands out as you go down. This distributes the
concentrated weight from the columns over a wide surface.
Ultimately, the entire weight of the building rests directly
on the hard clay material under the earth. In very heavy
buildings, the base of the spread footings rest on massive
concrete piers that extend all the way down to the earth's
bedrock layer.
One major advantage of the steel skeleton structure is that
the outer walls -- called the curtain wall -- need only
to support their own weight. This lets architects open the
building up as much as they want, in stark contrast to the
thick walls in traditional building construction. In many
skyscrapers, especially ones built in the 1950s and '60s, the
curtain walls are made almost entirely of glass, giving the
occupants a spectacular view of their city.
Making it Functional In the last section, we
saw that new iron and steel manufacturing processes opened up
the possibility of towering buildings. But this is only half
the picture. Before high-rise skyscrapers could become a
reality, engineers had to make them practical.
The Empire State Building's 73 elevators can
move 600 to 1,400 feet (183 to 427 meters) per minute.
At the maximum speed, you can travel from the lobby to
the 80th floor in 45
seconds.
Once you get more than five or six floors, stairs become a
fairly inconvenient technology. Skyscrapers would never have
worked without the coincident emergence of elevator
technology. Ever since the first passenger elevator was
installed in New York's Haughwout Department Store in 1857,
elevator shafts have been a major part of skyscraper design.
In most skyscrapers, the elevator shafts make up the
building's central core.
Figuring out the elevator structure is a balancing
act of sorts. As you add more floors to a building, you
increase the building's occupancy. When you have more people,
you obviously need more elevators or the lobby will fill up
with people waiting in line. But elevator shafts take up a lot
of room, so you lose floor space for every elevator you add.
To make more room for people, you have to add more floors.
Deciding on the right number of floors and elevators is one of
the most important parts of designing a building.
Building safety is also a major consideration in
design. Skyscrapers wouldn't have worked so well without the
advent of new fire-resistant building materials in the 1800s.
These days, skyscrapers are also outfitted with sophisticated
sprinkler equipment that puts out most fires before they
spread very far. This is extremely important when you have
hundreds of people living and working thousands of feet above
a safe exit.
Architects also pay careful attention to the comfort of
the building's occupants. The Empire
State Building, for example, was designed so its occupants
would always be within 30 feet (ft) of a window. The
Commerzbank building in Frankfurt, Germany has tranquil indoor
garden areas built opposite the building's office areas, in a
climbing spiral structure. A building is only successful when
the architects have focused not only on structural stability,
but also usability and occupant satisfaction.
Wind Resistance In addition to the vertical
force of gravity, skyscrapers also have to deal with the
horizontal force of wind. Most skyscrapers can easily
move several feet in either direction, like a swaying tree,
without damaging their structural integrity. The main problem
with this horizontal movement is how it affects the people
inside. If the building moves a substantial horizontal
distance, the occupants will definitely feel it.
The most basic method for controlling horizontal sway is to
simply tighten up the structure. At the point where the
horizontal girders attach to the vertical column, the
construction crew bolts and welds them on the top and bottom,
as well as the side. This makes the entire steel super
structure move more as one unit, like a pole, as opposed to a
flexible skeleton.
The Chrysler Building in New York
City.
For taller skyscrapers, tighter connections don't really do
the trick. To keep these buildings from swaying heavily,
engineers have to construct especially strong cores through
the center of the building. In the Empire
State Building, the Chrysler Building and other
skyscrapers from that era, the area around the central
elevator shafts is fortified by a sturdy steel truss, braced
with diagonal beams. Most recent buildings have one or more
concrete cores built into the center of the building.
Making buildings more rigid also braces them against
earthquake damage. Basically, the entire building moves with
the horizontal vibrations of the earth, so the steel skeleton
isn't twisted and strained. While this helps protect the
structure of the skyscraper, it can be pretty rough on the
occupants, and it can also cause a lot of damage to loose
furniture and equipment. Several companies are developing new
technology that will counteract the horizontal movement to
dampen the force of vibration. To learn more about these
systems, check out How
Smart Structures Will Work.
Some buildings already use advanced wind-compensating
dampers. The Citicorp Center in New York, for example, uses a
tuned mass damper. In this complex system, oil hydraulic
systems push a 400-ton concrete weight back and forth on
one of the top floors, shifting the weight of the entire
building from side to side. A sophisticated computer system
carefully monitors how the wind is shifting the building and
moves the weight accordingly. Some similar systems shift the
building's weight based on the movement of giant pendulums.
Vertical Variations As we've seen in the
previous sections, skyscrapers come in all shapes and sizes.
The steel skeleton concept makes for an extremely flexible
structure. The columns and girders are something like giant
pieces in an erector set. The only real limit is the
imagination of the architects and engineers who put the pieces
together.
Photo courtesy Wayne Lorentz: Glass, Steel
& Stone The
distinctive chrome-nickel-steel crown of the 1,046-foot
(319-meter) Chrysler Building is a classic example of
art deco
architecture.
