PHILADELPHIA — You know what's chicer than spending a ton on a landmark building? Spending a ton and barely showing it. When other museums and cultural institutions have turned to Frank Gehry, the Canadian Angeleno and 92-year-old grandmaster of torquing titanium, he has summoned up buildings both inventive and ostentatious: curves of metal at the Guggenheim Bilbao or Disney Hall in Los Angeles, or billowing sails of glass at the Fondation Louis Vuitton in Paris. But here in Philadelphia, where he was tasked to reimagine one of the country's oldest and most significant museums, he has left the stainless steel and the kinematics software at home. Fifteen whole years after the Philadelphia Museum of Art engaged Gehry for an expansion and renovation of its Beaux-Arts home at the top of the Benjamin Franklin Parkway, the first part of the work is complete — and discreet. His Core Project, as the museum calls it, has cleared out and reshaped the underground guts of its Greek Revival home to produce 20,000 additional square feet of galleries, along with a refreshed entrance and an atrium with potential for performances and gatherings in post-pandemic days. It's cost $233 million so far, and this is just part one; next will come additional new galleries underground, and a window puncturing the eastern staircase (you know, the one from "Rocky"). You'll see Gehry's hushed interventions first via the western entrance — which I still think of as the back of the museum, although it's been the primary access for years now. (The eastern entrance, off the parkway and up the steps, is closed for now.) It has more inviting glass doors and proper ramps for wheelchair access. The west lobby, called Lenfest Hall, has been given larger windows, and been denuded of the postmodern ticket booths designed by the museum's previous architects, Robert Venturi and Denise Scott Brown. The lobby's east wall has been torn down, and an auditorium has been ripped out to make way for a new central atrium, clad in the same honey-toned limestone that the museum's initial architects used in 1928. Here you'll see Gehry's only concession to showiness, in the form of a Piranesian switchback staircase leading to the basement level. Even that is outshone, though, by the splendid vaulted walkway leading off from it, decked out in Guastavino tile and re-emerging after decades as back of house.