Frank Lloyd Wright’s Solomon R. Guggenheim Museum is undergoing restoration of its exterior to fix cracks and other structural defects, using techniques and materials unavailable during the architect’s lifetime.

The Guggenheim Foundation said the findings of an exhaustive two-year condition assessment of the 1959 edifice concluded that it is in good condition, “requiring only limited structural interventions.” Those include filling exterior cracks, exposing and treating corroding steel, and repairing and protecting all concrete work.

The restoration work, which began several months ago, is projected to cost $29 million and be completed in time for the Manhattan museum’s 50th birthday celebration in 2009.

The assessment process included removing 11 coats of exterior paint, revealing hundreds of cracks, primarily from seasonal temperature fluctuations.

“Frank Lloyd Wright, who was always at the cutting edge of technology and frequently ahead of his time, is also notorious for the failures of his buildings, often structural in nature,” an architect with Wank Adams Slavin Associates LLP, one of several firms involved in the assessment, Pamela Jerome, said in a statement released by the museum yesterday. “Although some remedial reinforcing is necessary, our investigation revealed that Wright’s radical design for the Guggenheim was irreproachable.”

The foundation said all decisions regarding the restoration had to consider the iconic building simultaneously as an art form, architecture and structure.

“Structurally this presented several challenges that required creative solutions, including the use of carbon fiber fabric” as a reinforcing material that resists temperature and wind, it said. “Wright was a proponent of new construction methods and materials so it is believed he would approve of the use of this material.”

The museum remains open during the restoration; much of its interior was restored in 1992.