The twin towers of the World Trade Center rise above the New York skyline in this file photo from June 23, 1999. Both buildings were destroyed on Sept. 11, 2001 after two airliners were hijacked by terrorists and crashed into the towers.By Eric Uhlfelder and William Abrams/ For The Star-Ledger
Leslie Robertson was in Hong Kong, having dinner with colleagues, when he heard the news.
The iconic twin towers framing the New York skyline that he had helped design so long ago were under attack.
From the other side of the world, he quickly found a TV where images of Lower Manhattan were streaming the stunning scene of the World Trade Center, burning, yet still standing.
"I didn’t think the towers would fall. As designed, the weight of the floors above where the planes had hit was being transferred around the holes to the columns below," he said. "But as I watched, fearing for all those still inside, a decade of work flashbacked across my mind."
Ten years after the buildings were lost, he quietly carries with him an unresolved anguish.
"I was ready to pack my bags, (after the attacks) not because I felt I let anybody down, but simply due to the suffering associated with my work," he said.
With a career that spans five decades, Robertson was the lead structural engineer of the World Trade Center responsible for conceiving and executing the design and overseeing the work of engineers, draftsmen and technicians that allowed the towers to rise higher than any building before them.
In many ways, Robertson is an anachronism. He seems to belong to the grand Victorian age of innovation. His deportment is calm and pleasant. He enjoys entertaining friends, listening carefully, speaking softly and precisely.
Leslie Robertson was a structural engineer at the World Trade Center.He lives in Manhattan with his wife, SawTeen See, who is also an engineer and a managing partner in Leslie E. Robertson Associates. He and his wife enjoy classical music and the vibrant cultural scene of the city.
He has a wide range of interests. Since his college days, he demonstrates against wars and was active in organizing bus trips to Washington, D.C,, to rallies in protest of the Iraq war. He has also been active in the civil rights movement and has advocated for women’s rights over the years.
At 83, his eyes still radiate passionate interest in things around him and ambitions still to be realized. And he fully embraces all the technological advantages that have accompanied him into the 21st century.
Ten years after 9/11, his career continues to thrive and he still travels the world to oversee projects he’s designing in the Middle and Far East.
A graduate of the University of California at Berkeley, he wasn’t sure early on where he was heading. His first job was as a mathematician, solving problems in electrical engineering. But his work on electrical towers quickly led him into structures, and he soon took up structural engineering.
And in 1962, he was engineering the biggest project in the country.
LOOKING UP
The World Trade Center was born out of despair and extraordinary ambition to revive Lower Manhattan.
In the 1960s, cities were dying as the affluent fled to the suburbs. The big businesses that stayed in New York were moving out of the more constrained spaces of Lower Manhattan’s early 20th-century skyscrapers in favor of the more modern, larger office spaces of Midtown.
Chase Manhattan Bank Chairman David Rockefeller, founder of the Downtown-Lower Manhattan Development Corporation, along with his brother, New York Gov. Nelson Rockefeller, and the Port Authority of New York and New Jersey envisioned a massive project to return attention to the city and Lower Manhattan.
The Port Authority — which was in charge of the project — successfully lobbied for the 10-million-square-foot development to be shifted from the East River to the Hudson, providing its PATH trains running beneath the Hudson River direct access.
View full sizeConstruction workers on the top floors of the World Trade Center when the twin towers were under construction. The twin towers were built in the late 1960s and early 1970s. The design of the structures, then tallest in the world, was tested to withstand a hit from a Boeing 707, significantly smaller than the planes used by terrorists on 9/11.In 1962, the authority selected Minoru Yamasaki as the architect. After considering various models, the authority eventually decided that two 110-story towers would anchor the 16-acre site.
Yamasaki selected the Seattle-based engineering firm of Worthington, Skilling, Helle and Jackson to design the structure. The partner-in-charge was John Skilling.
Robertson, then 34, was selected to make it work.
"We had already worked well with Yama on a half a dozen projects," explained Robertson, "so he felt very comfortable calling on us to develop ideas for the trade center."
"What we provided Yama," Robertson recalled, "was something he never dreamed of — closely-spaced small columns on the perimeter of the building made into a gravity and lateral-force system that was very redundant and robust. It was exactly what was needed for his architecture."
