On the seismic response of existing RC frames retrofitted by combining member- and structure-level techniques

Seismic assessment analyses of existing reinforced concrete (RC) structures generally unveil the weaknesses of their response under earthquake induced actions and, moreover, represents the basis for designing appropriate retrofitting solutions. Retrofitting techniques can be grouped into two broad classes collecting the so-called member-level and structure-level techniques. On the one hand, the former are primarily aimed at enhancing strength and/or ductility of single members of the existing structures by means of alternative technical solutions, such as confinement, jacketing and so on. On the other hand, the latter reduce the seismic demand on the existing structure by introducing new members and/or sub-structures, such as shear walls or steel bracings that work in parallel with the existing structural system. Although several researches have been carried out for selecting the most efficient and cost-effective solution for seismic retrofitting, the definition of a rational strategy for obtaining the optimal retrofitting solution is still an open issue. However, any optimisation procedure is based on the definition of appropriate objective functions which should be considered to define the optimal solution, possibly on a multi-criteria basis. This paper is intended as a contribution to highlight the multi-faced aspects of the seismic response of existing buildings possibly retrofitted by combining member- and structure-level techniques and their importance in the possible definition of a multi-criteria optimisation strategy for retrofitting existing RC structures. With this aim, the conceptual strategy proposed is based on the three following assumptions [1]: - structure-level techniques do not modify the displacement capacity of existing structures; - the structural capacity can only be increased by means of member-level techniques; - the existing structure and the substructure representing the structure-level technique under consideration are described by two independent capacity curves: they work in parallel and define the capacity curve of the retrofitted structure. In this paper a case study and the key parameters which define alternative retrofitting solutions are described. An existing four storey structure, designed for only gravitational loads, is considered. The strengthening intervention of the structure was conceived by considering X-shaped steel bracings (as a structure-level technique) and FRP-based confinement of columns (as a member-level technique). Three combinations of such interventions are examined in this study: - Solution N.1 considers only the structure-level technique and the structure is retrofitted with steel bracings characterised by a sufficient lateral stiffnees; moreover, columns near the steel bracings are confined with steel members at 1st, 2nd and 3rd storey; - Solution N.2 is based on a combination of member- and structure-level techniques: indeed, the steel sub-structure is designed for developing stiffness lower than the Solution N.1 and a greater ductility is obtained by FRP-confining of the columns with insufficient ductility; - Solution N.3 is only based on FRP-confinement to obtain a global capacity greater than demand. Finally, the results of a series of nonlinear time history analyses aimed at pointing out different aspects of the seismic response of the alternative retrofitting solutions are outlined and discussed.