Partial factor methods for existing concrete structures

Abstract

For a large proportion of existing buildings and infrastructure, the design life has been reached or will be reached in the near future. This is because a huge part of the existing stock was built in the 1960s. These structures might need to be reassessed in order to investigate whether safety requirements are met. However, current practice on the assessment of existing concrete structures needs a thorough evaluation from a risk and reliability point of view. At present, existing structures are mostly verified using simplified procedures based on the partial factor method that are commonly applied in the design of new structures. Such assessments are often conservative and may lead to expensive upgrades. More realistic verification of the actual performance of existing structures can be achieved by probabilistic methods describing the uncertainties of the load and resistance variables with appropriate probabilistic models. Although reliability-based assessment of existing concrete structures has gained wide attention in the research field during the last decades, a consistent reliability-based assessment framework and a practically applicable codified approach that is compatible with the Eurocodes and accessible for common structural engineering problems in everyday practice is currently missing. Such an approach would allow for a more uniform, more objective and probably more widely applied assessment approach for existing concrete structures. In order to achieve such a framework, and considering that some new challenges had to be tackled that had not been covered by previous research efforts, in 2011 the former fib Special Activity Group 7 ‘Assessment and Interventions upon existing structures’ initiated Working Group 2A ‘Semi-probabilistic evaluation approach’ with the task of proposing technical guidelines on partial factor methods for existing concrete structures. In 2015 fib Special Activity Group 7 was reformed into fib Commission 3 on ‘Existing concrete structures’ and the task to finalise the bulletin – which was in an editorial phase – was included into the new working plan of its Task Group 3.1 ‘Reliability and safety evaluation: full-probabilistic and semi-probabilistic methods for existing structures’. It should be noted that this bulletin is based on and is consistent with the following background information: – The general principles of the assessment of existing structures provided by the JRC report (Luchinger et al. 2015) are adopted. – General guidance on the derivation of partial factors for concrete structures is provided in CEB Bulletin 128, CEB Bulletin 191 and CEB Bulletin 219. – The theoretical procedures of the probabilistic assessment of existing structures given by the JCSS publication (Diamantidis et al. 2001) are adapted v to provide operational guidance for semi-probabilistic assessment of existing reinforced concrete structures. – Probabilistic methods and recommendations on probabilistic modelling provided by the Joint Committee on Structural Safety (JCSS PMC 2001) are followed; however, appropriate modifications are proposed when the distinction between the design of new structures and the assessment of existing structures needs to be made.