Nicholas Gianopulos, 93, a Philadelphia structural engineer who figured out how to make some of the 20th century's greatest buildings stand up, and who kept some of the city's most important historic structures from falling down, died Saturday, July 21, at his home in Gladwyne. He had been under hospice care, said daughter Christiana.

Mr. Gianopulos was one of the founders of Keast & Hood, a Center City engineering firm that became the go-to consultant for celebrated architects such as Louis Kahn, Robert Venturi, and Romaldo Giurgola, a group collectively known in the 1960s as the "Philadelphia School." Because of Mr. Gianopulos' ability to simplify complex structural problems, as well as his affable personality, the Philadelphia School architects returned to him again and again for help with such projects as Kahn's National Assembly building in Bangladesh and Venturi, Rauch & Scott Brown's Institute of Scientific Information in West Philadelphia.

By the early 1970s, Mr. Gianopulos had become increasingly interested in historic preservation work, and he used his skills to stabilize some of Philadelphia's defining colonial-era buildings, including the towers at Independence Hall and Christ Church, said David Hollenberg, an architect who began his career at Keast & Hood. At the time, many modernist architects turned up their noses at historic restoration.

Mr. Gianopulos loved old buildings, Hollenberg said.

"He used to say, 'They don't build them like they used to — and that's a good thing,' " Hollenberg said. At the same time, Mr. Gianopulos felt "his work on old buildings enriched his understanding" of all architecture.

The most dramatic moment in Mr. Gianopulos'career came in 1989, when his firm was overseeing renovations at the Academy of Music.

After the project engineer discovered a crack in a crucial roof truss, Mr. Gianopulos came over for a second look. He was so alarmed by what he saw that he advised the Academy's management to immediately evacuate the building even though there was a concert in progress. Undeterred by the closure, the tenor Luciano Pavarotti regrouped and moved that evening's performance of L'Elisir d'Amore  to the Cathedral Basilica of SS. Peter and Paul.

Although many expected it would take months to rebuild the Academy's roof, Mr. Gianopulos ordered an accelerated repair schedule.

"We worked 24/7 for two weeks," said Dean Doukakis, then a young engineer at Keast & Hood, and now a principal. The Academy reopened in less than two weeks.

After that, his daughter said, "he spent his whole life looking for cracks."

Mr. Gianopulos grew up in Philipsburg, Pa., the son of immigrant parents from Tripotamo, Greece. He excelled in high school, and his teachers encouraged him to enroll in a Pennsylvania State University extension course in engineering. That led to a summer job at a nearby firm "where he learned the importance of getting it right," said Isaac Kornblatt-Stier, a Harvard University design student who has done extensive research on the Philadelphia School.

World War II put his career on hold. At 18, he had been delcared 4F — unfit for military service — because he was deaf in one ear. To support the war effort, he took a job at Bell Aircraft Corp. in Buffalo, where he made parts for military planes, said Korblatt-Stier. But as more friends went off to fight, Mr. Gianopulos felt an obligation to serve. He managed to enlist in 1944 by pretending not to have a hearing problem, and soon shipped out with the Second Army Battalion.

Stationed in Belgium, he was a reconnaissance scout during the Battle of the Bulge, one of the bloodiest campaigns of the war, and received a Purple Heart, his daughter said. He spent the following year in occupied Berlin. Afterward he attended Penn State on the G.I. Bill.

After moving to Philadelphia in the early '50s to take a job an engineering firm, he met two people who would change his life: Antoinette Manos, who became his wife, and Kahn, who became an important collaborator.

Kahn was then working on the Yale Art Gallery, one of his breakthrough projects, but was having trouble getting the New Haven building department to approve its unusual ceiling design, which featured a web of triangular concrete openings. The situation became so tense that Yale threatened to take the project from him. Although another engineering firm was overseeing construction, Kahn sought Mr. Gianopulos' advice.

"Nick showed him how he could turn it into a more conventional structural system," Kornblatt-Stier said. Kahn grudgingly adjusted the design to Mr. Gianopulos' specification. Later Kahn introduced him to Venturi and Denise Scott Brown.

Mr. Gianopulos loved a good design, his daughter said. Family vacations often centered on tours of important buildings.

Once he took the family to Nantucket so he could inspect two houses that he had engineered for Venturi and Scott Brown. He was afraid the island's harsh wind had weakened the houses. Somehow, he gained permission to explore the attics to look for wear and tear and, of course, cracks. "He was very happy to see there were none," his daughter said.

Mr. Gianopulos lived in Gladwyne and commuted daily to his Center City office on the Route 44 bus, getting to know the regulars, until his retirement in 2009. He also spent nearly three decades teaching architecture students at the University of Pennsylvania. Virtually every student who studied there between 1964 and 1991 took a class with him, said Dominique Hawkins, a preservation architect.

"He would wander the halls and talk to students about their thesis projects," she recalled. "All he asked was that they make sure their buildings could stand up."

In addition to his daughter and wife of 55 years, Mr. Gianopulos is survived by another daughter, Elizabeth, and a son, Elia.

A funeral service will be held at 11 a.m. Friday, July 27, at St. Luke's Greek Orthodox Church, 35 N. Main Rd., Broomall. A viewing will precede the service from 9:30 to 11 a.m. at the church. Interment will be in West Laurel Hill Cemetery, Bala Cynwyd.