Structural engineer, Leslie Robertson, diagrams wind flow, a force he says is peskier than an earthquake for building designers.
PHOTO: MarkValenta


eslie Robertson 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.

"New 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."

Speaking 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.

"The 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.

According 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 hurricane season.

As 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.

How Downward Wind Puts Buildings in Motion

Illustration: Mark Valenta

n 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. "Incredibly dull."

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.

What 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."


High Floors vs. High Seas— What people experience
in skyscrapers...

Mark Valenta

  • Nausea — from the sway.

  • Elevators must either be slowed down or stopped all together.

  • Water sloshing around
    in sinks, glasses and
    toilet bowls.

  • Noise, creaks and groans. They may sound scary,
    but they're not harmful to the building or occupants.

  • In extreme cases, windows can pop out
    and exterior panels
    can break off.

    Leslie E. Robertson Associates

Before the first shovel breaks ground, scale models like this one are used in wind tunnel tests to map the dynamic movement of airflow.
PHOTO: Mark Valenta

A tapered building with rounded edges reduces wind loads, but the World Trade Center has neither.
PHOTO: Mark Valenta