## 3 Experimental Program  
### Test Specimens  
This section discusses the test specimens and their settings considered in this study to evaluate the structural performance of the beam-column connection of the proposed steel modular system subjected to lateral loading. The main difficulty of the test is that double beam and column members exist at the beam-column connection of the assembled modular units, and thus it is difficult to impose proper support conditions on the test specimens.  
Figure 4(a) illustrates the typical bending moment diagram of a two-story modular frame system with two spans, which is subjected to two concentrated horizontal forces. Since it is not easy to perform a full-scale test on the entire frame structure, it would be better to cut some portion of the structure around the inner beam-column connection and perform a test on it. According to this scheme, an ideal test setup, which is obtained by cutting the frame structure along the dashed line indicated in Figure 4(a), can be suggested, as shown in Figure 4(b). In the ideal test setup, roller supports exist at the end of the horizontal floor and ceiling beams, and it is quite difficult to create specimen details exactly describing the given roller support conditions.  
Instead, as an alternative, we propose the specimen setup, shown in Figure 4(c), where a sub-column member is added to the end of the ceiling beam and the two horizontal beam members are loosely connected by a vertical link plate. The details of the vertical link plate are illustrated in Figure 4(d). Under this setting, there may be an additional contribution to the horizontal displacement at the top of  
Figure 4: Blind bolt details.  
Figure 3: Fastening process of the blind bolt.  
the double main columns induced by the rigid body rotation of the entire specimen. This additional displacement must be subtracted from the horizontal displacement measured at the top of the double main columns. This issue will be discussed in more details in Section 4.1.  
In order to investigate the effectiveness of the beam-column connection of the proposed steel frame modular unit, two types of specimens (specimens IC_OB_F and IC_OB_B) were manufactured and tested. Specim IC_OB_B has four knee bracing members with a cross section of L50x50x6 to increase the bending stiffness of the beam-column connection. In contrast, specimen IC_OB_F does not have these components. Except the use of the knee bracings, the two specimens are identical as indicated in Table 1, which lists all the frame members and connection components used in the two specimens. The detailed drawings of specimens IC_OB_F and IC_OB_B are provided in Figures 6 and 7, respectively. Two types of steel materials (SS400 and SM490) were used for the test specimens as indicated in Table 2. Their yield and ultimate strengths were measured per ASTM A370 [22], and are also summarized in the table. Table 3 shows the material properties of blind bolts.  
Figure 5: Determination of the test setup.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Specimen** & **IC\_OB\_F** & **IC\_OB\_B** \\ \hline \multirow{4}{*}{Frame members} & Column & HSS200\(\times\)100\(\times\)6 & \\ \cline{2-3}  & Floor beam & C250\(\times\)100\(\times\)4 & \\ \cline{2-3}  & Ceiling beam & C150\(\times\)100\(\times\)4 & \\ \cline{2-3}  & Knee brace & L50\(\times\)50\(\times\)4 & N/A \\ \hline \multirow{4}{*}{Frame members} & Connection plate & PL-12 & \\ \cline{2-3}  & Front-side connection bolts & 10 F13T M20 blind bolts + 8 F10T M20 H/T bolts & \\ \cline{2-3}  & Connection components & Detail of module to module connection & \\ \cline{2-3}  &