Citation: Vo, D.H.; Do, V.H.; Tran, Q.V.; Nguyen, M.H.; Hoang, T.L. Reliability Analysis of Steel Bridge Girders Strengthened with CFRP Considering the Debonding of Adhesive Layer. Designs 2022, 6, 126. https://doi.org/10.3390/ designs6060126 Academic Editor: José António Correia Received: 23 November 2022 Accepted: 14 December 2022 Published: 15 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Reliability Analysis of Steel Bridge Girders Strengthened with CFRP Considering the Debonding of Adhesive Layer Duy Hung Vo , Viet Hai Do * , Quang Vy Tran, Minh Hai Nguyen and Trong Lam Hoang Faculty of Road and Bridge Engineering, The University of Danang–University of Science and Technology, 54 Nguyen Luong Bang Str., Danang 550000, Vietnam * Correspondence: dvhai@dut.udn.vn Abstract: One issue to consider while designing and constructing steel girders reinforced with carbon fiber-reinforced polymer (CFRP) plates in bridges is debonding failure. Previous studies showed that the parameters such as characteristics of material, load, adhesive, and CFRP plates have an effect on the failure probability of steel girder, which is represented by the reliability index. Therefore, this study analyzes the reliability indices of steel girders in bridges strengthened with CFRP plates to clarify the effects of debonding failure. Debonding and strength limit states are used to compare differences in reliability indices of different design scenarios. Strength and debonding margin the functions for the strength limit state and debonding limit state will be established in this study. The probability of failure is determined by a Monte Carlo simulation (MCS). It is found that the reliability index of debonding limit state is much lower than that of the strength limit state. This shows that the debonding failure should be considered significant in the reliability analysis of steel bridge girders strengthened with CFRP plate. Keywords: reliability index; steel bridge girders; adhesive layer; CFRP plate; debonding limit state; strength limit state 1. Introduction An increasing number of bridges are being rehabilitated using Carbon Fiber Reinforced Polymers (CFRP), which also extends their lifetime [1,2]. The primary benefits of utilizing CFRP plates are the project’s overall cost savings, high strength, lightweight construction, and longevity [3]. As a direct result of this, a significant amount of work [4–16] has been carried out to evaluate the potential applications of carbon fiber-reinforced plastic (CFRP) in repairing, recovering, and reinforcing steel girders. Currently, the properties of these plates have been significantly improved, especially since the CFRP elasticity- modulus is approximately twice as high as that of structural steel. Additionally, the stiffness can be increased dramatically by using various dimensions and strengths [17]. The debonding of the CFRP plate ends regarded as a significant failure mode, and its occurrence should be prevented in order to avoid an unwanted failure of the system. The debonding strength of steel beams reinforced with CFRP plates has already been studied in a number of studies [18,19]. Therefore, one of the most important things for the design of CFRP-reinforced steel bridge girders is to consider the bonding between CFRP, adhesive, and steel surfaces. In general, the adhesive layer features substantially impact debonding occurrence. In the research carried out by Stratford and Cadei [20], a computation was made to determine the maximum stress occurring in the adhesive layer at the point where the specimens failed. When debonding occurs suddenly, CFRP-strengthened steel girders lose their stiffness and strength capacity. In fact, debonding is one of the most common failure mechanisms relating to steel bridge girders strengthened with CFRP plates. This occurs as a result of the fact that the CFRP plate does not entirely adhere well to the steel bridge girder. Meng Liu and Mina Dawood conducted a reliability analysis for steel beams reinforced with Designs 2022, 6, 126. https://doi.org/10.3390/designs6060126 https://www.mdpi.com/journal/designs