At the time, Robertson’s firm was not a leading practitioner of skyscraper engineering. Its tallest structure to date was the 20-story IBM building in Seattle. But Robertson had already demonstrated an aptitude for solving challenging problems.
He had designed the foundations for the 5-mile-long Maracaibo bridge in Venezuela, which got some notice because he opted to use 36-inch-deep wide-flange steel beams, instead of steel wire, to fortify the concrete footings.
He was one of the first engineers to apply a cross-braced wall in a 13-story IBM building in Pittsburgh, which required less steel than traditional building frames. It was also an early example of the column-free interior design that was to be a key feature of the World Trade Center.
For US Steel’s headquarters, also in Pittsburgh, he designed liquid-cooled columns that met the company’s demand to leave the building’s exterior structural steel exposed.
He would throw himself into projects, often studying problems differently than others. For example, he would ride on the tops of elevators, he said, in a variety of skyscrapers — including the Empire State and Helmsley buildings in New York — to see how elevator shafts were reacting over time to various stresses.
OUTSIDE IN
The task facing his group of about 30 engineers, draftsmen and technicians was beyond anything builders had ever contemplated.
Skyscrapers had topped out in 1931 with the Empire State Building’s 102 floors. This limit, which had prevailed for 40 years, was based on accepted engineering practice. Robertson was going to rewrite that limitation with each tower containing more than twice the floor space contained in the Empire State Building.
"To me, the trade center was a matter of expanding the basic ideas of structure," Robertson said.
Each of the towers’ 110 stories was nearly an acre. They married 200,000 tons of steel, 425,000 cubic yards of concrete, more than 43,000 windows, 198 miles of heating and cooling ducts, 12,000 miles of electric cables and more than 100 elevators.
A hijacked jet airliner is moments away from crashing into one of New York's World Trade Center twin towers in this file photo from Sept. 11, 2001.Robertson’s biggest concern in engineering the trade center was the wind, the force of which was even greater than the downward load of the buildings.
"Not only did we have to contend with the turbulence generated by setting two tall buildings next to one another," he said, "we were also confronted with various and extreme wind pressures exerted by neighboring skyscrapers and proximity to the Hudson and East rivers, and the Atlantic Ocean, which was just a few miles to the south."
Wind analyses involved teaming with leading wind expert, Alan Davenport of the University of Western Ontario, and constructing a 1:500-scale model of Lower Manhattan, Robertson said.
The model included the World Trade Center and was inserted into the most sophisticated wind tunnel of the time, said Robertson, housed at Colorado State University.
Unlike traditional skyscrapers based on a grid of columns and beams set apart at 30-foot intervals, Robertson’s and Skilling’s solution thoroughly reconfigured the columns. He designed a dense row of columns around the perimeter of each tower and another set of columns circling the buildings’ core. The two sets of columns were then connected by prefabricated floor trusses.
This created far more open floor space than traditional skyscrapers. "But with this structural configuration," explained Robertson, "the exterior walls carried extraordinary weight, requiring them to be even more robust than traditional skeletal walls to counter the lateral force of the wind."
Though the towers appeared identical from the outside, Robertson found that he could further strengthen the towers’ wind resistance by rotating one tower 90 degrees. Because the cores were rectangular and set inside each building’s square plan, the distance between the core and the exterior walls was longer on two sides. This lent to increased lateral stiffness where it was most effective.
To control the sense of movement inside, Robertson’s team, along with the 3M Corporation, invented unique viscoelastic dampers — structural shock absorbers. Nearly 11,000 of them helped regulate motion by absorbing energy generated by the movement of the columns and floor trusses.
WORST CASE
The World Trade Center project marked the first time that computer modeling was used to forecast how a structure would perform, according to Robertson, leading to more precise design specifications of materials and construction. And Robertson used this technology to confirm his structures could withstand a hit by the largest plane of the time — a Boeing 707. (The planes that hit the towers were more fuel-laden Boeing 767s.)
Testing such a horrific hypothesis comes down to two basic conditions: removing a series of adjacent columns and floor trusses and seeing how the buildings absorb the energy of the jet. Robertson says tests revealed that if a plane was flying at approach speed when it struck one of the towers, it would remain standing.
