## 2 Concept of the Modularized Steel Plate with Bolt Connections  
A previous study [24] introduced a novel retrofitting technique for RC beams utilizing modularized steel plates. This method entails encasing the RC beam with a steel plate, which establishes a void between the beam and steel plate that is subsequently filled with non-shrinkage mortar to address concerns regarding mortar shrinkage and to ensure workability during construction. This approach amalgamates the concrete jacketing technique with a retrofitting method employing steel plates or FRP. The jacketing procedure augments the rigidity and strength of the existing RC beam, whereas the steel plate bolsters its ductility. Figure 1 is a schematic representation of the components associated with modularized steel plates. The proposed technique is conceptualized in a singular-unit format to simplify construction. A single unit comprises a Z-shaped side plate, an L-shaped lower plate, and a bottom plate. The Z-shaped side plate, affixed to the RC beam side, enhances its ductility and strength. Each lower plate is L-shaped, comprising two pieces that form one component. Therefore, since lower plates are attached at both ends of the beam width, if the beam width changes due to construction errors, they can be attached according to the beam width, accommodating for construction errors. These plates are interconnected using bolts, with their dimensions determined by the intended enlargement section, specifically the depth of the newly added beam. The bottom plate enveloping the base of the added beam ensures ductility, especially since it resides in the tension zone. This plate is equipped with a vertically welded grid, serving as the tensile reinforcing bar and improving bond capacity in the added beam, seamlessly integrating into the slot at the base of the Z-shaped side plate. The statement that vertically welded grids can serve the function of tensile reinforcing bars within the added beam was supported by the findings derived from the author's previous research [24]. The bond between the steel plate and the existing RC beam is realized using chemical anchors. For ease of handling, the modularized steel plate retrofitting approach is segmented into units, each reinforcing the RC beam. A previous study [24] observed stress concentration at the interconnections of units when subjected to flexural load. To rectify this, the current research introduces an enhanced detail to fortify the inter-unit joints, as illustrated in Figure 2. This novel detail is a joint steel plate connected to the Z-shaped side plate, with Z-shaped plates of adjacent units bolted together. Contrary to the initial joint design, which solely anchored the Z-shaped side plate to the RC beam without inter-module connections, the revised method interlinks the Z-shaped side plates, ensuring cohesive behavior across modules. This enhancement retains the original modularized steel plate components, presenting the benefits of straightforward design and fabrication. The construction procedure for the modularized steel plate is sequentially delineated below. Notably, the construction solely relies on chemical anchors or high-strength bolts, obviating the need for specialized labor. This not only results in cost savings but also expedites the construction process due to its simplicity. 1. The surface preparation of the concrete, for example, removing surface contaminants, is conducted, and later, the L-shaped lower plate, with its protrusion, is affixed to the RC beam underside utilizing chemical anchors;
2. The L-shaped lower plate and the Z-shaped side plate are interconnected using high-strength bolts. The bottom plate is slotted into the designated groove at the Z-shaped side plate base. After aligning the bottom plate with the Z-shaped side plate, they are secured using high-strength bolts;  
Figure 1: Proposed modularized steel plate [24]. Figure 2: Proposed connection detail. 3. The Z-shaped side plate is anchored to the concrete beam side via chemical anchors;
4.