# Seismic Design Procedure for Low-Rise Cold-Formed Steel-Special Bolded Moment Frames
## 4 Test Results  
### Global Responses  
Figure 11 shows the applied load on the vertical axis and the storey drift ratio on the horizontal axis. The mechanism loads corresponding to bolt slippage (\(V_{s}\)) and beam yielding (\(V_{b}\)) are indicated for each specimen. The corresponding mechanism loads are summarised in Table 3. The specimens OS_01 to BS_02 were designed to carry out the proposed design procedure. Therefore, these specimens exhibited a plateau strength, and strength hardening occurred once the storey drift ratio became large (see Figure 11a-d). These response characteristics were similar to those in a previous study [4]. The plateau strength corresponded fairly well to the slip strength (\(V_{S}\)), and the \"pre-tensioned (PT)\" joint type exhibited a greater plateau strength than that of the \"sruig-tigthened (ST)\" joint type. The cross-sectional profile of the beam was the same for all specimens. Therefore, the load at which the beam yielded (\(V_{b}\)) was a fixed value for the OS and BS specimens, as shown in Table 3. It is clear in each figure that strength deterioration began when the load reached the beam yield strength or when the storey drift ratio exceeded 4.0%. The PT joint-type specimens (OS-1 and BS-1) showed strength deterioration when the storey drift ratio was less than 4.0%. According to the design phase, the expected moment at the bolted connection was assumed to be equal to the effective beam yield strength. Therefore, lateral torsional buckling (LTB) occurred in the beam when the load reached this criterion. Additionally, the overstrength margin for the PT joint-type bolted connection was smaller than that for the ST joint type. Furthermore, the storey drift ratio that could be accommodated to avoid strength deterioration was also smaller for the PT joint type than for the ST joint type. The strength of the ST joint-type specimen (OS-2) was degraded when the storey drift ratio exceeded 4.0%. This was due to the LTB, and the contact between the column and the beam led to the beam being overloaded. Once the load reached the beam strength, LTB occurred at the beam, and out-of-plane deformation accumulated along with strength deterioration. From these observations, it can be concluded that the snugly tightened joint, as required by the AISI [1], showed an advantage in both its overstrength capacity and acceptable storey drift ratio. Specimen BS_03, which had a bolt slippage strength greater than the beam yield strength, did not show a plateau strength (see Figure 11e). Once the load reached the beam yielding strength (\(V_{b}\)), strength deterioration started rapidly due to the beam's LTB. According to the CFS-SBFM design concept, the beam and column must remain essentially elastic. Otherwise, drastic strength deterioration may occur due to instability in the member, leading to a lack of high system ductility. In OS_01, LTB of the beam was observed at a storey drift ratio of 3.0% (see Figure 12a), where the applied load reached the beam yielding strength. At a storey drift ratio of 4.0%, the beam flange started to contact the column face (see Figure 12b), as the gap between the beam and column was designed to accommodate this storey drift ratio. After the beam contacted the column, the beam was overloaded, and local buckling (LB) of the beam web was observed (see Figure 12c). After observing LTB, LB, and strength deterioration, the loading was terminated. In OS_02, a significant LTB of the beam was observed at a storey drift ratio of 5.0% (see Figure 13a). At a storey drift ratio of 6.0%, the accumulated lateral torsional deformation became large (see Figure 13b), and LB of the beam flange was simultaneously observed (see Figure 13c). Significant LB deformation was mainly due to the contact between the column and beam. In BS_01, LTB of the beam occurred at a storey drift ratio of 4.0% (see Figure 14a), and LB of the web and flange was observed after the beam contacted the column face, which was similar to the situation with OS_01 (see Figure 14b).