Turkish Airlines uses the Ataturk Airport in Istanbul for flights and maintenance operations. The company asked for a seismic performance evaluation for one of its hangars at the Istanbul Ataturk Airport. The hangar’s structural system consisted of a 60m cantilever reinforced concrete shell roof carried by steel cables and supports. A performance evaluation revealed that the roof system did not have adequate stiffness in the orthogonal direction to the span. In order to limit the lateral displacement of the roof, additional damping by means of viscous dampers was presented as a retrofit solution. Miyamoto Turkey planned to attach the dampers between the roof and construct additional reinforced concrete shear walls on both sides of the roof system.