Seismic evaluation of ordinary RC buildings retrofitted with externally bonded FRPs using a reliability-based approach

Abstract

Despite the extensive literature on reinforced concrete (RC) members retrofitted with fiber-reinforced polymer (FRP) composites, few studies have employed a reliability-based approach to evaluate the seismic performance of RC buildings in terms of their collapse capacity and ductility. In this study, the performance of a poorly-confined RC building structure is investigated for different FRP retrofitting schemes using different configurations and combinations of wrapping and flange-bonded FRPs, as two well-established techniques. A nonlinear pushover analysis is then implemented with a computational reliability analysis based on Latin Hypercube Sampling (LHS) to determine the collapse capacity and ductility of the case-study structure. The variations in material properties and applied loads are examined using a rational probabilistic procedure. The results demonstrate the effectiveness of the reliability approach as it is capable of providing reliable and accurate comparisons between the retrofitting schemes implemented. In addition, the failure modes of the original and retrofitted frames are scrutinized for a more detailed study. It was found that the failure mode of the case-study building is remarkably dependent on the variations of both the input parameters and the adopted retrofitting scheme.

Introduction

The advantageous properties of externally bonded fiber-reinforced polymer (FRP) composites such as high tensile strength, corrosion resistance, and ease of installation have nominated them as a distinct option for seismic retrofitting of reinforced concrete (RC) structures. Over the past two decades, FRP composites have received wide acceptance worldwide and many research endeavors have conducted to explore the different aspects of their application and to address the associated challenges. The results of these studies have been beneficially employed in developing design codes and guidelines for real field applications.

In general, FRP composites can be implemented to enhance both the loading capacity and ductility of RC structures using different retrofitting schemes. Flexural retrofitting is a widely used approach that would serve not only to increase the loading capacity of members and structures but also under certain circumstances, to relocate plastic hinges in beams away from the column interface as well [1], [2], [3]. The latter capability can change the failure mode from a column-sway to a beam-sway one. A viable scheme to enhance flexural capacity is the flange-bonded FRPs in which FRP composites can be applied to critical regions of both beams and columns without any extreme damage to the slab. However, debonding is still a major issue of concern that can decrease the efficiency of such externally bonded FRP flexural retrofits. As a remedy, newly proposed anchorage systems are reliable solutions to overcome this undesirable failure mode [1], [4], [5], [6].

In an experimental investigation, Eslami and Ronagh [1] investigated the retrofitting of damaged/undamaged RC Beam-Column sub-assemblages using the flange-bonded FRP composites. Their results showed that the proposed technique when combined with a grooving anchorage system was effective in both increasing the flexural capacity and relocating plastic hinges away from the column interface. Compared to the pre-cracked control specimen, the retrofitted joints led to an increase of 30% in ultimate capacity. As expected, however, the displacement ductility factor of the retrofitted joint reportedly decreased due to the increased reinforcement ratio provided by the externally bonded FRPs.

Flange-bonded FRPs have also been used to improve the lateral behavior of an 8-story RC building by Ronagh and Eslami [7]. Comparison of the capacity curves obtained from nonlinear pushover analysis confirmed increments of 43% and 80% in the capacity of the selected building as a result of using glass FRP (GFRP) and carbon FRP (CFRP) composites, respectively. Although CFRP was observed to outperform GFRP in terms of lateral capacity, the former was associated with a lower ductility. Eslami et al. [8] used the nonlinear time-history analysis to find that CFRP flange-bonded retrofits were able to reduce the maximum inter-story drift of the same 8-story building under near-fault ground motions from 3.3% in the as-built to 2.4% in the retrofitted building.

On a par with flexural retrofitting scenario, FRP wraps can be effectively used for additional confinement in regions prone to plastic deformations. Towards this, Di Ludovico et al. [9] investigated the seismic performance of a 3-story RC building retrofitted with a combination of wrapped and web-bonded glass FRPs (GFRPs). The results obtained from their pseudo-dynamic loading showed that the retrofitted building was able to resist by around 50% higher peak ground accelerations (PGA). Further, the findings demonstrated the efficiency of FRP composites in improving the global performance of the building in terms of both ductility and energy dissipation capacity. Similar conclusions have been also reported elsewhere [10], [11], [12].

The studies cited above used deterministic procedures to investigate the performance of FRP retrofitted RC structures neglecting the fact that these structures might behave differently due to unpredicted uncertainties in loads, material properties, and geometry. In reaction to this shortcoming, some studies used reliability-based approaches in the element scale to study the performance of RC members such as beams [13], [14], [15], [16] and columns [17], [18], [19] retrofitted with externally bonded FRP composites. However, few studies were focusing on implementation of such reliability-based approaches to upgrade the performance of RC structures at the macro level.

To achieve a predefined performance in RC structures, Zou et al. [20] proposed a reliability-based algorithm to optimize the amount of FRP confinement in a building. Focusing solely on earthquake uncertainties, their study ignored structural uncertainties such as those associated with material properties in order to reduce the size of the optimization problem to a reasonable level. Using a reliability-based approach, Ali et al. [21] compared the effects of FRP confinement (in columns) with those of a combined flexural (in beams and columns) and confining (in columns) FRPs in a 3-story RC building. For these two retrofitting options, fragility curves were compared based on the maximum lateral drift of the top story. The results confirmed a notable increase (32%) in the reliability of the RC building treated with both flexural and confining FRPs, while sole application of the FRP wraps was only able to improve the reliability of the building by approximately 16%. Although their study demonstrated the effectiveness and significance of reliability assessment, little attention was paid to the collapse capacity of the building. A probabilistic procedure seems necessary when seeking an accurate judgment of the collapse capacity of the structure and a rationale comparison of the different retrofitting solutions.

The above considerations were the main motivation beyond the current study aimed at investigating the seismic behavior of ordinary RC buildings retrofitted with externally bonded FRP composites. To achieve this objective, the flexural and confining FRP composites, as the most prevalent techniques in FRP applications, were used in different configurations and combinations. As a case-study building, a 4-story ordinary RC building was considered and the results were examined in terms of collapse capacity, displacement ductility, and failure mode.