## 6 Concluding Remarks  
In this study, methods for connecting modular units were investigated. The following conclusions about the structural performance and behavior of the connections were drawn through experiments on modular units connected with straight and cross-shaped connector plates.  
1. It is necessary to consider connections between 4 and 8 modular units simultaneously; connector plates, interlocking devices, and welding are currently used for such connections, but welding should be avoided to improve the interior and exterior finish of the modular units.
2. As modular structures become taller, connection methods that can be applied from within the modular units should be used to ensure construction safety. It is desirable to keep the error to within 2 mm per 100 m in height.  
Figure 10: Drift angle of upper and lower columns: (a) K-SS; (b) K-SC.  
3. For connections between modular units using straight and cross-shaped connector plates with \(125\times 125\times 9\) mm rectangular tube columns and \(200\times 75\times 6\) mm bent channel beams, a plastic hinge formed at the beams; this was used to determine the internal force and the story drift angle was measured up to 0.04 rad.
4. The cross-shaped connector plate in the longitudinal direction showed excellent stiffness compared to the straight connector plate; the actual drift angle of the upper and lower columns, analyzed on the basis of a 0.04 rad drift angle, was smaller for the cross-shaped connector plate than for the straight connector plate by around 0.0014 rad.
5. Even if the upper and lower beam webs are bolted using a cross-shaped connector plate, the composite effect of the upper and lower beams is minimal, and thus it is better to perform structural analysis by modeling the ceiling and floor beams independently during the design phase. As well as by using connector plates between modular units, the effect of torsion can be minimized by installing shim plates in areas where upper-lower and left-right beams meet.
6. It should be noted that the results presented in this study are a preliminary investigation in order to evaluate a general behavior of the connections. As a recommendation for future work, more detailed and larger experiments including the behavior of bolts are recommended to enhance the reliability of the results and to robustly establish a general behavior of the connections proposed in this study.  
All authors have contributed to the development of the research and in the elaboration of this article. Particularly, [PERSON] contributed to the methodology, the investigation and the conceptualization; [PERSON] and [PERSON] carried the experimental research and draft the manuscript; [PERSON] contributed the methodology and edited the manuscript. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
This research was supported by a grant (2020-MOIS31-012) from the Fundamental Technology Development program for Extreme Disaster Response funded by the Ministry of Interior and Safety, and a grant (20-RERP-B082884-07) from the Housing Environment Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government.  
The authors declare no competing interest.  
All datasets generated in this study are available from the corresponding author upon reasonable request.