Conceptual design formulation and comparative assessment of a seismic retrofit procedure for existing RC structures based on combining member-and structure-level techniques

Several reinforced concrete structures built in Europe in the first decades after WWII are seismically deficient, as they were generally built without taking into proper consideration the seismic action. Therefore, they generally lack in both global conception and structural details. Consequently, nowadays seismic retrofitting of existing structures is a task of common relevance for structural engineers. In general, such challenge could be addressed combining member- and structural-level techniques, although no general conceptual design strategy is defined with the aim to support engineers in selecting the best retrofitting solution, as several combinations of the two aforementioned techniques are theoretically possible to achieve the target structural safety levels. The present paper proposes a general conceptual design procedure that derives from approaching the seismic retrofitting issue as a constrained optimization problem controlled by three design parameters, which describe the memberand structure-level interventions, and two constraint relationships, the latter being related to meeting the performance objectives at both serviceability and ultimate limit states. Based on this general formulation, three alternative options are considered for the seismic retrofitting of an RC frame assumed as a case study: they are based on either adopting only structure- or member- level techniques, or combining them according to aforementioned conceptual design procedure. Finally, a comparative assessment of the three solutions is present with the aim to highlight pros and cons of each one of them. More specifically, various relevant aspects, such as codetype checks, reliability analyses and the evaluation of the possible environmental impacts deriving from the implementation of the three alternative solutions, are taken into consideration. The preliminary results discussed in the present paper show that, generally speaking, a proper combination of member- and structure-level techniques may lead to a more cost-effective solution, although it might not represent the "optimal" solution from the environmental standpoint.