[2] He later went on a road trip and ran out of funds in Seattle, where he then joined the Seattle-based structural and civil engineering firm Worthington and Skilling in 1958.
[2] When Seattle-born American architect Minoru Yamasaki won the competition to design the World Trade Center, Robertson and his firm Worthington, Skilling, Helle, and Jackson (WSHJ) got the engineering contract.
[2] In addition to the World Trade Center, he was involved in structural engineering and design for other skyscrapers, including the U.S. Steel Headquarters in Pittsburgh, the Shanghai World Financial Center, and the Bank of China Tower in Hong Kong designed by the Chinese-American architect I. M. Pei, and Puerta de Europa in Madrid.
[2][5] Further, Robertson engineered the building of museums in Seattle, Portland, Maine, and Berlin in addition to theaters and bridges.
[2] He also helped coordinate the 1978 repair of New York City's Citigroup Center, which had been built with bolted joints that placed it in serious danger of collapse during a high wind.
[9] Robertson died from multiple myeloma at his home in San Mateo, California, on February 11, 2021, the day before his 93rd birthday.