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INTRODUCTION

Structural fragility/reliability has been used for assessing seismic risk in constructed facilities such as highway bridges. Fragility (also known as vulnerability) is defined as a conditional probability that a structure exceeds a prescribed damage state when subjected to various levels of natural or man-made hazards. Reliability is, on the other hand, defined as the extent of achieving the given functionality of a structure exposed to distressful hazards. Previous studies have shown that use of either empirical or analytical vulnerability functions can be regarded as one of the standard approaches for seismic fragility estimation [1-9], while analytical vulnerability functions are dominantly employed because in-situ data are limitedly available in most cases [1-6]. Conventional methodologies for generating analytical vulnerability functions include the statistical extrapolation of a structure's performance along with Monte Carlo simulation (MCS) [2-6]. Seo et al. [5] estimated the structural fragility of steel momentframe structures using MCS-based response surface metamodels (RSM). The joint RSM-MCS method enables the efficient fragility assessments of a group of steel moment-frame structures when compared to the conventional methodologies. Ghosh et al. [6] utilized metamodels combined with MCS to evaluate existing bridges subjected to seismic load. Although the concept of structural fragility and corresponding reliability are proven to be robust in evaluating the performance of civil structures, it has not fully been integrated into the resiliency appraisal of retrofitted structural members.

Over the past couple of decades, fiber reinforced polymer (FRP) composites have been used for enhancing the behavior of deteriorated reinforced concrete members, including several advantages such as favorable strength-toweight ratio, non-corrosive characteristics, and reduced long-term maintenance costs [10]. A number of studies on FRP-strengthening were concerned with the performance evaluation of various structural systems in static, fatigue, and seismic loadings [11]. The reliability-oriented assessment of such a strengthening method was, however, rarely reported, particularly structural fragility accounting for critical failure modes such as FRP-debonding. This paper proposes a theoretical framework for examining the debonding vulnerability of reinforced concrete beams strengthened with FRP sheets. Of interest is a relationship between FRP-debonding and performance reliability. An RSM model was built using a large number of laboratory test data compiled from published literature, encompassing geometric and material parameters, in order to predict the effective strain of FRP. It is worth noting that the effective strain controls the response of a strengthened beam in such a way that FRP-debonding failure takes place when the strain of the strengthening system exceeds its effective strain. The experimentally validated RSM model coupled with MCS was implemented to generate the debonding fragility of FRP-strengthened beams, which was compared with a conventional fragility approach, followed by quantifying performance reliability.