Rehabilitation of existing structures is an issue of a great economic significance. Many existing structures, including industrial heritage buildings and bridges, do not fulfil the requirements of currency valid standards. They have been designed and executed according to past standards and are often affected by severe environmental influences that may cause deterioration and gradual loss of reliability. The assessment of existing structures differs from the designing of new structures primarily due to increased cost of safety measures, different remaining working life and additional information on actual structural conditions that may be obtained from inspections, tests and measurements on a finished structure. These differences need to be adequately reflected in reliability verification. At present, existing structures are mostly verified using the partial factor method, commonly accepted for the designing of new structures. However, such assessments are often conservative and may lead to expensive upgrades. The explicit consideration of uncertainties related to the most significant parameters and the reflection of new information may be of great importance, and may lead to significant economic benefits. In general, probabilistic reliability analysis provides an effective tool for assessment, particularly owing to better description of uncertainties related to the assessment and straightforward reflection of in-situ observations and information about the satisfactory past performance of the structure. While numerous principles and methods are common to reliability analyses of new and existing structures, some of them are significantly different. These include the updating of models of basic variables and failure probabilities considering outcomes of inspections and knowledge about survived load effects, and specification of appropriate target reliabilities.
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