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This paper presents the flexural behavior of notched steel beams repaired with carbon fiber-reinforced polymer (CFRP) strips. A combined experimental and computational approaches are used to examine local plasticity near the damage and the effects of CFRP-repair. A modeling approach is proposed to take into account the bond-slip behavior of CFRP-steel interface. The experimentally validated models are further used to conduct a parametric study addressing various engineering properties of CFRP composites and adhesives. The CFRP-repair is shown to restore the strength of the damaged beam. The CFRP strip relieves the stress concentration resulting from the presence of the notch, reducing the high local plasticity. The parametric study confirms the improved effectiveness of high modulus CFRP (i.e., exceeding 150 GPa) in affecting repairs of steel members. Under static loading conditions, the stiffness of the adhesive bond line influences the local behavior of the CFRP-steel interface but has little effect on the overall member behavior.

Due to its high load-bearing capacity, high-strength steel is progressively more used in present engineering applications. However, the current design codes methodology for the lateral-torsional buckling (LTB) ultimate capacity of laterally unrestrained steel beams is based on experiments limited to room temperature and mild steel. A numerical analysis of steel beams’ ultimate bearing capacity in bending was carried out to evaluate the lateral-torsional bending of different high-strength steels, namely, Q460, Q690, and Q960 steel at high temperatures. As a comparison, the lateral-torsional bending of Q235 steel beam was also studied. Finite element models (FEMs) were generated using Abaqus with taking into account initial geometrical imperfection and residual stress. The validation of the FEMs has been established by comparing the ultimate flexural resistances obtained from previous tests with those from FEM. A number of parametric studies were taken to investigate the effects of different factors such as various span lengths, residual stress, steel grades, temperatures, and cross-section size on the flexural-torsional buckling (FTB) behavior of laterally unrestrained steel beams. Furthermore, the overall bending capacity of laterally unrestrained steel beams has been shown to be deteriorating at high temperatures. In addition, based on findings from the FEMs, design curves were proposed for evaluating the FTB of high-strength steel beams at elevated temperatures. The results were compared with the design curves given by design codes of GB 51249-2017 and EN 1993-1-2.

The mechanical characteristics of high-strength steel (HSS) at high temperatures vary significantly from those of mild steel, so fire design regulations derived from mild steel cannot be transferred to HSS structures. Therefore, this paper investigates the fire resistance of beams made of HSSs, which will facilitate the design and application of HSS structures. Due to the heat dissipation of the concrete floor, steel beams are generally heated from three sides, and the temperature distribution in the steel beam cross-section is nonuniform. For such cases, the fire resistance of HSS beams is assessed using finite element models (FEMs) with nonuniform temperature distribution. A Ramberg–Osgood model was used to predict stress–strain relationships of Q460, Q690, and Q960 HSSs at elevated temperatures, and the model was verified by material tests. In the FEM, the proposed stress–strain relationships of HSSs at elevated temperatures, initial geometric imperfections and residual stress are considered for better accuracy. After verifying the established model against experimental results, the influence of nonuniform temperature distribution, load pattern, steel grade, and slenderness ratio on the resistance of HSS beams is investigated by conducting parametric analyses using the verified FEM. It is found that the critical bending moment of steel beams at elevated temperatures associated with lateral-torsional buckling highly depends on the reduction of elastic modulus of steel. By comparing FEM results with EN 1993-1-2, it is evident that EN 1993-1-2 is unsuitable for assessing the fire resistance of HSS beams. Therefore, the expression specified in EN 1993-1-2 is modified by fitting the finite analysis results to more accurately evaluate the fire resistance of HSS beams considering the nonuniform temperature distribution.