Leslie Robertson, the structural engineer known for his innovative work on skyscrapers around the globe, died on February 11th, a day before his 93rd birthday, of multiple myeloma. Revered and beloved by the architects with whom he collaborated, Robertson may remain best known for engineering the Twin Towers of the World Trade Center (WTC) with architect Minoru Yamasaki. Its unconventional design created wideopen floors for office tenants by bringing densely spaced steel columns to the perimeter as well as by creating a robust steel-framed core. Other firsts included damping devices incorporated into the structural system and the development of a new partition called a shaft wall that eliminated masonry at the core, according to the Skyscraper Museum. The towers, completed in 1973, were said by Robertson to be able to withstand the impact of a 707—but the horrific attack on 9/11, by two 767s, loaded with thousands of gallons of fuel that fed the conflagrations, was unimaginable. It left Robertson grief-stricken. Many experts noted that because of how long each tower stood before falling, thousands of people were able to escape. 

Born in California, Robertson earned a B.S. in engineering at Berkeley in 1952. When Yamasaki won the competition for the WTC 10 years later, Robertson’s firm got the engineering contract and his skyscraper career was launched. “I was a kid,” he told The New York Times. “We had never done a real high-rise before.” The Twin Towers gave Robertson the chance to do pioneering research, such as helping develop the first boundary-layer wind tunnel and conducting the first experiments on people’s sensitivity to the sway of buildings, Engineering News Record reported.