David Childs, the lead architect of the One World Trade Center skyscraper that rose from the site where the Twin Towers collapsed in New York City during the 9/11 attacks, has died. He was 83.

Childs died on Wednesday in Pelham, New York, from Lewy body dementia, which had been diagnosed in September, his son, Nicholas Childs said.

While he was perhaps best known for his work on One World Trade Center, considered to be the tallest building in the Western Hemisphere, Childs also was instrumental in other important projects, including a new master plan for the National Mall in Washington, D.C., an expansion of Dulles International Airport in Virginia and the 7 World Trade Center building in Manhattan, according to his firm, Skidmore, Owings & Merrill.

“David’s contribution to the firm was extensive and profound, and we will always be grateful to David for his leadership, his impact, and his friendship,” Skidmore, Owings & Merrill said in a statement. “We will miss him dearly and extend our condolences and deepest sympathies to his family.”

A fond memory that Nicholas Childs has was when his father drew a rendering of what the One World Trade Center property would look like while they were out having lunch in New York, some years after 9/11 but well before the plans were finalized.

“He picked up a paper napkin, took out a pen, and drew what became the ultimate design of the building on the napkin for me,” Nicholas Childs said in a phone interview Friday, adding that he still has the drawing.

He said his father was a civic-minded architect who would often use a quote by 20th century American-German architect Ludwig Mies van der Rohe – “God is in the details.”

“He cared deeply about those details and making something beautiful,” Nicholas Childs said. “But he also wanted to make sure, I think as any great architect, that it was a balance of form and function, that it worked for people.”

The 1,776-foot-tall (540-meter-tall) One World Trade Center, once known as the Freedom Tower, is the centerpiece of the redevelopment of ground zero, along with the memorial pools placed where the Twin Towers stood. Opened in 2014, it sports a steel and glass structure that rises into the sky with a tapered, eight-triangle body, topped by a 408-foot-tall (124-meter-tall) needle.

The final design was produced after contentious negotiations between Childs and Daniel Libeskind, who designed the overall plan for the site, and arguments between government officials over what should be built at ground zero and objections by some relatives of 9/11 victims to the design of the Trade Center memorial.

Libeskind had drawn the first plans for building, a twisting glass skyscraper with an off-center spire meant to invoke the Statue of Liberty.

Childs produced a sleeker version of Libeskind’s design, then reworked it after police expressed concerns that the building was not sturdy enough to withstand a truck bomb.

In announcing the new design plans, Childs said the One World Trade Center tower is “iconic, simple and pure in its form, a memorable form that will reclaim the resilience and the spirit of our democracy.”

He also called it “the safest building in the world,” with features including wider staircases, a separate stairway and elevator for firefighters, “blast-resistant glazing” and more public stairways with direct access to the street.

Childs was born in Princeton, New Jersey, in 1941 and grew up in his early childhood years in Washington, D.C., before moving to Mount Kisco, New York. He attended the private Deerfield Academy in Deerfield, Massachusetts, before going to Yale University in New Haven, Connecticut, where he earned a bachelor’s degree and a master’s degree in architecture.

He was hired by Skidmore, Owings & Merrill in 1971 for its Washington, D.C., office, where he worked on projects including a new master plan for the National Mall, the Constitution Gardens park along the mall and the Four Seasons Hotel.

He transferred to the firm’s New York City office in 1984 and had a hand in significant projects in the city, including 1 Worldwide Plaza in the Hell’s Kitchen section of Manhattan and the Deutsche Bank Center, formerly called the Time Warner Center, with its twin skyscraper towers at Columbus Circle.

Childs also worked on the U.S. embassy in Ottawa, Canada, the Canary Wharf development in London and King Abdulaziz International Airport in Jeddah, Saudi Arabia.

In 2004, he was honored with a Rome Prize for architecture, awarded by the American Academy in Rome for innovative work in the arts and humanities.

Childs is survived by his wife, Annie; three children, Nicholas, Joshua and Jocelyn; and several grandchildren.

The family is planning a small, private memorial in the coming weeks, followed by a larger service around May, Nicholas Childs said.