## 3 Experimental Study  
In order to verify the effectiveness of the hybrid ML and MU technique, first, an experimental model of a bolted joint single-story steel planer frame structure as shown in Figure 3 is considered. The detailed fabrication and experimental procedure are explained in the experimental modeling section given below. A single-storey planer frame structure as depicted in Figure 3 is utilized to estimate the level of severity of joint damage using the experimental data. The small-scale steel frame model was developed in the laboratory and the whole experimental study was conducted in the control environment of the Experimental Mechanics Laboratory of the Civil Engineering Department at IIT Bombay. The properties of the steel frame are [PERSON]'s modulus = 2.1 GPa, Poisson's ratio = 0.3, and mass density = 7850 kg/m\({}^{3}\). The width and depth of the members are 0.032 m and 0.006 m, respectively. The height of the column and the length of the beam are 0.5 m and 0.3 m, respectively. In this context, the members of the above-mentioned sizes and dimensions were available in the laboratory and utilized for the fabrication of the frame. In order to make the beam-column connections, angle sections and nut-bolts were used. At the side of the column face as well as at the side of the beam face of a joint, one angle and four M-5 nut-bolts are used for the proper beam-column connection. In order to generate fixed support for the frame, two angles are placed at the inner and outer faces of the column and tightly connected at the base using nut-bolts. The experimental study was carried out for unloosed conditions, i.e., all bolts are in full-tight condition (UL) and also for different loosening conditions at different connections (BL1 to BL6). The unloosened and different loosened conditions are further explained in detail in Table 2 and Figure 4. It is better to be mentioned here that unloosened condition means all bolts are fully-tight represents bolts are tightened using hand plus turned the bolts half using a spanner and the hand-tight represents that the bolts are tightened using hands. \begin{table}
\begin{tabular}{c c} \hline \hline
**Condition of Structure** & **Details of Loosening** \\ \hline UL & Bolts are fully tight \\ BL1 & Right column-beam connection, bolts @ column face is hand tight \\ BL2 & Right column-beam connection, bolts @ column face is full loose \\ BL3 & Left column-beam connection, bolts @ beam face is hand tight \\ BL4 & Right column-beam connection, bolts @ beam face is full loose \\ BL5 & Left column-beam connection, bolts @ column face is hand tight \\ BL6 & Left column-beam connection, bolts @ column face is full loose \\ BL7 & Left column-beam connection, 2 bolts @ column face is full loose \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental study case details. Figure 3: **(a)** Experimental steel planer frame model on the rigid platform and data logger, **(b)** schematic diagram showing the location of sensors and excitation points. A B & K made B & K 8202-8200, 31.6 mv/N impact hammer was employed to provide an impact at the top corner of the right beam-column connection as described in Figure 2b. The strain time responses were registered using 12-contact type strain sensors (_FLA-3_), from SG1 to SG6. Here, the contact type strain gauges were fixed on either side of the column/beam members at 35 mm from the beam-column joints as shown in Figure 3. At SG1, 1-strain gauge is placed on the outer-face of the member and another strain gauge is placed inner-face of the member. Strain gauges are fitted in SG2 to SG6 position similarly. HBM-made MGC-plus data logger was utilized to collect the strain time history data with 600 Hz sampling frequency. In order to approximately localize joint damage using the ML approach, strain time history data were categorized into \"healthy\" and \"loosened\" cases. In the experimental case of UL, all 12 strain gauges were provided with healthy data, thus, producing 120 healthy data sets in 10 trials.