Keywords: seismic retrofitting, reinforced concrete building, irregularity, nonlinear static analyses, nonlinear dynamic analyses.

This paper presents a seismic retrofitting strategy for improved strength and ductility of under-designed plan-wise irregular reinforced concrete (RC) buildings. A mixed retrofitting intervention based on both fibre reinforced polymer (FRP) wrapping and RC jacketing applied to selected columns was proposed and investigated by numerical analyses with the aim of enhancing the seismic performance of a four-storey plan-wise irregular RC building designed for gravity loads. Retrofitting was aimed at both reducing the torsional component of the seismic response and improving the local and global ductility of the structure. A displacement-based procedure using nonlinear static pushover analyses was adopted to assess the seismic performance of the structure in the original configuration and to select the retrofitting intervention. As a result of the asymmetry of the investigated structure, appropriate correction factors were computed in order to take into account the effects of torsion. The retrofitting intervention reduced the irregularity of the structure and the global response could be more accurately captured using pushover analyses. The simplified procedure pointed out that the retrofitted structure was able to withstand the displacement demand and thus to satisfy the limit state of significant damage.

Nonlinear dynamic analyses were carried out to verify the effectiveness of the retrofitting intervention strategy. The combination of the two approaches applied to selected columns significantly improved the seismic performance of the structure. The retrofitting intervention increased the stiffness of the structure and reduced the maximum inter-storey drifts. A decrease of the storey rotation was observed for the retrofitted structure compared to the bare counterpart. The demand-to-capacity ratio (DCR), i.e. the ratio of the chord rotation demand to the chord rotation capacity, was used to evaluate the damage level of the columns and to identify the most critical columns affecting the seismic performance of the structure. The maximum chord rotation demand was obtained by numerical analyses and the chord rotation capacity was computed according to Eurocode 8 [1]. In the original configuration high DCR values were registered for the columns located at the flexible edges (higher demands) and for the columns with higher axial loads (lower rotational capacity). A considerable decrease of the DCR values was observed for the columns of the retrofitted model, because the deformation demand was reduced and the columns were detailed for ductility as a result of the high level of confinement provided by the FRP wrapping.

1

CEN European Standard EN 1998-3, "Eurocode 8: Design of structures for earthquake resistance. Part 3: Assessment and retrofitting of buildings", European Committee for Standardization, Brussels, 2005.

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