engineer, Leslie Robertson, diagrams wind flow, a force he
says is peskier than an earthquake for building designers.
has been fighting wind for over 50 years. As a structural engineer
now Director of Design for his own engineering consulting
company Robertson has engineered buildings that are recognizable
the world over, such as the World Trade Center and the headquarters
for AT&T, now the Sony building.
York has the worst wind in the nation," says Robertson. "Tall
buildings are designed to withstand earthquakes, temperatures and
even the constant pelting of rain, but wind remains the peskiest."
from the conference room of his third floor office at 211 East 46th
Street, Robertson says that buildings can be tipped several inches
by wind called a "wind load" by engineers and then
oscillate five times that amount, swaying like a pin that has been
grazed by a bowling ball. On an average day, for example, the World
Trade Center sways about a foot between the windward and leeward
sides, and takes about eight seconds to complete one rotation.
building develops twisting and swaying," says Robertson. "So if
you're in a corner office, there are even higher levels of acceleration."
Which translates into a feeling akin to sea sickness.
to Robertson, New York City has some of the worst wind loads in
the nation, even stronger than Chicago or Los Angeles. This is because
of the occasional wind generated off of the Atlantic Ocean during
a result, buildings in New York City must be designed to be twice
as strong as similar buildings designed to withstand an earthquake
in Los Angeles.
Downward Wind Puts Buildings in Motion
order to ensure the safety of a building design, tests are conducted
in long wind tunnels using miniature scale models of buildings
just a couple feet tall that are placed among other buildings
that mimic city contours. A giant propeller blows air into the tunnel
as a computer collects data from "pressure taps" connected to the
model buildings. The process is not as exciting as it may sound. "Wind
tunnel testing is like going to the submarine races," says Robertson.
says that the World Trade Center was the first ever skyscraper to
be constructed using this type of "rational wind engineering." Prior
to that, buildings were tested autonomously in aeronautical wind
tunnels without other models, and as a result, did not predict wind
effects accurately. "The aeronautical tunnels did not replicate
the actual wind," says Robertson.
makes the World Trade Center even more unique is that unlike other
skyscrapers, it doesn't taper as it rises. "It puts more real estate
higher up than any other building," says Robertson, adding that
there is actually more usable area in the top floors than the bottom
floors. In comparison, the top floors of the Sears Tower in Chicago
are one ninth the area of its base. "Having the space in the World
Trade Center is a huge advantage," says Robertson. "It's a high
rent district . The perimeter at the top is 800 feet. The Sears
tower has about a third as much."
Floors vs. High Seas What people experience
from the sway.
Elevators must either be slowed down or stopped all together.
Water sloshing around
in sinks, glasses and
Noise, creaks and groans. They may sound scary,
but they're not harmful to the building or occupants.
extreme cases, windows can pop out
and exterior panels
can break off.
Leslie E. Robertson Associates
the first shovel breaks ground, scale models like this one
are used in wind tunnel tests to map the dynamic movement
PHOTO: Mark Valenta
tapered building with rounded edges reduces wind loads, but
the World Trade Center has neither.