## 5 Conclusions  
Modular bridge substructures comprise numerous connections between modules, with bracing members integrating CFT columns to enhance seismic performance. This study proposed an experimental program that addressed the issue of low-cycle fatigue failure in bracing connections, including component and full-scale tests. The following conclusions were drawn from the test results:  
1. The connection plates welded to the CFT columns must be made more flexible by extending the plate length to prevent premature fracture of the steel tubes at the welding site.
2. The moment-rotation curves derived from the component tests can be utilized to design a modular pier with a dog bone-shaped flexible bracing member, which offers greater strength compared with stiffer bracing. The plastification location in the connection can be designed by altering the details to achieve the desired behavior.  
Figure 17: Comparison of bracing behavior of SMP and FMP.  
3. Enhanced connection details significantly decreased the stress concentration at the welded section of the connection, improving the energy absorption capacity of the modular pier. The full-scale modular pier demonstrated lower strain values compared with the previous stiffer bracing connection specimen, allowing for the initiation of the yielding of the bracing members before the CFT columns without premature fracture. Therefore, the seismic performance of the modular pier can be designed by adjusting the bracing members and their connection details.  
Although the effects of size and material properties on CFT columns are not considered in this study, their influence is undoubtedly significant. While many existing studies have investigated these effects on single CFT columns, it remains uncertain whether their findings are applicable to the complex behavior of the multi-column CFT piers and bracing connections presented in this study. In addition, this study presents a rough connection design, and the results may vary depending on the materials used and the connection details not considered. For example, the same steel grade was used for the bracing and CFT columns in this study, but if different steel grades were used, the connection stress development patterns may be different. In order to present a connection detail-design standard that reflects various actual field conditions, an in-depth study on the connection details of CFT columns for piers should be conducted. Therefore, future research is needed to verify these effects for rational design.  
Conceptualization, D.K.; methodology, D.K.; validation, D.K. and C.-H.J.; formal analysis, D.K. and C.-H.J.; investigation, D.K. and C.-H.J.; resources, D.K. and C.-H.J.; data curation, D.K. and C.-H.J.; writing--original draft preparation, D.K.; writing--review and editing, D.K. and C.-H.J.; visualization, D.K. and C.-H.J.; funding acquisition, K.-T.P. All authors have read and agreed to the published version of the manuscript.  
This research was carried out under the KICT Research Program (project no. 20240142-001, Development of DNA-based smart maintenance platform and application technologies for aging bridges) funded by the Ministry of Science and ICT.  
Not applicable.  
All data, models, and code generated or used during the study appear in the published article.  
Author [PERSON] was employed by the company DL E&C Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.