Seismic Assessment and Retrofitting of an Under-Designed RC Frame Through a Displacement-Based Approach

Introduction

Several reinforced concrete (RC) buildings were designed according to older Codes in high-seismicity European countries. These structures are very likely to experience severe damage or collapse during moderate seismic events. Therefore, simplified procedures to evaluate the seismic performance of existing and retrofitted buildings are needed, along with reliable and effective seismic upgrading techniques, (CEB-FIB, 2003; ATC, 1996; FEMA, 2000; Fajfar, 1999, 2000; Fajfar et al. 2005; Masi, 2003; Fardis et al., 2013; Rozman & Fajfar, 2009; Antonopoulos & Triantafillou, 2002; Valente, 2012, 2013a, 2013b). This study presents the results of numerical investigations on the seismic assessment and retrofitting of a RC structure, designed mainly for gravity loads without specific earthquake-resistant provisions, that was pseudo-dynamically tested at the JRC ELSA laboratory in Italy. Detailed numerical models were developed to reproduce the seismic response of the RC structure in the original configuration and non-linear dynamic analyses were performed. A displacement based procedure using non-linear static pushover analyses was used to evaluate the seismic performance of the original structure and the most relevant results are discussed. The effectiveness of the use of different approaches for the seismic performance assessment of other typologies of existing structures is presented by the authors in Milani and Valente (2015a, 2015b) and Valente and Milani (2016a, 2016b).

The displacement based procedure is also used for a preliminary design and evaluation of viable retrofitting interventions. In particular, three alternative retrofitting solutions for the RC structure are proposed and verified through a displacement based approach.

The first intervention scheme was defined on the basis of the results of the numerical analyses on the bare frame, adopting two different design strategies, like strength-only and ductility-only solutions. These selective interventions were applied to different members of the structure in order to improve its global and local seismic behavior. A strength-only intervention using RC jacketing was implemented in the strong column at the third and fourth stories of the structure to reduce the large difference in terms of flexural capacity. Moreover, a ductility-only intervention was accomplished at the first three stories of the structure, where a large inelastic deformation demand was expected. The intervention required the arrangement of external fiber reinforced polymer (FRP) composites, which provided an increase in concrete confinement of the columns.

The second retrofitting intervention was based on the addition of steel bracing, which can be considered as a very effective method for global strengthening of buildings, (Youssefa et al., 2007; Mazzolani, 2008; Di Sarno & Elnashai, 2009; Giannuzzi et al., 2014). In particular, the use of eccentric steel bracings in the rehabilitation of existing RC structures is efficient in limiting inter-storey drifts and can provide a stable energy dissipation capacity.

The third intervention was carried out by casting a concrete shear wall to the full width of the short bay of the frame. This solution leads to significant increases in overall strength and stiffness for the retrofitted frame, when compared to that of the initial frame configuration. This intervention is efficient in controlling global lateral drift and thus reducing damage in structural members.

The efficacy of the three retrofitting solutions adopted for the RC structure was evaluated by non-linear dynamic analyses.