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The man who engineered the Twin Towers of the World Trade Center was recently honored by the National Building Museum with a $25,000 prize, but it has nothing to do with September 11th. Although the events of that day tend to take center stage anytime the towers are mentioned, Leslie Robertson is receiving the honor for a lifetime of engineering advancements in high-rise buildings, including three of the world’s six tallest.

Leslie Robertson
image: WGBH/NOVA

"I did a lot of things that I don’t think an older engineer would have bothered to do," he said in a recent NOVA documentary. "I was charging down a different highway."

Take, for instance, his implementation for the first time of a "space-frame megastructure and outrigger or hat system" in the United States Steel headquarters in Pittsburgh.

"Imagine an open umbrella standing on its handle," he explained to ScienCentral News. "If you let go of it, it would fall over, even if you keep the handle from moving laterally at the base. But if you tie the tips all around the top of the umbrella to the ground, it would be stable. And the hat system does exactly that."

Robertson also revolutionized the way walls around elevator shafts were made fire-resistant in high rise buildings, which typically had fire partitions constructed of brick or gypsum block.

"I used to just ride up and down elevators, riding on the top of the elevator cars," he says. "And I found that they leaked air terribly. And that’s bad because in wintertime it’s like a chimney. The hot air rises, encourages airflow, and uses a lot of energy. Also, if there were a fire it would spread smoke. So I got to thinking about how to make a partition that was stronger."

He created the shaftwall system now almost universally used for fire-resistive walls in high-rise buildings. Shaftwall is a particularly strong and fire-resistant type of sheetrock.

Some of his most important work has to do with protecting high-rise buildings from wind. The Bank of China Tower, in Hong Kong, uses a "composite megastructure space frame" in order to resist all loads imposed by typhoon winds and the weight of the building.

And wind was the most important factor when he decided to engineer the Twin Towers, which at the time was taller than any building ever built. Robertson created "mechanical damping units" to prevent the building from swaying and oscillating in the strong high winds. He explains the damping units as "devices that take the energy of oscillation and turn it into heat energy. So instead the heat is dissipated. Using a damper is kind of like the difference between a buggy—where you’re bouncing up and down the whole ride—and a Mercedes Benz, which cruises down the highway without a jolt. They’re like shock absorbers."

But the most noted chance he took was with the interior design of the World Trade Center towers, specifically with regard to where the support columns were placed. Robertson set out to create skyscrapers with more rentable office space than the current engineering techniques typically allowed. Standard skyscrapers of the day had support columns that were spaced evenly throughout the width of the building. Robertson decided instead to keep the columns in a main inner core (housing the elevators, emergency stairs and other building services), and move the rest of the support columns to the exterior walls. This difference allowed the interior to have more open space. And he made it work by connecting the inner core and the exterior columns with a series of steel trusses below every floor.

This design first proved strong in 1993, when a terrorist bomb exploded in the building’s parking garage. And as Robertson pointed out on NOVA, "The bombing, I think, created a lot of confidence in everyone’s mind that the Trade Center was pretty sturdy."

And while engineers who assessed the disaster of September 11th say the towers did a good job of standing long enough for thousands to escape, the force of two massive jets traveling at blinding speeds and carrying thousands of gallons of jet fuel proved to be too much. Leslie Robertson’s office on Broad Street in Manhattan used to have a view of the Twin Towers. But now the view is of ground zero.

"Ground zero is a very disturbing place for me," he says. "I mean I probably have more emotional attachment to it than maybe any other person now alive."

Elsewhere on the Web

NOVA News Minutes: Deconstructing the Towers’ Collapse (video) - produced by ScienCentral

World Trade Center Investigation Executive Summary - FEMA

Legislation Introduced to Remedy Problems Uncovered During WTC Investigation - House Committee on Science



by Brad Kloza


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