The earliest skyscrapers, built in the late 1800s, were
very basic boxes with simple stone and glass curtain walls. To
the architects who built these skyscrapers, the extreme height
was impressive enough. In the period around 1900, the
aesthetic began to change. Buildings got taller, and
architects added more extravagant gothic elements, hiding the
boxy steel structure underneath.
The art deco movement of the 1920s, '30s and '40s
extended this approach, creating buildings that stood as true
works of art. Some of the most famous skyscrapers, including
the Empire
State Building and the Chrysler Building (above), came out
of this era. Things shifted again in the 1950s, when
international style began to take hold. Like the
earliest skyscrapers, these buildings had little or no
ornamentation. They were made mostly with glass, steel and
concrete.
Photo courtesy Wayne Lorentz: Glass, Steel
& Stone The 738-foot
(225-meter) Chase Tower in Dallas is a good example of
the innovative design of the
1980s.
Since the 1960s, many architects have taken the skyscraper
to new and unexpected places. One of the most interesting
variations has been the combination of several vertical
skeleton sections -- or tubes -- into one building. The
Sears Tower in Chicago, the most famous example of this
approach, consists of nine aligned tubes that reach to
different heights. This gives the building an interesting
staggered appearance.
The Tallest
Tower
Ever since the first
towering skyscrapers at the end of the 1800s, cities and
corporations have been competing to build the world's
tallest. Right now, there is some debate over who holds
the record. Not everybody agrees on which structures
should be considered. Traditionally, the architectural
community defines a building as an enclosed structure
built primarily for occupancy. This excludes a lot of
extremely tall freestanding structures, such as
Toronto's 1,815-foot (ft) CN Tower, from the running.
Even within "traditional buildings," there is some
controversy. If you include rooftop antennas in the
total height measure, the Sears tower takes first prize
at 1,730 feet. Without including antenna height, the Petronas
Towers in Malaysia, built in 1997, win with 1,483 ft
each. The top part of this structure is only decorative,
however, and it just barely creeps into the record books
by the tips of its thin spires. Many Chicagoans point
out that their Sears Tower has the highest occupied
floor, at 1,431 ft, and the highest traditional roof, at
1,454 ft.
Which building is considered the highest?
Conventionally, decorative structures count toward
height, but antennas do not, giving the Petronas Towers
the official lead.
Onward and Upward The "world's tallest"
title passes regularly from skyscraper to skyscraper. This is
one of the most competitive contests in construction.
Architects and engineers heartily embrace the challenges of
building higher, and corporations and cities are always
attracted to the glory of towering over the competition. The
current champ is the Petronas Towers in Malaysia (see sidebar
in previous section).
By all accounts, the skyscraper race is far from over.
There are more than 50 proposed buildings that would break the
current record. The 1,550-ft 7 South Dearborn building,
nearing completion in Chicago, will squeak by the 1,483-ft Petronas
Towers. China is working on the Shanghai World
Financial Center, which it says will be something more
than 1,500 feet. The proposed pyramid-shaped World Center for
Vedic Learning in Jabalpur, India, will tower over the city at
2,222 ft. One of the most ambitious projects, Hong Kong's
4,029-ft Bionic Tower, will include 300 stories.
The 7 South Dearborn building and several other
conservative structures, are already in construction. But, the
more ambitious buildings in the group are only theoretical at
this time. Are they possible? According to some engineering
experts, the real limitation is money, not technology. Super
tall buildings would require extremely sturdy materials and
deep, fortified bases. Construction crews would need elaborate
cranes and pumping systems to get materials and concrete up to
the top levels. All told, putting one of these buildings up
could easily cost tens of billions of dollars.
Additionally, there would be logistical problems with the
elevators. To make the upper floors in a 200-story building
easily accessible, you would need a large bank of elevators,
which would take up a wide area in the center of the building.
One easy solution to this problem is to arrange the elevators
so they only go part way up the building. Passengers who want
to go the top would take an elevator halfway, get off and then
take another elevator the rest of the way.
Experts are divided about how high we can really go in the
near future. Some say we could build a mile-high (5,280 ft, or
1,609 m) building with existing technology, while others say
we would need to develop lighter, stronger materials, faster
elevators and advanced sway dampers before these buildings
were feasible. Speaking only hypothetically, most engineers
won't impose an upper limit. Future technology advances could
conceivably lead to sky-high cities, many experts say, housing
a million people or more.
Whether we'll actually get there is another question. We
might be compelled to build farther upward in the future,
simply to conserve land. When you build upward, you can
concentrate much more development into one area, instead of
spreading out into untapped natural areas. Skyscraper cities
would also be very convenient: More businesses can be
clustered together in a city, reducing commuting time.
But the main force behind the skyscraper race might turn
out to be basic vanity. Where monumental height once honored
gods and kings, it now glorifies corporations and cities.
These structures come from a very fundamental desire --
everybody wants to have the biggest building on the block.
This drive has been a major factor in skyscraper development
over the past 120 years, and it's a good bet it will continue
to push buildings up in the centuries to come.
