The Tallest Buildings in the World

By Eric Uhlfelder

Twenty years ago he was in Hong Kong working on the AIG tower. He was having dinner with colleagues when he had first heard the news.

It wasn’t the first time Leslie Robertson’s towers were attacked.

When he surveyed the damage the day after the 1993 bombing, he found the Towers had remained structurally uncompromised. Given their resilience, terrorists may then have realized a more brazen attack from above would be far more public and devastating.

When Robertson heard a second plane had hit the Trade Center, he got up from dinner and found a TV where images of Lower Manhattan were streaming across from the other side of the globe. “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 breaches to the columns below. But as I watched, fearing for all those still inside, a decade of work flashbacked across my mind.”

The World Trade Center was born out of despair and extraordinary ambition to revive Lower Manhattan. Chase Manhattan Bank Chairman David Rockefeller, founder of the Downtown Lower Manhattan Development Corporation, New York State Governor 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. In 1962, the Authority selected Minoru Yamasaki as the architect. After considering various models, the Authority eventually decided two 110-story towers would anchor the 16-acre site.

At the time, few outside the project knew the steel structure that made Yamasaki’s vision possible was conceived by a relatively unknown Seattle-based engineering firm, Worthington, Skilling, Helle and Jackson. The Partner-in-Charge was John Skilling. And the lead structural engineer was 34-year-old Leslie Robertson.

LESLIE ROBERTSON, the lead structural engineer of the Twin Towers. Photo credit: LERA.

“What we provided Yama,” Robertson recalls, “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.”

In many ways, Robertson was an anachronism. He seemed to have belonged to the grand Victorian age of innovation. When I interviewed him a decade ago, his deportment was calm and pleasant. He enjoyed entertaining friends, listening carefully, speaking softly and precisely. At 83, his eyes still radiated passionate interest in things around him and ambitions still to be realized. And he fully embraced all the advantages that have accompanied him into the 21st century.

Like youthful tech innovators, he often did things differently, unconstrained by accepted doctrine or norm. He approached problems with defiant logic, believing that man’s greatest accomplishments are achieved not simply through close adherence to principles, but in their judicious expansion.

The task facing his team was beyond anything builders had ever contemplated. “We had already worked well with Yama on a half a dozen projects,” recalled Robertson, “and to me the Trade Center was a matter of expanding the basic ideas of structure.”

It was to be a lot more. Robertson was going to rewrite that limitation with each tower containing nearly twice the floor space contained in the Empire State Building.

Unlike traditional skyscrapers based on a grid of columns set apart at 30-foot intervals, Robertson’s and Skilling’s solution thoroughly reconfigured the structure. He designed a dense row of columns around the perimeter of each tower and another set of columns circumscribing 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,” explains 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.” And having used computer modeling, Robertson confirmed his design could withstand a hit by the largest plane of the time—a Boeing 707.

After September 11th, there was extensive study of how the Towers performed and why they ultimately fell.

The National Institute of Standards and Technology’s investigation of the collapse of the World Trade Center—the official government study—concluded the high-speed impact of the jet knocked off the fireproofing that wrapped around the columns and beams, combined with the super hot fires, caused the towers to collapse. Without the inferno, it believes the Towers would likely have remained standing.

Berkeley Professor Robert Bea—one of the country’s leading forensic engineers who led investigations into the Challenger disaster, Katrina, and Macondo Oil Well blowout in the Gulf of Mexico, and who heads the Center for Catastrophic Risk Management—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.”

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 September 11th 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.” And maybe through embraces and tears shed with strangers he too partially came to terms with what he could not comprehend.

Robertson thought his career was over. “I was ready to pack up my bags, not because I felt I let anybody down, but simply due the suffering associated with my work. I thought afterwards I had no chance at securing new contracts and felt that even existing jobs were going to get pulled at any moment.” But to his surprise, he received an outpouring of support.

He went on to design some of the world’s tallest skyscrapers. The 100-story Shanghai World Financial Center rises 492 meters. A 100-story tower in Kuala Lumpur ascends 500 meters. And the Lotte Jamsil Tower in Seoul South Korea soars 123 stories or 555 meters.

Time had distanced that terrible morning. However, until his death this past February, Leslie Robertson had quietly carried with him an unresolved anguish: “I cannot escape the people who died there . . . it’s still to me up there in the air burning . . . and I cannot make that go away.”

This is an abridged version of Eric Uhlfelder’s original cover story of the South China Morning Post’s Sunday magazine published a decade ago.

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