However, the impact that a jet-fuel-accelerated fire would have on the integrity of the structures was never projected. The reason, according to Robertson: No one knew how to model such a fire.
The National Institute of Standards and Technology, which led the official government investigation of the towers’ collapse, concurred: "The computing resources and software necessary to conduct (such an) analyses did not exist in the 1960s."
After 9/11, there was extensive study of how the towers performed and why they ultimately fell. The Federal Emergency Management Agency and the American Society of Structural Engineers are among those that claim that the towers were well-designed because they stood as long as they did, giving the vast majority of occupants time to exit.
On the other hand, several engineers cited in the science documentary series NOVA, "Why the Towers Fell," say the search for efficiency may have produced structures that were more susceptible to progressive failure.
Ten years after the disaster, the subject remains contentious and difficult to discuss dispassionately.
Robert Bea, an engineering professor at the University of California, Berkeley, is one of the country’s leading forensic engineers. He led investigations into the Challenger disaster, Hurricane Katrina, and the Macondo Oil Well blowout in the Gulf of Mexico and who heads the Center for Catastrophic Risk Management. He describes Robertson’s design as excellent.
"One part of me as an engineer looks at that efficiency achieved and says: Well, that’s exactly what we should be doing.
"However, it has taken me my entire professional life to learn that anyone’s structural system cannot, will not be perfect. Things will not perform as you expect them to. Things will not be built as you expect them to. The system has to have a level of protection called robustness that allows it to tolerate damage and defect," Bea said.
UPPER LIMITS
The NOVA documentary questioned the resilience of the unique floor trusses after they lost their fireproofing (from the impact of the jets) and the integrity of their links with the interior and exterior columns.
Bea said several specially reinforced midbuilding truss systems may have reinforced the geometry of the building and possibly could have helped to keep portions of them standing.
Robertson says that his design met or exceeded all local code requirements, which the NIST confirmed. He contends that the strength of his buildings lay precisely in his exterior walls designed to withstand the wind, a key to which was the robustness of the connections between the floor trusses and the columns.
The NIST investigation found the floor trusses did not fail first. In fact, NIST believed when the trusses lost their lateral strength — because of the loss of fireproofing and the intense heat of the fire — they actually pulled the columns inward. This added further strain to the structures, which were already suffering from misalignment as a result of the impact of the planes.
But NIST did conclude that if the jets hadn’t knocked off the fireproofing of the steel, the towers would likely have remained standing, enduring both the airplane strike and the fires.
Robertson said no structure built for commercial purposes could possibly keep a jumbo jet flying at high speeds from penetrating its facade.
Robert Prieto, chairman of the venerable engineering firm Parsons Brinckerhoff, said, "Unless we are prepared to live in an engineered environment that resembles the complex of caves in Afghanistan, we will not design buildings to stop planes." And Robertson doesn’t believe there is any certainty that midstructure reinforcement could have saved the buildings once they were so severely compromised.
Duke University professor Henry Petroski, a longtime student of engineering failure, contends evolution in both nature and things manmade is a push toward greater efficiency and optimization of materials used. When the process encounters intolerable environmental conditions and structure suffers, it then corrects.
Should the possibility of airplanes being intentionally flown full throttle into buildings then become part of environmental conditions for which engineers must plan?
It certainly was not in 1960s when Leslie Robertson was designing the tallest buildings in the world. According to NIST, no building code today requires such defense.
The fate of the World Trade Center ultimately came down to a simple fact: The level of hate driving a handful of individuals exceeded the imagination, intelligence and commitment that had led tens of thousands of men and women to create two remarkable towers, which had the audacity to reach a quarter mile into the sky.
This is not much solace for the families of the thousands who perished on Sept. 11 or for Robertson. Many did come to him in the months that followed, looking for some kind of answer, which he knew he could not provide.
"The first was a young woman, perhaps 13 or 14 years old. Her brother was working on one of the high floors. We met in a park at the foot of Manhattan. The tears came as her body shook. And as we cried together, words were not required."
Eric Uhlfelder writes about finance and urban affairs, and has authored several books on European capital markets, architectural history and urban design.
William Abrams is currently completing a historical novel about the collapse of the Tay Bridge in Scotland during the 19th century, which had been the longest bridge ever built.