Many heritage structures do not fulfil requirements of present standards. Decisions about adequate construction interventions should be based on complex assessment of structural reliability. Such an assessment should take into account new utility plans, actual material properties and environmental influences, including degradation processes, satisfactory past performance and advanced modelling reflecting properties of historic materials that are often distinctly different from those assumed for

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... current construction materials. The submitted contribution focuses on reliability assessment of balcony girders of the Estates Theatre in Prague, listed as a Czech national heritage monument. Preliminary reliability assessment, based on conservative recommendations of standards for existing structures, reveals that the resistance of the wrought steel girders from the 19th century is insufficient. This is why a series of non-destructive tests supplemented by a single tensile test are performed in order to obtain information about the homogeneity of the historic material, its strength and modulus of elasticity. The results of the tests are then evaluated using statistical methods. The design values of basic variables are estimated, considering uncertainties in material properties, geometry and resistance models. In addition, chemical analysis is performed to identify material composition and verify a type of historic steel.