The list of potential hazards related to concrete elements and structures prestressed with the use of unbonded tendons, including the flat slabs, is long and fairly well recognized. In addition to the standard accidental events this list includes: mishandling during construction, small fire, local corrosion, loss of bond at the anchorage, second order effects, brittle fracture of elements, etc. Despite of these hazards related to unbonded post-tensioning, this type of structures are extensively promoted and used in practice thanks to the possibility of the large span floors and innovative character of this technology. The paper presents a proposal for the application of risk analysis to assess the robustness of structures with flat slabs prestressed with unbonded tendons. The adoption of variables that determine risk and robustness as fuzzy numbers assigned to linguistic variables are proposed. Numerical example is presented to demonstrate risk and robustness assessment of building structure with unbonded posttensioned slabs supported directly on columns.