Rat-trap bond brick masonry is commonly found in historic and heritage structures. During reinforcement, achieving complete unloading is often impractical, making it necessary to consider the impact of existing load. Reinforcement of brick masonry with steel plates offers advantages such as ease of construction, safety, reliability, and good reversibility. This study presents axial compression tests on six rat-trap bond brick masonry specimens (including one unreinforced specimen, one steel plate-reinforced specimen without preload, and four steel plate-reinforced specimens under preload). The results demonstrate that steel plate reinforcement effectively enhances the load-bearing capacity and ductility of rat-trap bond brick masonry while improving its failure mode. The preload ratio, defined as the ratio of the axial compressive load applied at strengthening point to the peak load of the unreinforced specimen, is found to have a certain impact on the reinforcement effectiveness. Additionally, increasing the steel plate thickness further enhances load-bearing capacity; however, compatibility must be taken into consideration. A finite element model was developed based on the experimental results and extended for further analysis. Furthermore, an axial load-bearing capacity formula for rat-trap bond brick masonry reinforced with steel plates under preload is proposed, incorporating existing calculation theories for steel plates and masonry.
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