Engineer who designed twin towers and was left with ‘a troubled heart’ after 9/11

Leslie Robertson turned on the television and watched live as his dream collapsed to the ground. In the Sixties he had engineered what he thoughtwas an indelible symbol of the Manhattan skyline, only to see the twin towers of the World Trade Center destroyed by terrorists on September 11, 2001.

His brilliant structural design for the towers had won him lucrative contracts in the Far East and he was in Hong Kong when he received a phone call from his wife. Robertson turned on the TV to see plumes of black smoke rising from the middle of one of the towers. Hundreds of calculations rushed through his brain in an attempt to work out if it would stay up. He soon received his answer. The southern tower collapsed 57 minutes after being hit by a hijacked Boeing 767 piloted by terrorists. The northern tower collapsed 1 hour 42 minutes after being hit by another. That day 2753 people died.

Robertson, who has died of blood cancer aged 92, recalled lying awake at night replaying his years as lead structural engineer of the towers that were completed in the early Seventies and hailed for their pure tube structure, enabling the vast interiors to be unencumbered by structural columns.

He never slept well again. Puffy black bags developed under his eyes. He kept thinking that if the towers had stayed up, if one of America’s symbols of power had not been destroyed, the Bush administration might not have launched its War on Terror in Afghanistan and Iraq.

‘‘Perhaps the lives of countless of our military men and women would not have been lost. Perhaps countless trillions of dollars would not have been wasted on war. Just perhaps, I could have continuedmy passage into and through old age, comfortably, without a troubled heart,’’ said Robertson, a veteran activist from the civil rights movement in the Sixties.

At ameeting of the National Council of Structural Engineers Associations weeks after the attack, he was asked: ‘‘Is there anything you wish you had done differently in the design of the building?’’ Robertson broke down and wept at the lectern.

The official investigation into the collapse by the National Institute of Standards and Technology found that, even though the towers conformed to all building codes, the impact of the aircraft was likely to have knocked the fireproofing off the lightweight steel floor trusses. The trusses lost their lateral strength as they began to melt in temperatures of more than 500C. As they sagged, they pulled away from the exterior columns, which began to fall inwards. Each floor fell on to the one beneath.

In the early 1960s the World Trade Center

‘‘Perhaps the lives of countless of our military men and women would not have been lost.’’

had been the first project to use computer modelling to predict the building’s response to a catastrophic event. Robertson even modelled the impact of an aircraft and concluded the buildings would not collapse, but his calculations did not factor in the burning aviation fuel. He was exonerated by the investigation: ‘‘The computing resources and software necessary to conduct [such] analyses did not exist in the 1960s,’’ it found.

In the months after the attack, eminent structural engineers sprang to Robertson’s defence, arguing that the towers stood long enough to allow most of the people below the crash sites – the 94th floor to the 99th floor in the northern tower, and the 78th floor to the 84th floor in the southern tower – to escape.

Robertson received hate mail, but while flying to Toronto he was upgraded to first class. When he asked why, the flight attendant replied: ‘‘I was in Tower 2, and I walked out.’’

Leslie Robertson was born in Manhattan Beach, California. He described his father, an inventor, as a ‘‘brilliant man who couldn’t stick at anything’’.

Leslie left school at 16 to join the navy as an electronics technician. He later studied civil engineering at the University of California, Berkeley, graduating in 1952. After several years working as amathematician, he felt bored so set off on a road trip across America. He ran out of money in Seattle and found work at the structural engineering company Worthington & Skilling, where he stayed.

In 1962 the architect Minoru Yamasaki won a design competition held by the Port Authority of New York for a vast office complex to regenerate rundown Lower Manhattan. Yamasaki called in Worthington & Skilling to help to realise his vision of twin modernist towers, each 110 storeys and a quarter of amile high.

Robertson, 34 at the time, had never worked on a skyscraper before. ‘‘I ... set out to do no less than change the principles of skyscraper design,’’ he told the New York Times. Some architecture critics thought the design bland, but showered Robertson’s structural engineering with praise.

His first two marriages ended in divorce. He is survived by third wife Saw-teen See, an engineer, and by two daughters and a son from the three marriages.

In the new millennium he returned to the World Trade Center site as structural engineer of 4World Trade Center, the first tower to rise there after 9/11.

Ground Zero remained sacred to him and he rarely refused requests to meet family members of those who had died in the towers. ‘‘The first was a young woman, perhaps 13 or 14 years old,’’ he recalled. ‘‘Her brother was working on one of the high floors. The tears came as her body shook. And aswe cried together, words were not required.’’– The Times