After a decade of museum design and other civic commissions being dominated by starchitects such as Frank Gehry, Daniel Libeskind, and Santiago Calatrava, two of the most sought-after architects at the moment, Tod Williams and Billie Tsien, project little in the way of ego, either in person or in their buildings.

Last month, their firm, Tod Williams Billie Tsien Architects, won the huge and complex job of designing the Barnes Foundation’s new home in Philadelphia. Here in New York, they designed the 53rd Street home of the American Folk Art Museum, which opened in 2001. In the coming months, plans will be released for two of their other commissions: the redesign of the Harmony Atrium, a public galleria that extends from Broadway to Columbus Avenue, between 62nd and 63rd Streets, and the new $60 million Lakeside Center, which will replace Prospect Park’s decrepit Wollman Rink.

While starchitects’ work tends to be intellectual and theory-driven, Mr. Williams’s and Ms. Tsien’s is emotional and sensuous. Rather than seeking big-budget commissions that will allow them to make dramatic, self-expressive statements, Mr. Williams and Ms. Tsien favor projects that pose certain problems or limitations. In the case of the Barnes, a judge ruled that the foundation could move to Philadelphia, in violation of its founding charter, only if the art is hung on the walls in the same configuration as in the foundation’s original home in Merion, Pa. This means that each gallery in the new building must be dimensionally identical to its counterpart in the old building.

Mr. Williams said, in a recent joint interview in the firm’s studio on Central Park South: “We’re interested in using our skills to make something that both resolves the problem and transcends it.” He added: “In some ways, the knottier the problem, the more powerful the result.”

They did not present an actual design to the Barnes Foundation’s board, just their ideas and their philosophy, which was enough for the board to decide the chemistry was right. The chairwoman, Aileen Roberts, said the board was impressed by their sensitivity to clients’ unique needs.

“They’re really hearing you and addressing your issues, and,” Ms. Roberts said, perhaps alluding to certain recent trends in museum design, “your issues tend to not be sculptural.”

A former trustee of the American Folk Art Museum, who was the chairman of the building committee, Sam Farber, said that, toward the end of its search process, the committee narrowed its search to three firms, each of which they asked to make a presentation.

“The others brought models,” he said. “What impressed us about Tod and Billie was they came in and said, ‘We really can’t tell you what we’re going to build, though we can tell you what kind of building we’d like to build. But we think it’s very important to talk to people at the museum [first], about what they think their museum is,'” he said. “They talked about their philosophy and didn’t make one drawing, and that really impressed us.”

Of course the boards of Barnes and of the Folk Art Museum visited several of the firm’s completed buildings, and their positive assessments of these played a major role in their decisions.

There is, though, something magical about the experience of listening to Mr. Williams and Ms. Tsien talk — something that makes you want to walk with them through a museum and hear them rhapsodize on the combination of sights and textures and smells. They say they are more often inspired by material culture and the natural world than by formal ideas. They collect a variety of art, from folk and “outsider” art to work by the contemporary Ghanaian sculptor El Anatsui. They also collect objects that are not art but which they find beautiful, such as a mastodon tusk, or a 150-year-old piece of African money, or a highly simplified African chair.

“We like the idea that something can be very beautiful and functional at the same time,” Ms. Tsien explained.

The president of Lincoln Center, Reynold Levy, said that part of the fun of working with them is trying to figure out who contributes what. “I don’t know who’s in charge of the melodies and who’s in charge of the words, but together they do something beautiful,” he said.

In their studio, Mr. Williams and Ms. Tsien pointed to a conference table they built together 20 years ago. Mr. Williams designed the geometrical base, made of anodized aluminum, and Ms. Tsien designed the glass top, in which a process of sandblasting produced a subtle, vaguely organic pattern of etchings. “Billie’s sensibility moves more toward the graphic, and mine more toward the tectonic,” Mr. Williams said.

Ms. Tsien, who is six years younger than Mr. Williams, came to work for him in 1977, after getting her architecture degree at UCLA. She had studied fine art as an undergraduate at Yale but found it “too subjective,” she said. She liked that architecture came with clear criteria on which to judge her work, and that it involved doing something for someone else. Soon after she came to work with Mr. Williams, they started dating; later, they married and she became a partner in the firm.

Today, the firm employs around 30 people. They work on about 10 projects at once, each of which lasts an average of three years. Besides their institutional projects, they usually have one or two residential projects and one or two short-term art projects. They designed the installation for the recent Louise Nevelson show at the Jewish Museum, and are currently designing an installation for SITE Sante Fe, a contemporary art space.

For the Barnes, Ms. Tsien said that they want to stay true to the institution’s identity as an educational foundation, rather than a museum. Asked how that translates into the architecture, she said: “Today, when museums are built, the first thing is commerce: the store, the café.”
“We accept that they are necessary,” Mr. Williams said. “The question is: Are they front and center?”

At the Harmony Atrium, which is a privately owned public space now being managed by Lincoln Center for the Performing Arts, Mr. Williams and Ms. Tsien have had to wrestle with competing interests. Lincoln Center, which commissioned their design, wants the atrium to serve as a kind of entry point for Lincoln Center audiences, with a centralized discount ticket facility and information about current programming. But the space is also intended to serve the local community. The firm’s project architect, Felix Ade, said that the landmarks commissioner, Amanda Burden, rejected the first plans as oriented too much around Lincoln Center.

Mr. Williams and Ms. Tsien’s final design incorporates their interest in nature. There will be two large “green walls,” covered with plants, and areas of falling water. The stone floor will be bluish, and there will be benches made of moss-green marble. “We see them as islands in a stream — floating green islands,” Ms. Tsien said.

The roof will contain recessed skylights with spotlights hidden in them. The space will “be bright and welcoming but also have the ability to change its mood,” Mr. Williams said.

Lakeside Center, a project of the nonprofit Prospect Park Alliance, is to be a 38,000-square-foot recreation facility, with two skating rinks, a café, gift shop, lockers, and space to rent skates and pedal boats for use on the nearby lake.

An uncovered rink will be used for free-form skating in the winter and as a water garden in the summer. The other rink, which is intended for hockey in the winter, will be covered, but open on the sides so that, as Mr. Williams noted, skaters “will be able to smell the pine trees.” In the summer, it will be available for other sporting events, weddings, and film screenings.

Assuming the Barnes Foundation’s plans survive the latest legal challenge, that commission may turn Mr. Williams and Ms. Tsien into architectural celebrities. Before that, though, New Yorkers will get to know them through their projects here: public spaces that are beautiful and functional, and elevating and humble, at the same time.